Обсуждение: Isolating a record column from a PL-Pgsql function call ?

Gauthier, Dave <dave.gauthier@intel.com> schrieb:

> Hi:
>
>
>
> I have a PL-Pgsql function that returns a record of a type that have 4 elements
> (columns).  Let’s say the first element/column is called “id_num”.  Is there a
> way I can specify the id_num element of the record returned from iside a query?
>
>
>
> For example, select col1 from thetable where my_plpgsql_fn(col2).id_num = 123;

I think, you should rewrite your function to hand over the id_num as
additional parameter. But i'm not sure if i understand you corrently. Is
this a setof-returning function?

Please, show us your real function, maybe someone can help you more.




Andreas
--
Really, I'm not out to destroy Microsoft. That will just be a completely
unintentional side effect.                              (Linus Torvalds)
"If I was god, I would recompile penguin with --enable-fly."   (unknown)
Kaufbach, Saxony, Germany, Europe.              N 51.05082°, E 13.56889°

The example I gave was overly simplistic.  I actually want to look at more than one of the columns in the returned
record,so rewritting it to return an int won't address the need for the other columns.  And no, it does not return a
set/arrayof records.  It returns just one record.   

If I run the function outside of a query, it returns something like...

"(myvcharval1,myvcharval2,myintval)"

Notice the parens, the commas and the lack of single quotes around the "myvcharval(x)" values.

I suppose I could do some funky string dissection with this, but I was wondering if there was something more
straignt-forward.

-dave

-----Original Message-----
From: pgsql-general-owner@postgresql.org [mailto:pgsql-general-owner@postgresql.org] On Behalf Of Andreas Kretschmer
Sent: Tuesday, December 16, 2008 2:51 PM
To: pgsql-general@postgresql.org
Subject: Re: [GENERAL] Isolating a record column from a PL-Pgsql function call ?

Gauthier, Dave <dave.gauthier@intel.com> schrieb:

> Hi:
>
>
>
> I have a PL-Pgsql function that returns a record of a type that have 4 elements
> (columns).  Let's say the first element/column is called "id_num".  Is there a
> way I can specify the id_num element of the record returned from iside a query?
>
>
>
> For example, select col1 from thetable where my_plpgsql_fn(col2).id_num = 123;

I think, you should rewrite your function to hand over the id_num as
additional parameter. But i'm not sure if i understand you corrently. Is
this a setof-returning function?

Please, show us your real function, maybe someone can help you more.




Andreas
--
Really, I'm not out to destroy Microsoft. That will just be a completely
unintentional side effect.                              (Linus Torvalds)
"If I was god, I would recompile penguin with --enable-fly."   (unknown)
Kaufbach, Saxony, Germany, Europe.              N 51.05082°, E 13.56889°

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Gauthier, Dave <dave.gauthier@intel.com> schrieb:

> Hi:
>
>
>
> I have a PL-Pgsql function that returns a record of a type that have 4 elements
> (columns).  Let’s say the first element/column is called “id_num”.  Is there a
> way I can specify the id_num element of the record returned from iside a query?
>
>
>
> For example, select col1 from thetable where my_plpgsql_fn(col2).id_num = 123;

Okay, i think, i got it:

test=# create function x(in int, out a int, out b int) returns record as $$begin a:=1;b:=2;return; end; $$language
plpgsql;
CREATE FUNCTION
Time: 0.412 ms

test=*# select 5 where 3=(select b from x(2));
 ?column?
----------
(0 rows)

Time: 0.335 ms
test=*# select 5 where 2=(select b from x(2));
 ?column?
----------
        5
(1 row)



Andreas
--
Really, I'm not out to destroy Microsoft. That will just be a completely
unintentional side effect.                              (Linus Torvalds)
"If I was god, I would recompile penguin with --enable-fly."   (unknown)
Kaufbach, Saxony, Germany, Europe.              N 51.05082°, E 13.56889°

And it's a bit more complicated at that. Here's why....

