Обсуждение: WIP: Triggers on VIEWs

Here is a WIP patch implementing triggers on VIEWs, as outlined in the
proof-of-concept here:
http://archives.postgresql.org/pgsql-hackers/2010-08/msg00160.php

The new triggers allowed on a VIEW are:
1). Statement-level BEFORE INSERT/UPDATE/DELETE
2). Row-level INSTEAD OF INSERT/UPDATE/DELETE
3). Statement-level AFTER INSERT/UPDATE/DELETE

The new INSTEAD OF trigger type may only be used with VIEWs, and may
only be row-level. It does not support the WHEN or FOR UPDATE OF
column_list options.

There are still a number of things left todo:
 - extend ALTER VIEW with enable/disable trigger commands
 - much more testing
 - documentation

and then there's the question of what to do about the concurrency
issues raised by Marko Tiikkaja. Currently it works like Oracle (i.e.,
no locking).

Regards,
Dean

On 15 August 2010 18:38, Dean Rasheed <dean.a.rasheed@gmail.com> wrote:
> There are still a number of things left todo:
>  - extend ALTER VIEW with enable/disable trigger commands

On further reflection, I wonder if the ability to disable VIEW
triggers is needed/wanted at all. I just noticed that while it is
possible to disable a RULE on a TABLE, it is not possible to do so on
VIEW. This certainly makes sense for the _RETURN rule, although
possibly some people might have a use for disabling other rules on
views. The situation with triggers is similar - disabling an INSTEAD
OF trigger would be pointless, and could only lead to errors when
updating the view. Some people may have a use case for disabling
BEFORE and AFTER statement triggers on views, but I suspect that the
number of such cases is small, and I'm tempted to omit this, for now
at least.

Thoughts?
- Dean

Dean Rasheed <dean.a.rasheed@gmail.com> writes:
> On 15 August 2010 18:38, Dean Rasheed <dean.a.rasheed@gmail.com> wrote:
>> There are still a number of things left todo:
>> �- extend ALTER VIEW with enable/disable trigger commands

> On further reflection, I wonder if the ability to disable VIEW
> triggers is needed/wanted at all.

AFAIK the only real use for disabling triggers is in connection with
trigger-based replication systems.  A view wouldn't be carrying
replication-related triggers anyway, so I think this could certainly
be left out of a first cut.

You aren't saying that you can see some reason why this couldn't be
added later, if wanted, correct?
        regards, tom lane

On 16 August 2010 18:50, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Dean Rasheed <dean.a.rasheed@gmail.com> writes:
>> On 15 August 2010 18:38, Dean Rasheed <dean.a.rasheed@gmail.com> wrote:
>>> There are still a number of things left todo:
>>>  - extend ALTER VIEW with enable/disable trigger commands
>
>> On further reflection, I wonder if the ability to disable VIEW
>> triggers is needed/wanted at all.
>
> AFAIK the only real use for disabling triggers is in connection with
> trigger-based replication systems.  A view wouldn't be carrying
> replication-related triggers anyway, so I think this could certainly
> be left out of a first cut.
>
> You aren't saying that you can see some reason why this couldn't be
> added later, if wanted, correct?
>

Yes. It should be easy to add later if wanted. I just don't see much
use for it, and I don't want to add more to an already quite big
patch, if it's not really needed.

Regards,
Dean

On 15 August 2010 18:38, Dean Rasheed <dean.a.rasheed@gmail.com> wrote:
> Here is a WIP patch implementing triggers on VIEWs ... <snip>
>
> There are still a number of things left todo:
>  - extend ALTER VIEW with enable/disable trigger commands
>  - much more testing
>  - documentation
>

Attached is an updated patch with more tests and docs, and a few minor
code tidy ups. I think that the INSTEAD OF triggers part of the patch
is compliant with Feature T213 of the SQL 2008 standard. As discussed,
I don't plan to add the syntax to allow triggers on views to be
disabled at this time, but that should be easy to implement, if there
is a use case for it.

Comments welcome.

