Обсуждение: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

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CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Corey Huinker
Дата:

Attached is my work in progress to implement the changes to the CAST() function as proposed by Vik Fearing.

This work builds upon the Error-safe User Functions work currently ongoing.

The proposed changes are as follows:

CAST(expr AS typename)
    continues to behave as before.

CAST(expr AS typename ERROR ON ERROR)
    has the identical behavior as the unadorned CAST() above.

CAST(expr AS typename NULL ON ERROR)
    will use error-safe functions to do the cast of expr, and will return NULL if the cast fails.

CAST(expr AS typename DEFAULT expr2 ON ERROR)
    will use error-safe functions to do the cast of expr, and will return expr2 if the cast fails.

There is an additional FORMAT parameter that I have not yet implemented, my understanding is that it is largely intended for DATE/TIME field conversions, but others are certainly possible.
CAST(expr AS typename FORMAT fmt DEFAULT expr2 ON ERROR)

What is currently working:
- Any scalar expression that can be evaluated at parse time. These tests from cast.sql all currently work:

VALUES (CAST('error' AS integer));
VALUES (CAST('error' AS integer ERROR ON ERROR));
VALUES (CAST('error' AS integer NULL ON ERROR));
VALUES (CAST('error' AS integer DEFAULT 42 ON ERROR));

SELECT CAST('{123,abc,456}' AS integer[] DEFAULT '{-789}' ON ERROR) as array_test1;

- Scalar values evaluated at runtime.

CREATE TEMPORARY TABLE t(t text);
INSERT INTO t VALUES ('a'), ('1'), ('b'), (2);
SELECT CAST(t.t AS integer DEFAULT -1 ON ERROR) AS foo FROM t;
 foo
-----
  -1
   1
  -1
   2
(4 rows)


Along the way, I made a few design decisions, each of which is up for debate:

First, I created OidInputFunctionCallSafe, which is to OidInputFunctionCall what InputFunctionCallSafe is to InputFunctionCall. Given that the only place I ended up using it was stringTypeDatumSafe(), it may be possible to just move that code inside stringTypeDatumSafe.

Next, I had a need for FuncExpr, CoerceViaIO, and ArrayCoerce to all report if their expr argument failed, and if not, just past the evaluation of expr2. Rather than duplicate this logic in several places, I chose instead to modify CoalesceExpr to allow for an error-test mode in addition to its default null-test mode, and then to provide this altered node with two expressions, the first being the error-safe typecast of expr and the second being the non-error-safe typecast of expr2.

I still don't have array-to-array casts working, as the changed I would likely need to make to ArrayCoerce get somewhat invasive, so this seemed like a good time to post my work so far and solicit some feedback beyond what I've already been getting from Jeff Davis and Michael Paquier.

I've sidestepped domains as well for the time being as well as avoiding JIT issues entirely.

No documentation is currently prepared. All but one of the regression test queries work, the one that is currently failing is:

SELECT CAST('{234,def,567}'::text[] AS integer[] DEFAULT '{-1011}' ON ERROR) as array_test2;

Other quirks:
- an unaliased CAST ON DEFAULT will return the column name of "coalesce", which internally is true, but obviously would be quite confusing to a user.

As a side observation, I noticed that the optimizer already tries to resolve expressions based on constants and to collapse expression trees where possible, which makes me wonder if the work done to do the same in transformTypeCast/ and coerce_to_target_type and coerce_type isn't also wasted.

Вложения

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Tom Lane
Дата:
Corey Huinker <corey.huinker@gmail.com> writes:
> The proposed changes are as follows:
> CAST(expr AS typename)
>     continues to behave as before.
> CAST(expr AS typename ERROR ON ERROR)
>     has the identical behavior as the unadorned CAST() above.
> CAST(expr AS typename NULL ON ERROR)
>     will use error-safe functions to do the cast of expr, and will return
> NULL if the cast fails.
> CAST(expr AS typename DEFAULT expr2 ON ERROR)
>     will use error-safe functions to do the cast of expr, and will return
> expr2 if the cast fails.

While I approve of trying to get some functionality in this area,
I'm not sure that extending CAST is a great idea, because I'm afraid
that the SQL committee will do something that conflicts with it.
If we know that they are about to standardize exactly this syntax,
where is that information available?  If we don't know that,
I'd prefer to invent some kind of function or other instead of
extending the grammar.

            regards, tom lane

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
vignesh C
Дата:
On Tue, 20 Dec 2022 at 04:27, Corey Huinker <corey.huinker@gmail.com> wrote:
>
>
> Attached is my work in progress to implement the changes to the CAST() function as proposed by Vik Fearing.
>
> This work builds upon the Error-safe User Functions work currently ongoing.
>
> The proposed changes are as follows:
>
> CAST(expr AS typename)
>     continues to behave as before.
>
> CAST(expr AS typename ERROR ON ERROR)
>     has the identical behavior as the unadorned CAST() above.
>
> CAST(expr AS typename NULL ON ERROR)
>     will use error-safe functions to do the cast of expr, and will return NULL if the cast fails.
>
> CAST(expr AS typename DEFAULT expr2 ON ERROR)
>     will use error-safe functions to do the cast of expr, and will return expr2 if the cast fails.
>
> There is an additional FORMAT parameter that I have not yet implemented, my understanding is that it is largely
intendedfor DATE/TIME field conversions, but others are certainly possible. 
> CAST(expr AS typename FORMAT fmt DEFAULT expr2 ON ERROR)
>
> What is currently working:
> - Any scalar expression that can be evaluated at parse time. These tests from cast.sql all currently work:
>
> VALUES (CAST('error' AS integer));
> VALUES (CAST('error' AS integer ERROR ON ERROR));
> VALUES (CAST('error' AS integer NULL ON ERROR));
> VALUES (CAST('error' AS integer DEFAULT 42 ON ERROR));
>
> SELECT CAST('{123,abc,456}' AS integer[] DEFAULT '{-789}' ON ERROR) as array_test1;
>
> - Scalar values evaluated at runtime.
>
> CREATE TEMPORARY TABLE t(t text);
> INSERT INTO t VALUES ('a'), ('1'), ('b'), (2);
> SELECT CAST(t.t AS integer DEFAULT -1 ON ERROR) AS foo FROM t;
>  foo
> -----
>   -1
>    1
>   -1
>    2
> (4 rows)
>
>
> Along the way, I made a few design decisions, each of which is up for debate:
>
> First, I created OidInputFunctionCallSafe, which is to OidInputFunctionCall what InputFunctionCallSafe is to
InputFunctionCall.Given that the only place I ended up using it was stringTypeDatumSafe(), it may be possible to just
movethat code inside stringTypeDatumSafe. 
>
> Next, I had a need for FuncExpr, CoerceViaIO, and ArrayCoerce to all report if their expr argument failed, and if
not,just past the evaluation of expr2. Rather than duplicate this logic in several places, I chose instead to modify
CoalesceExprto allow for an error-test mode in addition to its default null-test mode, and then to provide this altered
nodewith two expressions, the first being the error-safe typecast of expr and the second being the non-error-safe
typecastof expr2. 
>
> I still don't have array-to-array casts working, as the changed I would likely need to make to ArrayCoerce get
somewhatinvasive, so this seemed like a good time to post my work so far and solicit some feedback beyond what I've
alreadybeen getting from Jeff Davis and Michael Paquier. 
>
> I've sidestepped domains as well for the time being as well as avoiding JIT issues entirely.
>
> No documentation is currently prepared. All but one of the regression test queries work, the one that is currently
failingis: 
>
> SELECT CAST('{234,def,567}'::text[] AS integer[] DEFAULT '{-1011}' ON ERROR) as array_test2;
>
> Other quirks:
> - an unaliased CAST ON DEFAULT will return the column name of "coalesce", which internally is true, but obviously
wouldbe quite confusing to a user. 
>
> As a side observation, I noticed that the optimizer already tries to resolve expressions based on constants and to
collapseexpression trees where possible, which makes me wonder if the work done to do the same in transformTypeCast/
andcoerce_to_target_type and coerce_type isn't also wasted. 