I discovered that I can refer to the function as a relation in the from clause...

  select y.col1 from my_plpgsql_fn(789) y

but I need to pass the function something from outside, sort of like...

  select x.col_a, y.col1
  from mytable x, my_plpgsql_fn(a.val) y
  where x.col_b = 'foo';

When I try this one, I get a "ERROR Function expression in FROM cannot refer to other relations of same query level"


I could accomplish what I need to because Im actually doing all this in the Perl/DBI.  And in Perl, I can just run the
"x"query in a loop, then the "my_plpgsql_fn" call inside the loop.  This will accomplish the same thing, but it'll be 2
queriesinstead of one (and it'll be far from elegant!) 

-dave





-----Original Message-----
From: pgsql-general-owner@postgresql.org [mailto:pgsql-general-owner@postgresql.org] On Behalf Of Gauthier, Dave
Sent: Tuesday, December 16, 2008 3:03 PM
To: Andreas Kretschmer; pgsql-general@postgresql.org
Subject: Re: [GENERAL] Isolating a record column from a PL-Pgsql function call ?

The example I gave was overly simplistic.  I actually want to look at more than one of the columns in the returned
record,so rewritting it to return an int won't address the need for the other columns.  And no, it does not return a
set/arrayof records.  It returns just one record.   

If I run the function outside of a query, it returns something like...

"(myvcharval1,myvcharval2,myintval)"

Notice the parens, the commas and the lack of single quotes around the "myvcharval(x)" values.

I suppose I could do some funky string dissection with this, but I was wondering if there was something more
straignt-forward.

-dave

-----Original Message-----
From: pgsql-general-owner@postgresql.org [mailto:pgsql-general-owner@postgresql.org] On Behalf Of Andreas Kretschmer
Sent: Tuesday, December 16, 2008 2:51 PM
To: pgsql-general@postgresql.org
Subject: Re: [GENERAL] Isolating a record column from a PL-Pgsql function call ?

Gauthier, Dave <dave.gauthier@intel.com> schrieb:

> Hi:
>
>
>
> I have a PL-Pgsql function that returns a record of a type that have 4 elements
> (columns).  Let's say the first element/column is called "id_num".  Is there a
> way I can specify the id_num element of the record returned from iside a query?
>
>
>
> For example, select col1 from thetable where my_plpgsql_fn(col2).id_num = 123;

I think, you should rewrite your function to hand over the id_num as
additional parameter. But i'm not sure if i understand you corrently. Is
this a setof-returning function?

Please, show us your real function, maybe someone can help you more.




Andreas
--
Really, I'm not out to destroy Microsoft. That will just be a completely
unintentional side effect.                              (Linus Torvalds)
"If I was god, I would recompile penguin with --enable-fly."   (unknown)
Kaufbach, Saxony, Germany, Europe.              N 51.05082°, E 13.56889°

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"Gauthier, Dave" <dave.gauthier@intel.com> writes:
> I have a PL-Pgsql function that returns a record of a type that have 4 elements (columns).  Let's say the first
element/columnis called "id_num".  Is there a way I can specify the id_num element of the record returned from iside a
query?

> For example, select col1 from thetable where my_plpgsql_fn(col2).id_num = 123;

You're just missing some parentheses:

select col1 from thetable where (my_plpgsql_fn(col2)).id_num = 123;

as indeed is documented here:
http://www.postgresql.org/docs/8.3/static/sql-expressions.html#AEN1679

There are messy syntactic reasons for requiring these parens, which
I don't recall the details of at the moment ...

            regards, tom lane

On Tue, Dec 16, 2008 at 01:03:05PM -0700, Gauthier, Dave wrote:
> The example I gave was overly simplistic.  I actually want to look at
> more than one of the columns in the returned record, so rewritting it
> to return an int won't address the need for the other columns.  And
> no, it does not return a set/array of records.  It returns just one
> record.
>
> If I run the function outside of a query, it returns something like...
>
> "(myvcharval1,myvcharval2,myintval)"
>
> Notice the parens, the commas and the lack of single quotes around the
> "myvcharval(x)" values.

This is how a value of type record is serialized.  When you're inside PG
it knows about the structure of the value and gives nice syntax to pull
the thing apart in (reasonably) nice ways.