Regards,
Dean

On Sep 5, 2010, at 3:09 AM, Dean Rasheed wrote:

> On 15 August 2010 18:38, Dean Rasheed <dean.a.rasheed@gmail.com> wrote:
>> Here is a WIP patch implementing triggers on VIEWs ... <snip>
>>
>> There are still a number of things left todo:
>>  - extend ALTER VIEW with enable/disable trigger commands
>>  - much more testing
>>  - documentation
>>
>
> Attached is an updated patch with more tests and docs, and a few minor


At least for me, there are some portions of the docs which could use some formatting changes in order to not be
confusinggrammatically; e.g., this was confusing to me on the first read: 

-    trigger name.  In the case of before triggers, the
+    trigger name.  In the case of before and instead of triggers, the

I realize this lack of formatting was inherited from the existing docs, but this would make more sense to me if this
(andpresumably the other related instances of "before" and "after") were set apart with <literal></> or similar.  This
isalready in use in some places in this patch, so seems like the correct markup. 

Regards,

David
--
David Christensen
End Point Corporation
david@endpoint.com

On 7 September 2010 02:03, David Christensen <david@endpoint.com> wrote:
>
> On Sep 5, 2010, at 3:09 AM, Dean Rasheed wrote:
>
>> On 15 August 2010 18:38, Dean Rasheed <dean.a.rasheed@gmail.com> wrote:
>>> Here is a WIP patch implementing triggers on VIEWs ... <snip>
>>>
>>> There are still a number of things left todo:
>>>  - extend ALTER VIEW with enable/disable trigger commands
>>>  - much more testing
>>>  - documentation
>>>
>>
>> Attached is an updated patch with more tests and docs, and a few minor
>
>
> At least for me, there are some portions of the docs which could use some formatting changes in order to not be
confusinggrammatically; e.g., this was confusing to me on the first read: 
>
> -    trigger name.  In the case of before triggers, the
> +    trigger name.  In the case of before and instead of triggers, the
>
> I realize this lack of formatting was inherited from the existing docs, but this would make more sense to me if this
(andpresumably the other related instances of "before" and "after") were set apart with <literal></> or similar.  This
isalready in use in some places in this patch, so seems like the correct markup. 
>

Yeah, I think you're right. That chapter is the first place in the
manual where the concepts of before/after/instead of are introduced.
The first time it mentions them it uses <firstterm>, but then it
doesn't use any markup for them for the remainder of the chapter. It
looks like the rest of the manual uses <literal>+uppercase wherever
they're mentioned, which does help readability, so it would make sense
to bring the rest of that chapter into line with that convention.

Thanks for looking at this.

Cheers,
Dean

On 7 September 2010 08:43, Dean Rasheed <dean.a.rasheed@gmail.com> wrote:
> On 7 September 2010 02:03, David Christensen <david@endpoint.com> wrote:
>>
>> On Sep 5, 2010, at 3:09 AM, Dean Rasheed wrote:
>>
>>> On 15 August 2010 18:38, Dean Rasheed <dean.a.rasheed@gmail.com> wrote:
>>>> Here is a WIP patch implementing triggers on VIEWs ... <snip>
>>>>
>>>> There are still a number of things left todo:
>>>>  - extend ALTER VIEW with enable/disable trigger commands
>>>>  - much more testing
>>>>  - documentation
>>>>
>>>
>>> Attached is an updated patch with more tests and docs, and a few minor
>>
>>
>> At least for me, there are some portions of the docs which could use some formatting changes in order to not be
confusinggrammatically; e.g., this was confusing to me on the first read: 
>>
>> -    trigger name.  In the case of before triggers, the
>> +    trigger name.  In the case of before and instead of triggers, the
>>
>> I realize this lack of formatting was inherited from the existing docs, but this would make more sense to me if this
(andpresumably the other related instances of "before" and "after") were set apart with <literal></> or similar.  This
isalready in use in some places in this patch, so seems like the correct markup. 
>>
>
> Yeah, I think you're right. That chapter is the first place in the
> manual where the concepts of before/after/instead of are introduced.
> The first time it mentions them it uses <firstterm>, but then it
> doesn't use any markup for them for the remainder of the chapter. It
> looks like the rest of the manual uses <literal>+uppercase wherever
> they're mentioned, which does help readability, so it would make sense
> to bring the rest of that chapter into line with that convention.
>
> Thanks for looking at this.
>
> Cheers,
> Dean
>

Here's an updated version with improved formatting and a few minor
wording changes to the triggers chapter.