CFBot shows some compilation errors as in [1], please post an updated
version for the same:
[02:53:44.829] time make -s -j${BUILD_JOBS} world-bin
[02:55:41.164] llvmjit_expr.c: In function ‘llvm_compile_expr’:
[02:55:41.164] llvmjit_expr.c:928:6: error: ‘v_resnull’ undeclared
(first use in this function); did you mean ‘v_resnullp’?
[02:55:41.164] 928 | v_resnull = LLVMBuildLoad(b, v_reserrorp, "");
[02:55:41.164] | ^~~~~~~~~
[02:55:41.164] | v_resnullp
[02:55:41.164] llvmjit_expr.c:928:6: note: each undeclared identifier
is reported only once for each function it appears in
[02:55:41.164] llvmjit_expr.c:928:35: error: ‘v_reserrorp’ undeclared
(first use in this function); did you mean ‘v_reserror’?
[02:55:41.164] 928 | v_resnull = LLVMBuildLoad(b, v_reserrorp, "");
[02:55:41.164] | ^~~~~~~~~~~
[02:55:41.164] | v_reserror
[02:55:41.165] make[2]: *** [<builtin>: llvmjit_expr.o] Error 1
[02:55:41.165] make[2]: *** Waiting for unfinished jobs....
[02:55:45.495] make[1]: *** [Makefile:42: all-backend/jit/llvm-recurse] Error 2
[02:55:45.495] make: *** [GNUmakefile:21: world-bin-src-recurse] Error 2

[1] - https://cirrus-ci.com/task/6687753371385856?logs=gcc_warning#L448

Regards,
Vignesh

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Andrew Dunstan
Дата:
On 2023-01-02 Mo 10:57, Tom Lane wrote:
> Corey Huinker <corey.huinker@gmail.com> writes:
>> The proposed changes are as follows:
>> CAST(expr AS typename)
>>     continues to behave as before.
>> CAST(expr AS typename ERROR ON ERROR)
>>     has the identical behavior as the unadorned CAST() above.
>> CAST(expr AS typename NULL ON ERROR)
>>     will use error-safe functions to do the cast of expr, and will return
>> NULL if the cast fails.
>> CAST(expr AS typename DEFAULT expr2 ON ERROR)
>>     will use error-safe functions to do the cast of expr, and will return
>> expr2 if the cast fails.
> While I approve of trying to get some functionality in this area,
> I'm not sure that extending CAST is a great idea, because I'm afraid
> that the SQL committee will do something that conflicts with it.
> If we know that they are about to standardize exactly this syntax,
> where is that information available?  If we don't know that,
> I'd prefer to invent some kind of function or other instead of
> extending the grammar.


+1


cheers


andrew

--
Andrew Dunstan
EDB: https://www.enterprisedb.com

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Corey Huinker
Дата:



On Mon, Jan 2, 2023 at 10:57 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
Corey Huinker <corey.huinker@gmail.com> writes:
> The proposed changes are as follows:
> CAST(expr AS typename)
>     continues to behave as before.
> CAST(expr AS typename ERROR ON ERROR)
>     has the identical behavior as the unadorned CAST() above.
> CAST(expr AS typename NULL ON ERROR)
>     will use error-safe functions to do the cast of expr, and will return
> NULL if the cast fails.
> CAST(expr AS typename DEFAULT expr2 ON ERROR)
>     will use error-safe functions to do the cast of expr, and will return
> expr2 if the cast fails.

While I approve of trying to get some functionality in this area,
I'm not sure that extending CAST is a great idea, because I'm afraid
that the SQL committee will do something that conflicts with it.
If we know that they are about to standardize exactly this syntax,
where is that information available?  If we don't know that,
I'd prefer to invent some kind of function or other instead of
extending the grammar.

                        regards, tom lane

I'm going off the spec that Vik presented in https://www.postgresql.org/message-id/f8600a3b-f697-2577-8fea-f40d3e18bea8@postgresfriends.org which is his effort to get it through the SQL committee. I was alreading thinking about how to get the SQLServer TRY_CAST() function into postgres, so this seemed like the logical next step.

While the syntax may change, the underlying infrastructure would remain basically the same: we would need the ability to detect that a typecast had failed, and replace it with the default value, and handle that at parse time, or executor time, and handle array casts where the array has the default but the underlying elements can't.

It would be simple to move the grammar changes to their own patch if that removes a barrier for people.
 

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Tom Lane
Дата:
Corey Huinker <corey.huinker@gmail.com> writes:
> On Mon, Jan 2, 2023 at 10:57 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> While I approve of trying to get some functionality in this area,
>> I'm not sure that extending CAST is a great idea, because I'm afraid
>> that the SQL committee will do something that conflicts with it.

> I'm going off the spec that Vik presented in
> https://www.postgresql.org/message-id/f8600a3b-f697-2577-8fea-f40d3e18bea8@postgresfriends.org
> which is his effort to get it through the SQL committee.

I'm pretty certain that sending something to pgsql-hackers will have
exactly zero impact on the SQL committee.  Is there anything actually
submitted to the committee, and if so what's its status?

            regards, tom lane

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Vik Fearing
Дата:
On 1/3/23 19:14, Tom Lane wrote:
> Corey Huinker <corey.huinker@gmail.com> writes:
>> On Mon, Jan 2, 2023 at 10:57 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> While I approve of trying to get some functionality in this area,
>>> I'm not sure that extending CAST is a great idea, because I'm afraid
>>> that the SQL committee will do something that conflicts with it.
> 
>> I'm going off the spec that Vik presented in
>> https://www.postgresql.org/message-id/f8600a3b-f697-2577-8fea-f40d3e18bea8@postgresfriends.org
>> which is his effort to get it through the SQL committee.
> 
> I'm pretty certain that sending something to pgsql-hackers will have
> exactly zero impact on the SQL committee.  Is there anything actually
> submitted to the committee, and if so what's its status?

I have not posted my paper to the committee yet, but I plan to do so 
before the working group's meeting early February.  Just like with 
posting patches here, I cannot guarantee that it will get accepted but I 
will be arguing for it.

I don't think we should add that syntax until I do get it through the 
committee, just in case they change something.
-- 
Vik Fearing

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Tom Lane
Дата:
Vik Fearing <vik@postgresfriends.org> writes:
> I have not posted my paper to the committee yet, but I plan to do so 
> before the working group's meeting early February.  Just like with 
> posting patches here, I cannot guarantee that it will get accepted but I 
> will be arguing for it.

> I don't think we should add that syntax until I do get it through the 
> committee, just in case they change something.

Agreed.  So this is something we won't be able to put into v16;
it'll have to wait till there's something solid from the committee.

            regards, tom lane

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
"Gregory Stark (as CFM)"
Дата:
On Tue, 3 Jan 2023 at 14:16, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Vik Fearing <vik@postgresfriends.org> writes:
>
> > I don't think we should add that syntax until I do get it through the
> > committee, just in case they change something.
>
> Agreed.  So this is something we won't be able to put into v16;
> it'll have to wait till there's something solid from the committee.

I guess I'll mark this Rejected in the CF then. Who knows when the SQL
committee will look at this...

-- 
Gregory Stark
As Commitfest Manager

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Isaac Morland
Дата:
On Mon, 19 Dec 2022 at 17:57, Corey Huinker <corey.huinker@gmail.com> wrote:

Attached is my work in progress to implement the changes to the CAST() function as proposed by Vik Fearing.

CAST(expr AS typename NULL ON ERROR)
    will use error-safe functions to do the cast of expr, and will return NULL if the cast fails.

CAST(expr AS typename DEFAULT expr2 ON ERROR)
    will use error-safe functions to do the cast of expr, and will return expr2 if the cast fails.

Is there any difference between NULL and DEFAULT NULL? 

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Corey Huinker
Дата:


On Tue, Mar 28, 2023 at 2:53 PM Gregory Stark (as CFM) <stark.cfm@gmail.com> wrote:
On Tue, 3 Jan 2023 at 14:16, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> Vik Fearing <vik@postgresfriends.org> writes:
>
> > I don't think we should add that syntax until I do get it through the
> > committee, just in case they change something.
>
> Agreed.  So this is something we won't be able to put into v16;
> it'll have to wait till there's something solid from the committee.

I guess I'll mark this Rejected in the CF then. Who knows when the SQL
committee will look at this...

--
Gregory Stark
As Commitfest Manager

Yes, for now. I'm in touch with the pg-people on the committee and will resume work when there's something to act upon.
 

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Corey Huinker
Дата:
On Tue, Mar 28, 2023 at 3:25 PM Isaac Morland <isaac.morland@gmail.com> wrote:
On Mon, 19 Dec 2022 at 17:57, Corey Huinker <corey.huinker@gmail.com> wrote:

Attached is my work in progress to implement the changes to the CAST() function as proposed by Vik Fearing.

CAST(expr AS typename NULL ON ERROR)
    will use error-safe functions to do the cast of expr, and will return NULL if the cast fails.

CAST(expr AS typename DEFAULT expr2 ON ERROR)
    will use error-safe functions to do the cast of expr, and will return expr2 if the cast fails.

Is there any difference between NULL and DEFAULT NULL? 