To answer your question, if you have a function like:

  CREATE FUNCTION foo(p INT, OUT x INT, OUT y INT) RETURNS RECORD ...

Then I think you want to do something like:

  SELECT a, x, y
  FROM (
    SELECT t.a, (foo(t.b)).*
    FROM tbl t
    WHERE t.c = 10) z;

The reason you have to do this, and not have the function in the FROM
clause, is because each item in the FROM clause is independent.  There's
some standard syntax to say when this isn't true, but PG doesn't know
about it yet.

One other, recently reported[1], caveat is that PG currently evaluates
the function for each parameter returned from the SELECT statement it
appears in; so in the above example it'll get called twice.  It seems
possible to work around this by doing the following:

  SELECT a, (foo).x, (foo).y
  FROM (
    SELECT t.a, foo(t.b)
    FROM tbl t
    WHERE t.c = 10) z;

As Tom said in another response, the parens are a bit annoying but
needed to keep things un-ambiguous.  I think it's something to do with
schemas; for example the "(foo).x" above is really short for "z.foo.y".
This is then ambiguous whether you're referring to what's written
above, or referring to table "foo" in schema "z".  PG's fix for this
was to introduce the brackets and this, in combination with allowing
non-ambiguous column names to be referenced without a table name,
means we end up with the strange "(foo)" syntax.  At least that's my
understanding.


  Sam

 [1] http://archives.postgresql.org/pgsql-hackers/2008-12/msg00483.php

Wheeew! OK, that does work.
This knowlege creates new options for doing other stuff on my plate.

Thanks to all who responded !

-dave

-----Original Message-----
From: pgsql-general-owner@postgresql.org [mailto:pgsql-general-owner@postgresql.org] On Behalf Of Sam Mason
Sent: Wednesday, December 17, 2008 6:45 AM
To: pgsql-general@postgresql.org
Subject: Re: [GENERAL] Isolating a record column from a PL-Pgsql function call ?

On Tue, Dec 16, 2008 at 01:03:05PM -0700, Gauthier, Dave wrote:
> The example I gave was overly simplistic.  I actually want to look at
> more than one of the columns in the returned record, so rewritting it
> to return an int won't address the need for the other columns.  And
> no, it does not return a set/array of records.  It returns just one
> record.
>
> If I run the function outside of a query, it returns something like...
>
> "(myvcharval1,myvcharval2,myintval)"
>
> Notice the parens, the commas and the lack of single quotes around the
> "myvcharval(x)" values.

This is how a value of type record is serialized.  When you're inside PG
it knows about the structure of the value and gives nice syntax to pull
the thing apart in (reasonably) nice ways.

To answer your question, if you have a function like:

  CREATE FUNCTION foo(p INT, OUT x INT, OUT y INT) RETURNS RECORD ...

Then I think you want to do something like:

  SELECT a, x, y
  FROM (
    SELECT t.a, (foo(t.b)).*
    FROM tbl t
    WHERE t.c = 10) z;

The reason you have to do this, and not have the function in the FROM
clause, is because each item in the FROM clause is independent.  There's
some standard syntax to say when this isn't true, but PG doesn't know
about it yet.

One other, recently reported[1], caveat is that PG currently evaluates
the function for each parameter returned from the SELECT statement it
appears in; so in the above example it'll get called twice.  It seems
possible to work around this by doing the following:

  SELECT a, (foo).x, (foo).y
  FROM (
    SELECT t.a, foo(t.b)
    FROM tbl t
    WHERE t.c = 10) z;

As Tom said in another response, the parens are a bit annoying but
needed to keep things un-ambiguous.  I think it's something to do with
schemas; for example the "(foo).x" above is really short for "z.foo.y".
This is then ambiguous whether you're referring to what's written
above, or referring to table "foo" in schema "z".  PG's fix for this
was to introduce the brackets and this, in combination with allowing
non-ambiguous column names to be referenced without a table name,
means we end up with the strange "(foo)" syntax.  At least that's my
understanding.


  Sam

 [1] http://archives.postgresql.org/pgsql-hackers/2008-12/msg00483.php

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