Regards,
Dean

--On 5. September 2010 09:09:55 +0100 Dean Rasheed 
<dean.a.rasheed@gmail.com> wrote:

I had a first look on your patch, great work!

> Attached is an updated patch with more tests and docs, and a few minor
> code tidy ups. I think that the INSTEAD OF triggers part of the patch
> is compliant with Feature T213 of the SQL 2008 standard. As discussed,

Reading the past discussions, there was some mention about the RETURNING 
clause.
I see Oracle doesn't allow its RETURNING INTO clause with INSTEAD OF 
triggers (at least my 10g XE instance here doesn't allow it, not sure about 
newer versions). I assume the following example might have some surprising 
effects on users:

CREATE TABLE foo(id integer);
CREATE VIEW vfoo AS SELECT 'bernd', * FROM foo;    

CREATE OR REPLACE FUNCTION insert_foo() RETURNS trigger
LANGUAGE plpgsql
AS $$       BEGIN INSERT INTO foo VALUES(NEW.id);RETURN NEW;
END; $$;

CREATE TRIGGER insert_vfoo INSTEAD OF INSERT ON vfoo       FOR EACH ROW EXECUTE PROCEDURE insert_foo();

INSERT INTO vfoo VALUES('helmle', 2) RETURNING *; text  | id
--------+----helmle |  2
(1 row)

SELECT * FROM vfoo;text  | id
-------+----bernd |  2
(1 row)

This is solvable by a properly designed trigger function, but maybe we need 
to do something about this?

> I don't plan to add the syntax to allow triggers on views to be
> disabled at this time, but that should be easy to implement, if there
> is a use case for it.

I really don't see a need for this at the moment. We don't have DISABLE 
RULE either. I'm going to post some additional comments once i've 
understand all things.

-- 
Thanks
Bernd

On 2010-09-23 1:16 AM, Bernd Helmle wrote:
> INSERT INTO vfoo VALUES('helmle', 2) RETURNING *;
>    text  | id
> --------+----
>   helmle |  2
> (1 row)
>
> SELECT * FROM vfoo;
>   text  | id
> -------+----
>   bernd |  2
> (1 row)
>
> This is solvable by a properly designed trigger function, but maybe we need
> to do something about this?

I really don't think we should limit what people are allowed to do in 
the trigger function.

Besides, even if the RETURNING clause returned 'bernd' in the above 
case, I think it would be even *more* surprising.  The trigger function 
explicitly returns NEW which has 'helmle' as the first field.


Regards,
Marko Tiikkaja

On 23 September 2010 00:26, Marko Tiikkaja
<marko.tiikkaja@cs.helsinki.fi> wrote:
> On 2010-09-23 1:16 AM, Bernd Helmle wrote:
>>
>> INSERT INTO vfoo VALUES('helmle', 2) RETURNING *;
>>   text  | id
>> --------+----
>>  helmle |  2
>> (1 row)
>>
>> SELECT * FROM vfoo;
>>  text  | id
>> -------+----
>>  bernd |  2
>> (1 row)
>>
>> This is solvable by a properly designed trigger function, but maybe we
>> need
>> to do something about this?
>
> I really don't think we should limit what people are allowed to do in the
> trigger function.
>
> Besides, even if the RETURNING clause returned 'bernd' in the above case, I
> think it would be even *more* surprising.  The trigger function explicitly
> returns NEW which has 'helmle' as the first field.
>

Yes, I agree. To me this is the least surprising behaviour. I think a
more common case would be where the trigger computed a value (such as
the 'last updated' example). The executor doesn't have any kind of a
handle on the row inserted by the trigger, so it has to rely on the
function return value to support RETURNING.