What I think you're asking is: is there a difference between these two statements:

SELECT CAST(my_string AS integer NULL ON ERROR) FROM my_table;

SELECT CAST(my_string AS integer DEFAULT NULL ON ERROR) FROM my_table;

And as I understand it, the answer would be no, there is no practical difference. The first case is just a convenient shorthand, whereas the second case tees you up for a potentially complex expression. Before you ask, I believe the ON ERROR syntax could be made optional. As I implemented it, both cases create a default expression which then typecast to integer, and in both cases that expression would be a const-null, so the optimizer steps would very quickly collapse those steps into a plain old constant.


 

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
jian he
Дата:
hi.
more preparation work has been committed.

1. SQL/JSON patch [1] added keyword ERROR
2. CoerceViaIo, CoerceToDomain can be evaluated error safe. see commit [2].
3. ExprState added ErrorSaveContext point, so before calling ExecInitExprRec
set valid ErrorSaveContext for ExprState->escontext we should evaluate
expression error softly.
see commit [2] also.

I only found oracle implement, [3].
Based on my reading of [4], it seems CAST(EXPRESSION AS TYPE DEFAULT
def_expr ON ERROR)
is not included in SQL:2023.

anyway, just share my POC based on the previous patch in this thread.
it will work for domain over composite, composite over domain.
example:
CREATE DOMAIN d_char3_not_null as char(3) NOT NULL;
CREATE TYPE comp_domain_with_typmod AS (a d_char3_not_null, b int);
SELECT CAST('(,42)' AS comp_domain_with_typmod DEFAULT NULL ON ERROR);
--return NULL


[1]:
https://git.postgresql.org/cgit/postgresql.git/diff/src/backend/parser/gram.y?id=6185c9737cf48c9540782d88f12bd2912d6ca1cc
[2]: https://git.postgresql.org/cgit/postgresql.git/commit/?id=aaaf9449ec6be62cb0d30ed3588dc384f56274bf
[3] https://docs.oracle.com/en/database/oracle/oracle-database/23/sqlrf/CAST.html
[4] https://peter.eisentraut.org/blog/2023/04/04/sql-2023-is-finished-here-is-whats-new
Вложения

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Vik Fearing
Дата:
On 22/07/2025 03:59, jian he wrote:
> Based on my reading of [4], it seems CAST(EXPRESSION AS TYPE DEFAULT
> def_expr ON ERROR)
> is not included in SQL:2023.
>
> [4]https://peter.eisentraut.org/blog/2023/04/04/sql-2023-is-finished-here-is-whats-new


It was accepted into the standard after 2023 was released.  I am the 
author of this change in the standard, so feel free to ask me anything 
you're unsure about.

-- 

Vik Fearing

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
jian he
Дата:
On Tue, Jul 22, 2025 at 2:45 PM Vik Fearing <vik@postgresfriends.org> wrote:
>
> It was accepted into the standard after 2023 was released.  I am the
> author of this change in the standard, so feel free to ask me anything
> you're unsure about.
>

is the generally syntax as mentioned in this thread:
CAST(source_expression AS target_type DEFAULT default_expression ON ERROR)

if so, what's the restriction of default_expression?

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Vik Fearing
Дата:
On 22/07/2025 12:19, jian he wrote:
> On Tue, Jul 22, 2025 at 2:45 PM Vik Fearing <vik@postgresfriends.org> wrote:
>> It was accepted into the standard after 2023 was released.  I am the
>> author of this change in the standard, so feel free to ask me anything
>> you're unsure about.
>>
> is the generally syntax as mentioned in this thread:
> CAST(source_expression AS target_type DEFAULT default_expression ON ERROR)
>
> if so, what's the restriction of default_expression?


The actual syntax is:


<cast specification> ::=
     CAST <left paren>
         <cast operand> AS <cast target>
         [ FORMAT <cast template> ]
         [ <cast error behavior> ON CONVERSION ERROR ]
         <right paren>


"CONVERSION" is probably a noise word, but it is there because A) Oracle 
wanted it there, and B) it makes sense because if the <cast error 
behavior> fails, that is still a failure of the entire CAST.


The <cast error behavior> is:


<cast error behavior> ::=
     ERROR
   | NULL
   | DEFAULT <value expression>


but I am planning on removing the NULL variant in favor of having the 
<value expression> be a <contextually typed value specification>.  So it 
would be either ERROR ON CONVERSION ERROR (postgres's current behavior), 
or DEFAULT NULL ON CONVERSION ERROR.


An example of B) above would be: CAST('five' AS INTEGER DEFAULT 'six' ON 
CONVERSION ERROR).  'six' is no more an integer than 'five' is, so that 
would error out because the conversion error does not happen on the 
operand but on the default clause. CAST('five' AS INTEGER DEFAULT 6 ON 
CONVERSION ERROR) would work.

-- 

Vik Fearing

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Vik Fearing
Дата:
On 22/07/2025 14:26, Vik Fearing wrote:
> The <cast error behavior> is:
>
> <cast error behavior> ::=
>     ERROR
>   | NULL
>   | DEFAULT <value expression>
>
> but I am planning on removing the NULL variant in favor of having the 
> <value expression> be a <contextually typed value specification>.  So 
> it would be either ERROR ON CONVERSION ERROR (postgres's current 
> behavior), or DEFAULT NULL ON CONVERSION ERROR. 


Sorry, I meant <implicitly typed value specification>.


The point being that CAST(ARRAY['1', '2', 'three'] AS INTEGER ARRAY 
DEFAULT NULL ON CONVERSION ERROR) will give you (CAST NULL AS INTEGER 
ARRAY) and *not* ARRAY[1, 2, NULL].

-- 

Vik Fearing

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Corey Huinker
Дата:


On Tue, Jul 22, 2025 at 2:45 AM Vik Fearing <vik@postgresfriends.org> wrote:

On 22/07/2025 03:59, jian he wrote:
> Based on my reading of [4], it seems CAST(EXPRESSION AS TYPE DEFAULT
> def_expr ON ERROR)
> is not included in SQL:2023.
>
> [4]https://peter.eisentraut.org/blog/2023/04/04/sql-2023-is-finished-here-is-whats-new


It was accepted into the standard after 2023 was released.  I am the
author of this change in the standard, so feel free to ask me anything
you're unsure about.


That's excellent news. I was already planning on retrying this for v19, but I'll try sooner now.
 

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
jian he
Дата:
On Tue, Jul 22, 2025 at 8:26 PM Vik Fearing <vik@postgresfriends.org> wrote:
>
> The actual syntax is:
>
>
> <cast specification> ::=
>      CAST <left paren>
>          <cast operand> AS <cast target>
>          [ FORMAT <cast template> ]
>          [ <cast error behavior> ON CONVERSION ERROR ]
>          <right paren>
>
>
> "CONVERSION" is probably a noise word, but it is there because A) Oracle
> wanted it there, and B) it makes sense because if the <cast error
> behavior> fails, that is still a failure of the entire CAST.
>
>
> The <cast error behavior> is:
>
> <cast error behavior> ::=
>      ERROR
>    | NULL
>    | DEFAULT <value expression>
>
>
> but I am planning on removing the NULL variant in favor of having the
> <value expression> be a <contextually typed value specification>.  So it
> would be either ERROR ON CONVERSION ERROR (postgres's current behavior),
> or DEFAULT NULL ON CONVERSION ERROR.
>
>
> An example of B) above would be: CAST('five' AS INTEGER DEFAULT 'six' ON
> CONVERSION ERROR).  'six' is no more an integer than 'five' is, so that
> would error out because the conversion error does not happen on the
> operand but on the default clause. CAST('five' AS INTEGER DEFAULT 6 ON
> CONVERSION ERROR) would work.
>

hi.

> <cast error behavior> ::=
>      ERROR
>    | NULL
>    | DEFAULT <value expression>

for <value expression>
I disallow it from returning a set, or using aggregate or window functions.
For example, the following three cases will fail:

+SELECT CAST('a' as int DEFAULT sum(1) ON CONVERSION ERROR); --error
+SELECT CAST('a' as int DEFAULT sum(1) over() ON CONVERSION ERROR); --error
+SELECT CAST('a' as int DEFAULT ret_setint() ON CONVERSION ERROR) --error
(ret_setint function is warped as (select 1 union all select 2))

for array coerce, which you already mentioned, i think the following
is what we expected.
+SELECT CAST('{234,def,567}'::text[] AS integer[] DEFAULT '{-1011}' ON
CONVERSION ERROR);
+  int4
+---------
+ {-1011}
+(1 row)

I didn't implement the [ FORMAT <cast template> ] part for now.
please check the attached regress test and tests expected result.
Вложения

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
jian he
Дата:
On Tue, Jul 22, 2025 at 8:26 PM Vik Fearing <vik@postgresfriends.org> wrote:
>
>
> On 22/07/2025 12:19, jian he wrote:
> > On Tue, Jul 22, 2025 at 2:45 PM Vik Fearing <vik@postgresfriends.org> wrote:
> >> It was accepted into the standard after 2023 was released.  I am the
> >> author of this change in the standard, so feel free to ask me anything
> >> you're unsure about.
> >>
> > is the generally syntax as mentioned in this thread:
> > CAST(source_expression AS target_type DEFAULT default_expression ON ERROR)
> >
> > if so, what's the restriction of default_expression?
>
>
> The actual syntax is:
>
>
> <cast specification> ::=
>      CAST <left paren>
>          <cast operand> AS <cast target>
>          [ FORMAT <cast template> ]
>          [ <cast error behavior> ON CONVERSION ERROR ]
>          <right paren>
>
> "CONVERSION" is probably a noise word, but it is there because A) Oracle
> wanted it there, and B) it makes sense because if the <cast error
> behavior> fails, that is still a failure of the entire CAST.
>
>
> The <cast error behavior> is:
>
>
> <cast error behavior> ::=
>      ERROR
>    | NULL
>    | DEFAULT <value expression>
>
>

hi.

just want to confirm my understanding of ``[ FORMAT <cast template> ]``.

SELECT CAST('2022-13-32' AS DATE FORMAT 'YYYY-MM-DD' DEFAULT NULL ON
CONVERSION ERROR);
will return NULL.
because  ``SELECT to_date('2022-13-32', 'YYYY-MM-DD');``
will error out, so the above query will fall back to the DEFAULT
expression evaluation.

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Vik Fearing
Дата:


On 24/07/2025 03:22, jian he wrote: 
+SELECT CAST('a' as int DEFAULT sum(1) ON CONVERSION ERROR); --error
+SELECT CAST('a' as int DEFAULT sum(1) over() ON CONVERSION ERROR); --error


This seems like an arbitrary restriction.  Can you explain why this is necessary?  Those same expressions are allowed as the <cast operand>.


+SELECT CAST('a' as int DEFAULT ret_setint() ON CONVERSION ERROR) --error
(ret_setint function is warped as (select 1 union all select 2))


This makes sense to me.


for array coerce, which you already mentioned, i think the following
is what we expected.
+SELECT CAST('{234,def,567}'::text[] AS integer[] DEFAULT '{-1011}' ON
CONVERSION ERROR);
+  int4
+---------
+ {-1011}
+(1 row)


Yes, that looks correct to me.


I didn't implement the [ FORMAT <cast template> ] part for now.


That is fine, since it's separate feature


please check the attached regress test and tests expected result.


Except for the weird restriction on the default value, this all looks good to me (with the usual caveat that I am not an expert in C).


Are you planning to also implement the <castable predicate>?

-- 

Vik Fearing

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Vik Fearing
Дата:
On 24/07/2025 15:44, jian he wrote:
> just want to confirm my understanding of ``[ FORMAT <cast template> ]``.
>
> SELECT CAST('2022-13-32' AS DATE FORMAT 'YYYY-MM-DD' DEFAULT NULL ON
> CONVERSION ERROR);
> will return NULL.
> because  ``SELECT to_date('2022-13-32', 'YYYY-MM-DD');``
> will error out, so the above query will fall back to the DEFAULT
> expression evaluation.


That is correct.  Any error produced during typecasting will fall back 
to the DEFAULT value.  If not supplied, the behavior is ERROR ON ERROR 
as it currently is.


Any error produced while converting the DEFAULT value to the requested 
type is raised as an error.

-- 

Vik Fearing

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Corey Huinker
Дата:



I didn't implement the [ FORMAT <cast template> ] part for now.
please check the attached regress test and tests expected result.

Question about this:

+/*
+ * Push steps to evaluate a SafeTypeCastExpr and its various subsidiary expressions.
+ * We already handle CoerceViaIO, CoerceToDomain, and ArrayCoerceExpr error
+ * softly.  However, FuncExpr (e.g., int84) cannot be made error-safe.
+ * In such cases, we wrap the source expression and target type information into
+ * a CoerceViaIO node instead.
+ */

I'm not sure we _can_ just fall back to the CoerceViaIO if there is a defined cast from TypeA -> TypeB. I seem to recall there was some reason we couldn't do that, possibly to do with how it handled rounding, but I have no clear memory of it.

Aside from that, I like what you've done with making SafeTypeCastExpr be its own node type and not saddling regular typecasts with the overhead.

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
jian he
Дата:
On Thu, Jul 31, 2025 at 3:15 AM Corey Huinker <corey.huinker@gmail.com> wrote:
>
>
> Question about this:
>
> +/*
> + * Push steps to evaluate a SafeTypeCastExpr and its various subsidiary expressions.
> + * We already handle CoerceViaIO, CoerceToDomain, and ArrayCoerceExpr error
> + * softly.  However, FuncExpr (e.g., int84) cannot be made error-safe.
> + * In such cases, we wrap the source expression and target type information into
> + * a CoerceViaIO node instead.
> + */
>
> I'm not sure we _can_ just fall back to the CoerceViaIO if there is a defined cast from TypeA -> TypeB. I seem to
recallthere was some reason we couldn't do that, possibly to do with how it handled rounding, but I have no clear
memoryof it. 
>

indeed.
select ('11.1'::numeric::int);
return 11, but '11.1' string can not coerce to int 11. So in this
case, we can not use CoerceViaIO.

so we need to handle numeric source types with fractional points with
special care.
currently, this applies only to numeric, float4, and float8.
(hope this is all the corner case we need to catch...)

select castsource::regtype, casttarget::regtype, castfunc::regproc, castcontext
from   pg_cast pc
where  castsource::regtype = ANY('{numeric, float4, float8}'::regtype[])
and    castmethod = 'f';

only return 17 rows. one row is cast numreic to money, function numeric_cash.
numeric_cash seems more trickly to be error safe, because it will call
numeric_mul.
so I made these 16 function errors safe.
see v3-0001-make-some-numeric-cast-function-error-safe.patch
Вложения

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
jian he
Дата:
hi.

fix the regress tests failure in
https://api.cirrus-ci.com/v1/artifact/task/5894868779663360/testrun/build/testrun/regress/regress/regression.diffs
Вложения

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Corey Huinker
Дата:
so we need to handle numeric source types with fractional points with
special care.
currently, this applies only to numeric, float4, and float8.
(hope this is all the corner case we need to catch...)

I'm fairly certain that the committers won't like us special-casing the internal cast functions, as we would have to maintain these special cases as new core types are added, and it still bypasses the defined cast function for user-defined types, which could have similar issues similar to the rounding issue.

I think the way forward here is either to:

1.  add a second function definition to CAST. The potential syntax forr a second function gets clumsy, but might look something like this:

CREATE CAST (source_type AS target_type)
    WITH FUNCTION function_name [ (argument_type [, ...]) ]
    [ AS ASSIGNMENT | AS IMPLICIT ]
   [
    WITH SAFE FUNCTION function_name [ (argument_type [, ...]) ]
    [ AS ASSIGNMENT | AS IMPLICIT ]
   ]

That doesn't seem right to me, it seems easier to:

2. Modify the CAST definition to indicate whether the existing cast function has the regular function signature or a -Safe one. In cases where a CAST has a defined function but the safe flag is turned off, we would have to fail the query with an error like "Defined CAST function from srctype to desttype is not error-safe".

This would involve changing the syntax of CREATE CAST by adding an option SAFE, or ERROR SAFE, or similar:

CREATE CAST (source_type AS target_type)
    WITH [SAFE] FUNCTION function_name [ (argument_type [, ...]) ]
    [ AS ASSIGNMENT | AS IMPLICIT ]

We would add a new value to pg_cast.castmethod, 's' for "safe".

We could refactor all the numeric types to use the modified functions, so no special-case code there anymore, and it gives extension writers an incentive to (eventually) make their own cast functions error-safe.

While method 2 seems a lot cleaner, there may be a performance regression in the now error-safe typecast functions. If so, that might tip the balance to having two functions defined.

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
jian he
Дата:
On Mon, Aug 4, 2025 at 1:09 PM Corey Huinker <corey.huinker@gmail.com> wrote:
>>
>> so we need to handle numeric source types with fractional points with
>> special care.
>> currently, this applies only to numeric, float4, and float8.
>> (hope this is all the corner case we need to catch...)
>
>
> I'm fairly certain that the committers won't like us special-casing the internal cast functions, as we would have to
maintainthese special cases as new core types are added, and it still bypasses the defined cast function for
user-definedtypes, which could have similar issues similar to the rounding issue. 
>
It's not special-casing the internal cast functions.
It's how the cast being evaluated.
There are two ways: CoerceViaIO, FuncExpr.

generally if there is a pg_cast entry, postgres will use FuncExpr. but to safely
cast evaluation (DEFAULT ON CONVERSION ERROR) we can not use FuncExpr in some
cases. Because the FuncExpr associate function is not error safe.
So in v4, we try to use CoerceViaIO to evaluate the case, but it turns
out CoerceViaIO results are
not the same as FuncExpr.
one of the example is:
select ('11.1'::numeric::int);


In the end, it seems we need to make all these functions in the below
query error safe.
select castsource::regtype, casttarget::regtype, castfunc,
castcontext,castmethod, pp.prosrc, pp.proname from pg_cast pc join pg_proc pp on
pp.oid = pc.castfunc and pc.castfunc > 0
order by castsource::regtype;
It's a lot of work, but seems doable, after playing around with it.