I can confirm the latest Oracle (11g R2 Enterprise Edition) does not
support RETURNING INTO with INSTEAD OF triggers (although it does work
with its auto-updatable views), presumably because it's triggers don't
return values, but I think it would be a shame for us to not support
it.

Regards,
Dean

--On 23. September 2010 08:59:32 +0100 Dean Rasheed 
<dean.a.rasheed@gmail.com> wrote:

> Yes, I agree. To me this is the least surprising behaviour. I think a
> more common case would be where the trigger computed a value (such as
> the 'last updated' example). The executor doesn't have any kind of a
> handle on the row inserted by the trigger, so it has to rely on the
> function return value to support RETURNING.

I didn't mean to forbid it altogether, but at least to document 
explicitely, that the trigger returns a VIEW's NEW tuple, not the one of 
the base table (and may modify it). But you've already adressed this in 
your doc patches, so nothing to worry about further.

-- 
Thanks
Bernd

--On 8. September 2010 09:00:33 +0100 Dean Rasheed
<dean.a.rasheed@gmail.com> wrote:

> Here's an updated version with improved formatting and a few minor
> wording changes to the triggers chapter.

This version doesn't apply clean anymore due to some rejects in
plainmain.c. Corrected version attached.

--
Thanks

    Bernd

On 29 September 2010 20:18, Bernd Helmle <mailings@oopsware.de> wrote:
>
>
> --On 8. September 2010 09:00:33 +0100 Dean Rasheed
> <dean.a.rasheed@gmail.com> wrote:
>
>> Here's an updated version with improved formatting and a few minor
>> wording changes to the triggers chapter.
>
> This version doesn't apply clean anymore due to some rejects in plainmain.c.
> Corrected version attached.
>

Ah yes, those pesky bits have been rotting while I wasn't looking.
Thanks for fixing them!

Regards,
Dean

--On 30. September 2010 07:38:18 +0100 Dean Rasheed 
<dean.a.rasheed@gmail.com> wrote:

>> This version doesn't apply clean anymore due to some rejects in
>> plainmain.c. Corrected version attached.
>>
>
> Ah yes, those pesky bits have been rotting while I wasn't looking.
> Thanks for fixing them!

Basic summary of this patch:

* The patch includes a fairly complete discussion about INSTEAD OF triggers 
and their usage on views. There are also additional enhancements to the 
RULE documentation, which seems, given that this might supersede the usage 
of RULES for updatable views, reasonable.

* The patch passes regressions tests and comes with a bunch of its own 
regression tests. I think they are complete, they cover statement and row 
Level trigger and show the usage for JOINed views for example.

* I've checked against a draft of the SQL standard, the behavior of the 
patch seems to match the spec (my copy might be out of date, however).

* The code looks pretty good to me, there are some low level error messages 
exposing some implementation details, which could be changed (e.g. 
"wholerow is NULL"), but given that this is most of the time unexpected and 
is used in some older code as well, this doesn't seem very important.

* The implementation introduces the notion of "wholerow". This is a junk 
target list entry which allows the executor to carry the view information 
to an INSTEAD OF trigger. In case of DELETE/UPDATE, the rewriter is 
responsible to inject the new "wholerow" TLE and make the original query to 
point to a new range table entry (correct me, when i'm wrong), which is 
based on the view's query. I'm not sure i'm happy with the notion of 
"wholerow" here, maybe "viewrow" or "viewtarget" is more descriptive?

* I'm inclined to say that INSTEAD OF triggers have less overhead than 
RULES, but this is not proven yet with a reasonable benchmark.

I would like to do some more tests/review, but going to mark this patch as 
"Ready for Committer", so that someone more qualified on the executor part 
can have a look on it during this commitfest, if that's okay for us?

-- 
Thanks
Bernd

On 5 October 2010 21:17, Bernd Helmle <mailings@oopsware.de> wrote:
> Basic summary of this patch:
>

Thanks for the review.