I don't think we need to change the pg_cast catalog entry,
we just need to make these function (pg_cast.castmethod) errors safe.

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Corey Huinker
Дата:

In the end, it seems we need to make all these functions in the below
query error safe.
select castsource::regtype, casttarget::regtype, castfunc,
castcontext,castmethod, pp.prosrc, pp.proname from pg_cast pc join pg_proc pp on
pp.oid = pc.castfunc and pc.castfunc > 0
order by castsource::regtype;
It's a lot of work, but seems doable, after playing around with it.

It is do-able. But that's just the cast functions that are part of core postgres. 
 


I don't think we need to change the pg_cast catalog entry,
we just need to make these function (pg_cast.castmethod) errors safe.

That would break any user-defined cast functions that were not also error safe, which is to say all of them.

We need a way for user-defined cast functions to indicate whether or not they are error safe, and handle both situations accordingly (i.e. fail a CAST ON DEFAULT when the user-defined cast is not error-safe).
 

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
jian he
Дата:
On Tue, Aug 5, 2025 at 12:10 PM Corey Huinker <corey.huinker@gmail.com> wrote:
>>
>> In the end, it seems we need to make all these functions in the below
>> query error safe.
>> select castsource::regtype, casttarget::regtype, castfunc,
>> castcontext,castmethod, pp.prosrc, pp.proname from pg_cast pc join pg_proc pp on
>> pp.oid = pc.castfunc and pc.castfunc > 0
>> order by castsource::regtype;
>> It's a lot of work, but seems doable, after playing around with it.
>
> It is do-able. But that's just the cast functions that are part of core postgres.
>
hi.
it's doable for most of the data types.
but I found geometric type related cast function refactoring to error
safe is quite challenging,
so I skip that part refactoring.

>>
>> I don't think we need to change the pg_cast catalog entry,
>> we just need to make these function (pg_cast.castmethod) errors safe.
>
> That would break any user-defined cast functions that were not also error safe, which is to say all of them.
>
> We need a way for user-defined cast functions to indicate whether or not they are error safe, and handle both
situationsaccordingly (i.e. fail a CAST ON DEFAULT when the user-defined cast is not error-safe). 
>

I understand what you mean now.
CREATE CAST can use built-in functions too, we have no way to check
whether these CREATE CAST
associated functions are error safe or not.
for example:
CREATE CAST (jsonpath  AS bytea) WITH FUNCTION jsonpath_send (jsonpath
) AS ASSIGNMENT;
select '$'::jsonpath::bytea;

To avoid this situation, we have to add a new column to pg_cast to
indicate whether a cast function is error-safe.
It's unlikely a pg_cast entry has two functions, one is error safe, one is not,
adding pg_cast.casterrorsafefunc would not be that appropriate.

so I choose pg_cast.casterrorsafe would be fine.
pg_cast.casterrorsafe true means castfunc function is error safe, we
can use it as safe cast evaluation
(CAST... DEFAULT defexpr ON CONVERSION ERROR)

please check the attached V6 script:

v6-0001 to v6-0016 is about making existing pg_cast.castfunc function
error safe.
(commit message have associated query to explain the refactoring, as mentioned
above, geometric and money associated type not refactored yet)

v6-0017-make-ArrayCoerceExpr-error-safe.patch
v6-0018-CAST-expr-AS-newtype-DEFAULT-ON-ERROR.patch
is about (CAST... DEFAULT defexpr ON CONVERSION ERROR).
Вложения

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
jian he
Дата:
hi.

First of all, rebase v6.

select distinct castsource::regtype, casttarget::regtype, pp.prosrc
from pg_cast pc join pg_proc pp on pp.oid = pc.castfunc
join pg_type pt on pt.oid = castsource
join pg_type pt1 on pt1.oid = casttarget
and pc.castfunc > 0 and pt.typarray <> 0 and pt.typnamespace =
'pg_catalog'::regnamespace
and pt1.typnamespace = 'pg_catalog'::regnamespace
order by castsource::regtype, casttarget::regtype;

The above query will list all cast functions in PG_CATALOG schema that
need to be
refactored to be error-safe.  looking around, I found that the below source data
type, the cast function is already error safe:

boolean
name
"char"  ("char" to integer was handled in integer cast part)
regprocedure
regoper
regoperator
regclass
regtype
regconfig
regdictionary
regproc
oid
regnamespace
regrole
regcollation
regdatabase
int4range
numrange
tsrange
tstzrange
daterange
int8range
xid8
time without time zone
time with time zone

After applying the attached v7 patch patchset,
the below are cast functions that I didn't refactor to error safe yet.

select pc.castsource::regtype,
       pc.casttarget::regtype, castfunc::regproc, pp.prosrc
from pg_cast pc join pg_proc pp on pp.oid = pc.castfunc
join pg_type pt on pt.oid = castsource
join pg_type pt1 on pt1.oid = casttarget
and pc.castfunc > 0 and pt.typnamespace = 'pg_catalog'::regnamespace
and pt1.typnamespace = 'pg_catalog'::regnamespace and not pc.casterrorsafe;

 castsource | casttarget |       castfunc       |    prosrc
------------+------------+----------------------+--------------
 bigint     | money      | pg_catalog.money     | int8_cash
 integer    | money      | pg_catalog.money     | int4_cash
 numeric    | money      | pg_catalog.money     | numeric_cash
 money      | numeric    | pg_catalog."numeric" | cash_numeric
(4 rows)

The reason I don't refactor these cast functions related to money data
type  is because
1. I am not sure if the PGLC_localeconv() function is error safe or not.
2. refactoring such ``DirectFunctionCall1(numeric_int8, amount));``
requires more effort.

please check the attached v7 patch set:
01-error-safe-for-casting-bytea-to-other-types
02-error-safe-for-casting-bit-varbit-to-other-types
03-error-safe-for-casting-character-to-other-types
04-error-safe-for-casting-text-to-other-types
05-error-safe-for-casting-character-varying-to-other-types
06-error-safe-for-casting-inet-to-other-types
07-error-safe-for-casting-macaddr8-to-other-types
08-error-safe-for-casting-integer-to-other-types
09-error-safe-for-casting-bigint-to-other-types
10-error-safe-for-casting-numeric-to-other-types
11-error-safe-for-casting-float4-to-other-types
12-error-safe-for-casting-float8-to-other-types
13-error-safe-for-casting-date-to-other-types
14-error-safe-for-casting-interval-to-other-types
15-error-safe-for-casting-timestamptz-to-other-types
16-error-safe-for-casting-timestamp-to-other-types
17-error-safe-for-casting-jsonb-to-other-types
18-error-safe-for-casting-geometry-data-types
19-invent_some_error_safe_function.patch
20-CAST-expr-AS-newtype-DEFAULT-ON-ERROR.patch

Each patch includes a commit message explaining which function is being
refactored to be error-safe.

I also made some changes for
"20-CAST-expr-AS-newtype-DEFAULT-ON-ERROR.patch"

If the source expression is Const, it can be unknown type or other type.
* for UNKNOWN type, example:
SELECT CAST ('\x112233445566778899'::bytea AS int8 DEFAULT NULL ON
CONVERSION ERROR);
We use CoerceUnknownConstSafe to verify earlier that such Const can be
coerced to target data type or not.

* for other data type, example:
SELECT CAST ('\x112233445566778899'::bytea AS int8 DEFAULT NULL ON
CONVERSION ERROR);
we use evaluate_expr_safe to evaluate it error safe way earlier in
transformTypeSafeCast.
similar to transformPartitionBoundValue call evaluate_expr

The geometry cast functions ``(poly_circle(PG_FUNCTION_ARGS)`` have
problems making it error safe,
working on it.
Вложения

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
jian he
Дата:
On Fri, Oct 10, 2025 at 8:23 PM jian he <jian.universality@gmail.com> wrote:
> After applying the attached v7 patch patchset,
> the below are cast functions that I didn't refactor to error safe yet.
>
> select pc.castsource::regtype,
>        pc.casttarget::regtype, castfunc::regproc, pp.prosrc
> from pg_cast pc join pg_proc pp on pp.oid = pc.castfunc
> join pg_type pt on pt.oid = castsource
> join pg_type pt1 on pt1.oid = casttarget
> and pc.castfunc > 0 and pt.typnamespace = 'pg_catalog'::regnamespace
> and pt1.typnamespace = 'pg_catalog'::regnamespace and not pc.casterrorsafe;
>
>  castsource | casttarget |       castfunc       |    prosrc
> ------------+------------+----------------------+--------------
>  bigint     | money      | pg_catalog.money     | int8_cash
>  integer    | money      | pg_catalog.money     | int4_cash
>  numeric    | money      | pg_catalog.money     | numeric_cash
>  money      | numeric    | pg_catalog."numeric" | cash_numeric
> (4 rows)
>
> The reason I don't refactor these cast functions related to money data
> type  is because
> 1. I am not sure if the PGLC_localeconv() function is error safe or not.
> 2. refactoring such ``DirectFunctionCall1(numeric_int8, amount));``
> requires more effort.
>
> please check the attached v7 patch set:
> 01-error-safe-for-casting-bytea-to-other-types
> 02-error-safe-for-casting-bit-varbit-to-other-types
> 03-error-safe-for-casting-character-to-other-types
> 04-error-safe-for-casting-text-to-other-types
> 05-error-safe-for-casting-character-varying-to-other-types
> 06-error-safe-for-casting-inet-to-other-types
> 07-error-safe-for-casting-macaddr8-to-other-types
> 08-error-safe-for-casting-integer-to-other-types
> 09-error-safe-for-casting-bigint-to-other-types
> 10-error-safe-for-casting-numeric-to-other-types
> 11-error-safe-for-casting-float4-to-other-types
> 12-error-safe-for-casting-float8-to-other-types
> 13-error-safe-for-casting-date-to-other-types
> 14-error-safe-for-casting-interval-to-other-types
> 15-error-safe-for-casting-timestamptz-to-other-types
> 16-error-safe-for-casting-timestamp-to-other-types
> 17-error-safe-for-casting-jsonb-to-other-types
> 18-error-safe-for-casting-geometry-data-types
> 19-invent_some_error_safe_function.patch
> 20-CAST-expr-AS-newtype-DEFAULT-ON-ERROR.patch
>
> Each patch includes a commit message explaining which function is being
> refactored to be error-safe.
>

Summary of what this patch set is doing:
To implement the syntax:
CAST(source_expr AS target_type DEFAULT def_expr ON CONVERSION ERROR)

we need to ensure that the transformed cast expression is error-safe.
A transformed cast expression can be either a CoerceViaIO or a FuncExpr.
Since CoerceViaIO is already error-safe but FuncExpr cannot be
rewritten as CoerceViaIO
one of the example: SELECT ('11.1'::numeric::int)

So we need to refactor how pg_cast.castfunc works to make it error-safe.
We also need to query pg_cast to determine whether a given pg_cast.castfunc is
error-safe.  For this, a new field casterrorsafe is added to pg_cast.

Patches 01–18: Refactor all pg_cast.castfunc entries (except those with the
money data type) to be error-safe.
Patches 19–20: Implement support for CAST(... AS ... DEFAULT def_expr ON
CONVERSION ERROR)

Please check the attached v8.

It includes minor changes compared to v7:
mainly a small mistake fix in src/backend/jit/llvm/llvmjit_expr.c.
Additionally, one occurrence of NO_XML_SUPPORT(); was replaced with
errsave(escontext, …).
Вложения

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
jian he
Дата:
On Tue, Oct 14, 2025 at 10:00 AM jian he <jian.universality@gmail.com> wrote:
> Please check the attached v8.
>
hi.

please see the attached V9 patchset.
v9-0001 to v9-0018: refactor pg_cast.castfunc entries, make it error safe.
v9-0019:  CAST(... AS ... DEFAULT def_expr ON CONVERSION ERROR)

select pc.castsource::regtype,pc.casttarget::regtype,
castfunc::regproc, pp.prosrc
from pg_cast pc join pg_proc pp on pp.oid = pc.castfunc
join pg_type pt on pt.oid = castsource
join pg_type pt1 on pt1.oid = casttarget
and pc.castfunc > 0 and pt.typnamespace = 'pg_catalog'::regnamespace
and pt1.typnamespace = 'pg_catalog'::regnamespace and not pc.casterrorsafe;

 castsource | casttarget |       castfunc       |    prosrc
------------+------------+----------------------+--------------
 circle     | polygon    | pg_catalog.polygon   |
 bigint     | money      | pg_catalog.money     | int8_cash
 integer    | money      | pg_catalog.money     | int4_cash
 numeric    | money      | pg_catalog.money     | numeric_cash
 money      | numeric    | pg_catalog."numeric" | cash_numeric
(5 rows)

The above result shows type casts using functions which cannot be error safe.
Money type related casts still can not be error safe.

Cast from circle to polygon cannot be error safe because the associated cast
function (pg_cast.castfunc) is written in SQL
(see src/backend/catalog/system_functions.sql LINE 112).
It appears impossible to make SQL language functions error safe, because
fmgr_sql ignores fcinfo->context.


eval_const_expressions cannot be error safe, so we need to handle
source_expr as an UNKNOWN constant in an error safe beforehand.
For example, we need handle ('1' AS DATE) in an error safe way
for
SELECT CAST('1' AS date  DEFAULT '2011-01-01' ON ERROR);

Since we must handle the source_expr when it is an UNKNOWN constant in an
error safe way, we can apply the same handling when source_expr is a
Const whose type is not UNKNOWN.
For example:
SELECT CAST('[(1,2),(3,4)]'::path AS polygon DEFAULT NULL ON CONVERSION ERROR);

If source_expr is a Const and the cast expression is a FuncExpr, we can be
certain that all arguments (FuncExpr->args) are also Const; see the function
chain coerce_to_target_type → coerce_type → build_coercion_expression.

We don’t need to worry about deparsing the expression because struct
SafeTypeCastExpr includes source_expr, cast_expr, and default_expr.  Even if
cast_expr is NULL, we can still use source_expr to reconstruct the original CAST
expression.

so I introduced:
evaluate_expr_safe: error safe version of evaluate_expr
CoerceUnknownConstSafe: tests whether an UNKNOWN Const can be coerced to the
target type.
Вложения

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Corey Huinker
Дата:
The above result shows type casts using functions which cannot be error safe.
Money type related casts still can not be error safe.

Cast from circle to polygon cannot be error safe because the associated cast
function (pg_cast.castfunc) is written in SQL
(see src/backend/catalog/system_functions.sql LINE 112).
It appears impossible to make SQL language functions error safe, because
fmgr_sql ignores fcinfo->context.

It is inevitable that some typecast functions are not type safe, either because they cannot easily be made so, or because they come from an extension that has not yet made them so.


 
eval_const_expressions cannot be error safe, so we need to handle
source_expr as an UNKNOWN constant in an error safe beforehand.
For example, we need handle ('1' AS DATE) in an error safe way
for
SELECT CAST('1' AS date  DEFAULT '2011-01-01' ON ERROR);

Since we must handle the source_expr when it is an UNKNOWN constant in an
error safe way, we can apply the same handling when source_expr is a
Const whose type is not UNKNOWN.
For example:
SELECT CAST('[(1,2),(3,4)]'::path AS polygon DEFAULT NULL ON CONVERSION ERROR);

In the case you've presented here, the cast to type "path" doesn't need to be safe, but the cast path->polygon does. Or rather, if it isn't safe we should raise an error.
 

so I introduced:
evaluate_expr_safe: error safe version of evaluate_expr
CoerceUnknownConstSafe: tests whether an UNKNOWN Const can be coerced to the
target type.

Good. That's the missing bit I knew we needed to add. Sorry I wasn't able to find time.

Issue 1:
+++ b/doc/src/sgml/syntax.sgml
@@ -2106,6 +2106,10 @@ CAST ( <replaceable>expression</replaceable> AS <replaceable>type</replaceable>
     The <literal>CAST</literal> syntax conforms to SQL; the syntax with
     <literal>::</literal> is historical <productname>PostgreSQL</productname>
     usage.
+    It can also be written as:
+<synopsis>
+CAST ( <replaceable>expression</replaceable> AS <replaceable>type</replaceable> ERROR ON CONVERSION ERROR )
+</synopsis>

The wording implies that this syntax is a new shortcut to a previously established ON CONVERSION ERROR syntax, when it was the only way to do it until this patch.


Issue 2:
s/is historical/is the original/
s/It can be also be written as:/The equivalent ON CONVERSION ERROR behavior is:/

+ /*
+ * Use evaluate_expr_safe to pre-evaluate simple constant cast
+ * expressions early, in a way that tolter errors.

typo on tolter? This happens in 2 places.


Issue 3:
+ errhint("Currently CAST ... DEFAULT ON CONVERSION ERROR does not support this cast"),

Suggest: errhint("Explicit cast is defined but definition is not declared as safe")

This hint helps distinguish the fact that it's the cast function getting in the way of this cast-on-default working. If the cast had been defined as IO then we wouldn't have had a problem.

It is true that we presently have no means of declaring a cast safe aside from making it so in pg_proc.dat, but that's coming shortly.


 
Issue 4:

catversion.h is not updated. Which isn't a bad thing because that brings me to...


Issue 5:

I think 0019 is a bit big for a committer to digest all in one sitting. Currently it:

- introduces the type-safe cast node
- introduces the cast on default syntax
- redefines 
- adds in test cases for all safe functions defined in patches 1-18.

As tedious as it might be, I think we want to move this patch to the front, move all pg_cast.dat changes to their respective patches that introduce that datatype's safe typecast function, as well as the test cases that are made possible by that new safe typecast. That will make it easier to ensure that each new cast has test coverage.

Yes, that's going to be a lot of catversion bumps, requiring somebody to fudge the dates as we presently only allow for 10 catversion bumps in a day, but the committer will either combine a few of the patches or spread the work out over a few days.

Overall:

I like where this patchset is going. The introduction of error contexts has eliminated a lot of the issues I was having carrying the error state forward into the next eval step.

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
jian he
Дата:
On Thu, Nov 6, 2025 at 6:00 AM Corey Huinker <corey.huinker@gmail.com> wrote:
>
> Issue 5:
>
> I think 0019 is a bit big for a committer to digest all in one sitting. Currently it:
>
> - introduces the type-safe cast node
> - introduces the cast on default syntax
> - redefines
> - adds in test cases for all safe functions defined in patches 1-18.
>
> As tedious as it might be, I think we want to move this patch to the front, move all pg_cast.dat changes to their
respectivepatches that introduce that datatype's safe typecast function, as well as the test cases that are made
possibleby that new safe typecast. That will make it easier to ensure that each new cast has test coverage. 
>
> Yes, that's going to be a lot of catversion bumps, requiring somebody to fudge the dates as we presently only allow
for10 catversion bumps in a day, but the committer will either combine a few of the patches or spread the work out over
afew days. 
>

Currently, patches v9-0001 through v9-0018 focus solely on refactoring
pg_cast.castfunc.  This refactoring is already valuable on its own because it
establishes the infrastructure needed for error safe type casts.
As mentioned before, to make
CAST(source_expr AS target_type DEFAULT expr ON CONVERSION ERROR);
work,
we cannot just simply replace casting FuncExpr nodes with CoerceViaIO, since
type modifiers behave differently in these two Nodes.
(e.g., casting numeric 1.11 to integer is not equivalent to casting the literal
"1.11" to integer).

Also, are we settled on this new pg_cast column name (pg_cast.casterrorsafe)?
Overall, I think it's better not to touch pg_cast.dat in each of these
refactoring patches.

Without first refactoring pg_cast.castfunc (01 to 18), making CAST ...
DEFAULT as the first patch (0001)
won't work, since pg_cast.castfunc itself is not error safe yet, and we
have no way to test the CAST DEFAULT syntax.

So we have to *first* refactor pg_cast.castfunc, make it error safe
then implement CAST DEFAULT.
--------------------------------------------------
The CAST DEFAULT patch is large, I tried to split some newly created function
into a seperate patch.
see v10-0019-invent-some-error-safe-functions.patch.

SELECT CAST('five' AS INTEGER DEFAULT 6 ON CONVERSION ERROR);
is not hard to make it error safe. ('five' is a simple Cons node)

However, I found out, to make
SELECT CAST('five'::INTEGER AS INTEGER DEFAULT 6 ON CONVERSION ERROR);
error safe is hard.

('five'::INTEGER) is a TypeCast node, normally it will error out in
transformTypeCast->
coerce_to_target_type->coerce_type ```(if (inputTypeId == UNKNOWNOID
&& IsA(node, Const)))```
If we do not error out, then we need a Node to represent the failed cast, mainly
for deparse purposes.

that means for CAST(source_expr .... DEFAULT defexpr ON CONVERSION ERROR);
The only corner case we handle is when source_expr is a simple Const node.
In all other cases, source_expr is processed through transformExpr,
which does all
the normal parse analysis for a node.

--
jian
https://www.enterprisedb.com/
Вложения

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Corey Huinker
Дата:
As mentioned before, to make
CAST(source_expr AS target_type DEFAULT expr ON CONVERSION ERROR);
work,
we cannot just simply replace casting FuncExpr nodes with CoerceViaIO, since
type modifiers behave differently in these two Nodes.
(e.g., casting numeric 1.11 to integer is not equivalent to casting the literal
"1.11" to integer).

I wasn't suggesting that. I was suggesting that we move tests that use a given type's custom cast function into the same patch that makes that cast safe. This would mean that the initial syntax+nodes+executor patch starts out only with test cases known to not have custom functions.

 

Also, are we settled on this new pg_cast column name (pg_cast.casterrorsafe)?
Overall, I think it's better not to touch pg_cast.dat in each of these
refactoring patches.

I'm fine with it. I can see having 'f' and 's' both mean cast functions, but 's' means safe, but the extra boolean works too and we'll be fine with either method.
 

Without first refactoring pg_cast.castfunc (01 to 18), making CAST ...
DEFAULT as the first patch (0001)
won't work, since pg_cast.castfunc itself is not error safe yet, and we
have no way to test the CAST DEFAULT syntax.

So we have to *first* refactor pg_cast.castfunc, make it error safe
then implement CAST DEFAULT.

Makes sense.
 

However, I found out, to make
SELECT CAST('five'::INTEGER AS INTEGER DEFAULT 6 ON CONVERSION ERROR);
error safe is hard.

That's no problem at all. The CAST () function can't do anything about an input that's already an error before the CAST node executes. Nor should it. It should only concern itself with errors related to the actual casting of a value to the specified type.
 

('five'::INTEGER) is a TypeCast node, normally it will error out in
transformTypeCast->
coerce_to_target_type->coerce_type ```(if (inputTypeId == UNKNOWNOID
&& IsA(node, Const)))```
If we do not error out, then we need a Node to represent the failed cast, mainly
for deparse purposes.

We _must_ error out in that case, before the CAST executes.
 

that means for CAST(source_expr .... DEFAULT defexpr ON CONVERSION ERROR);
The only corner case we handle is when source_expr is a simple Const node.
In all other cases, source_expr is processed through transformExpr,
which does all
the normal parse analysis for a node.

I'll get to reviewing this patchset soon.

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Amul Sul
Дата:
On Tue, Nov 11, 2025 at 8:37 AM jian he <jian.universality@gmail.com> wrote:
>
> On Thu, Nov 6, 2025 at 6:00 AM Corey Huinker <corey.huinker@gmail.com> wrote:
> > [...]
>
> Currently, patches v9-0001 through v9-0018 focus solely on refactoring
> pg_cast.castfunc.

I had a quick look but haven't finished the full review due to lack of
time today. Here are a few initial comments:

v10-0003:

-   NO_XML_SUPPORT();
+   errsave(escontext,
+           errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+           errmsg("unsupported XML feature"),
+           errdetail("This functionality requires the server to be
built with libxml support."));
    return NULL;

Can use ereturn() instead of errsave() followed by return NULL.
--

10-0004:

+/* error safe version of textToQualifiedNameList */
+List *
+textToQualifiedNameListSafe(text *textval, Node *escontext)

If I am not mistaken, it looks like an exact copy of
textToQualifiedNameList(). I think you can simply keep only
textToQualifiedNameListSafe() and call that from
textToQualifiedNameList() with a NULL value for escontext. This way,
all errsave() or ereturn() calls will behave like ereport().

The same logic applies to RangeVarGetRelidExtendedSafe() and
makeRangeVarFromNameListSafe. These can be called from
RangeVarGetRelidExtended() and makeRangeVarFromNameList(),
respectively.
--

+   {
+       errsave(escontext,
+               errcode(ERRCODE_INVALID_NAME),
+               errmsg("invalid name syntax"));
+       return NIL;
+   }

I prefer ereturn() instead of errsave + return.
--

v10-0017

    for (i = 0; i < lengthof(messages); i++)
        if (messages[i].type == type)
-           ereport(ERROR,
+       {
+           errsave(escontext,
                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                     errmsg(messages[i].msg, sqltype)));
+           return;
+       }

Here, I think, you can use ereturn() to return void.
--

v10-0018

+   if (unlikely(result == 0.0) && val1 != 0.0 && !isinf(val2))
+   {
+       errsave(escontext,
+               errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
+               errmsg("value out of range: underflow"));
+
+       result = 0.0;
+       return result;
+   }

Here, you can use ereturn() to return 0.0. Similar changes are needed
at other places in the same patch.

Regards,
Amul

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Corey Huinker
Дата:
On Mon, Nov 10, 2025 at 11:36 PM Corey Huinker <corey.huinker@gmail.com> wrote:
As mentioned before, to make
CAST(source_expr AS target_type DEFAULT expr ON CONVERSION ERROR);
work,
we cannot just simply replace casting FuncExpr nodes with CoerceViaIO, since
type modifiers behave differently in these two Nodes.
(e.g., casting numeric 1.11 to integer is not equivalent to casting the literal
"1.11" to integer).

I wasn't suggesting that. I was suggesting that we move tests that use a given type's custom cast function into the same patch that makes that cast safe. This would mean that the initial syntax+nodes+executor patch starts out only with test cases known to not have custom functions.

With a second look at the patches, I think the organization is fine as-is.


Also, are we settled on this new pg_cast column name (pg_cast.casterrorsafe)?
Overall, I think it's better not to touch pg_cast.dat in each of these
refactoring patches.

I'm fine with it. I can see having 'f' and 's' both mean cast functions, but 's' means safe, but the extra boolean works too and we'll be fine with either method.

I can work on this part if you don't have time.


I'll get to reviewing this patchset soon.

Not as soon as I would have liked, but the patchset still applies without error, and survives all of my attempts to break it, including user defined functions that generate errors deterministically as well as non-deterministically, hoping to see it incorrectly use the default rather than reporting the error.

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
jian he
Дата:
On Mon, Nov 17, 2025 at 9:43 PM Amul Sul <sulamul@gmail.com> wrote:
>
> 10-0004:
>
> +/* error safe version of textToQualifiedNameList */
> +List *
> +textToQualifiedNameListSafe(text *textval, Node *escontext)
>
> If I am not mistaken, it looks like an exact copy of
> textToQualifiedNameList(). I think you can simply keep only
> textToQualifiedNameListSafe() and call that from
> textToQualifiedNameList() with a NULL value for escontext. This way,
> all errsave() or ereturn() calls will behave like ereport().
>
> The same logic applies to RangeVarGetRelidExtendedSafe() and
> makeRangeVarFromNameListSafe. These can be called from
> RangeVarGetRelidExtended() and makeRangeVarFromNameList(),
> respectively.
> --
>

hi.

List *
textToQualifiedNameList(text *textval)
{
    List       *namelist;
    rawname = text_to_cstring(textval);
    if (!SplitIdentifierString(rawname, '.', &namelist))
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_NAME),
                 errmsg("invalid name syntax")));
}

I don’t see any way to pass the escontext (ErrorSaveContext) without changing
the textToQualifiedNameList function signature.

There are around 30 occurrences of textToQualifiedNameList.
changing the function textToQualifiedNameList signature is invasive,
so I tend to avoid it.
I think it's easier to just duplicate textToQualifiedNameList
than changing the function textToQualifiedNameList signature.

Am I missing something?

--
jian
https://www.enterprisedb.com

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Corey Huinker
Дата:
> The same logic applies to RangeVarGetRelidExtendedSafe() and
> makeRangeVarFromNameListSafe. These can be called from
> RangeVarGetRelidExtended() and makeRangeVarFromNameList(),
> respectively.
> --
>

I don’t see any way to pass the escontext (ErrorSaveContext) without changing
the textToQualifiedNameList function signature.
...
Am I missing something?

I think we need to keep these separate. The execution paths that don't care about capturing errors shouldn't be slowed down by minority of paths that do. That may change in the future, but if it does, we'll be getting rid of a lot of internal functions with this type of difference.

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
Amul Sul
Дата:
On Fri, Nov 21, 2025 at 2:11 PM jian he <jian.universality@gmail.com> wrote:
>
> On Mon, Nov 17, 2025 at 9:43 PM Amul Sul <sulamul@gmail.com> wrote:
> >
> > 10-0004:
> >
> > +/* error safe version of textToQualifiedNameList */
> > +List *
> > +textToQualifiedNameListSafe(text *textval, Node *escontext)
> >
> > If I am not mistaken, it looks like an exact copy of
> > textToQualifiedNameList(). I think you can simply keep only
> > textToQualifiedNameListSafe() and call that from
> > textToQualifiedNameList() with a NULL value for escontext. This way,
> > all errsave() or ereturn() calls will behave like ereport().
> >
> > The same logic applies to RangeVarGetRelidExtendedSafe() and
> > makeRangeVarFromNameListSafe. These can be called from
> > RangeVarGetRelidExtended() and makeRangeVarFromNameList(),
> > respectively.
> > --
> >
>
> hi.
>
> List *
> textToQualifiedNameList(text *textval)
> {
>     List       *namelist;
>     rawname = text_to_cstring(textval);
>     if (!SplitIdentifierString(rawname, '.', &namelist))
>         ereport(ERROR,
>                 (errcode(ERRCODE_INVALID_NAME),
>                  errmsg("invalid name syntax")));
> }
>
> I don’t see any way to pass the escontext (ErrorSaveContext) without changing
> the textToQualifiedNameList function signature.
>
> There are around 30 occurrences of textToQualifiedNameList.
> changing the function textToQualifiedNameList signature is invasive,
> so I tend to avoid it.
> I think it's easier to just duplicate textToQualifiedNameList
> than changing the function textToQualifiedNameList signature.
>
> Am I missing something?
>

The change I was suggesting will be as below:

--- a/src/backend/utils/adt/varlena.c
+++ b/src/backend/utils/adt/varlena.c
@@ -2684,6 +2684,13 @@ name_text(PG_FUNCTION_ARGS)
  */
 List *
 textToQualifiedNameList(text *textval)
+{
+   textToQualifiedNameListSafe(textval, NULL);
+}
+
+/* error safe version of textToQualifiedNameList */
+List *
+textToQualifiedNameListSafe(text *textval, Node *escontext)
 {
    char       *rawname;
    List       *result = NIL;


We must try to avoid duplication whenever possible, as any bug fixes
or enhancements would need to be copied to multiple places, which is
often overlooked.

Regards,
Amul

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
jian he
Дата:
On Mon, Nov 24, 2025 at 11:38 AM Amul Sul <sulamul@gmail.com> wrote:
>
> The change I was suggesting will be as below:
>
> --- a/src/backend/utils/adt/varlena.c
> +++ b/src/backend/utils/adt/varlena.c
> @@ -2684,6 +2684,13 @@ name_text(PG_FUNCTION_ARGS)
>   */
>  List *
>  textToQualifiedNameList(text *textval)
> +{
> +   textToQualifiedNameListSafe(textval, NULL);
> +}
> +
> +/* error safe version of textToQualifiedNameList */
> +List *
> +textToQualifiedNameListSafe(text *textval, Node *escontext)
>  {
>     char       *rawname;
>     List       *result = NIL;
>
>
> We must try to avoid duplication whenever possible, as any bug fixes
> or enhancements would need to be copied to multiple places, which is
> often overlooked.
>
hi.
great idea!

I incorporated all of your ideas into v11.
I replaced all errsave to ereturn.
I aslo simplified T_SafeTypeCastExpr expression initialization, evaluation logic
within execExpr.c, execExprInterp.c.

but one thing I didn't touch: float8_div.

+static inline float8
+float8_div_safe(const float8 val1, const float8 val2, struct Node *escontext)

but we can change float8_div to:

static inline float8
float8_div(const float8 val1, const float8 val2)
{
   return float8_div_safe(val1, val2, NULL);
}
I am worried that entering another function would cause a minor performance
degradation.  And since these simple functions are so simple, keeping them
separated should not be a big problem.  also I placed float8_div,
float8_div_safe together.


--
jian
https://www.enterprisedb.com/
Вложения

Re: CAST(... ON DEFAULT) - WIP build on top of Error-Safe User Functions

От
jian he
Дата:
On Fri, Nov 21, 2025 at 3:32 PM Corey Huinker <corey.huinker@gmail.com> wrote:
>
>>>
>>> Also, are we settled on this new pg_cast column name (pg_cast.casterrorsafe)?
>>> Overall, I think it's better not to touch pg_cast.dat in each of these
>>> refactoring patches.
>>
>>
>> I'm fine with it. I can see having 'f' and 's' both mean cast functions, but 's' means safe, but the extra boolean
workstoo and we'll be fine with either method. 
>
>
> I can work on this part if you don't have time.

Do you mean change pg_cast.casterrorsafe from boolean to char?
Currently pg_cast.casterrorsafe works just fine.
if the cast function is not applicable, the castfunc would be InvalidOid.
also the cast function is either error safe or not, I don't see a
usage case for the third value.

attached v12-0004 fixes the xmlparse error, see
https://cirrus-ci.com/task/6181359384788992?logs=build#L1217.

all other patches remain the same as v11.

--
jian
https://www.enterprisedb.com/
Вложения

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