> * The patch includes a fairly complete discussion about INSTEAD OF triggers
> and their usage on views. There are also additional enhancements to the RULE
> documentation, which seems, given that this might supersede the usage of
> RULES for updatable views, reasonable.
>
> * The patch passes regressions tests and comes with a bunch of its own
> regression tests. I think they are complete, they cover statement and row
> Level trigger and show the usage for JOINed views for example.
>
> * I've checked against a draft of the SQL standard, the behavior of the
> patch seems to match the spec (my copy might be out of date, however).
>
> * The code looks pretty good to me, there are some low level error messages
> exposing some implementation details, which could be changed (e.g. "wholerow
> is NULL"), but given that this is most of the time unexpected and is used in
> some older code as well, this doesn't seem very important.
>

Hopefully that error should never happen, since it would indicate a
bug in the code rather than a user error.

> * The implementation introduces the notion of "wholerow". This is a junk
> target list entry which allows the executor to carry the view information to
> an INSTEAD OF trigger. In case of DELETE/UPDATE, the rewriter is responsible
> to inject the new "wholerow" TLE and make the original query to point to a
> new range table entry (correct me, when i'm wrong), which is based on the
> view's query. I'm not sure i'm happy with the notion of "wholerow" here,
> maybe "viewrow" or "viewtarget" is more descriptive?
>

That's a good description of how it works. I chose "wholerow" because
that matched similar terminology used already, for example in
preptlist.c when doing FOR UPDATE/SHARE on a view. I don't feel
strongly about it, but my inclination is to stick with "wholerow"
unless someone feels strongly otherwise.

> * I'm inclined to say that INSTEAD OF triggers have less overhead than
> RULES, but this is not proven yet with a reasonable benchmark.
>

It's difficult to come up with a general statement on performance
because there are so many variables that might affect it. In a few
simple tests, I found that for repeated small updates RULEs and
TRIGGERs perform roughly the same, but for bulk updates (one query
updating 1000s of rows) a RULE is best.

> I would like to do some more tests/review, but going to mark this patch as
> "Ready for Committer", so that someone more qualified on the executor part
> can have a look on it during this commitfest, if that's okay for us?
>

Thanks for looking at it. I hope this is useful functionality to make
it easier to write updatable views, and perhaps it will help with
implementing auto-updatable views too.

Cheers,
Dean


> --
> Thanks
>
>        Bernd
>

Bernd Helmle <mailings@oopsware.de> writes:
> I would like to do some more tests/review, but going to mark this patch as 
> "Ready for Committer", so that someone more qualified on the executor part 
> can have a look on it during this commitfest, if that's okay for us?

I've started looking at this patch now.  I think it would have been best
submitted as two patches: one to add the SQL-spec "INSTEAD OF" trigger
functionality, and a follow-on to extend INSTEAD OF triggers to views.
I'm thinking of breaking it apart into two separate commits along that
line, mainly because I think INSTEAD OF is probably committable but
I'm much less sure about the other part.

In the INSTEAD OF part, the main gripe I've got is the data
representation choice:

***************
*** 1609,1614 ****
--- 1609,1615 ----     List       *funcname;       /* qual. name of function to call */     List       *args;
/*list of (T_String) Values or NIL */     bool        before;         /* BEFORE/AFTER */
 
+     bool        instead;        /* INSTEAD OF (overrides BEFORE/AFTER) */     bool        row;            /*
ROW/STATEMENT*/     /* events uses the TRIGGER_TYPE bits defined in catalog/pg_trigger.h */     int16       events;
   /* INSERT/UPDATE/DELETE/TRUNCATE */
 

This pretty much sucks, because not only is this a rather confusing
definition, it's not really backwards compatible: any code that thinks
"!before" must mean "after" is going to be silently broken.  Usually,
when you do something that necessarily breaks old code, what you want
is to make sure the breakage is obvious at compile time.  I think the
right thing here is to replace "before" with a three-valued enum,
perhaps called "timing", so as to force people to take another look
at any code that touches the field directly.

Although we already have macros TRIGGER_FIRED_AFTER/TRIGGER_FIRED_BEFORE
that seem to mask the details here, the changes you had to make in
contrib illustrate that the macros' callers could still be embedding this
basic mistake of testing "!before" when they mean "after" or vice versa.
I wonder whether we should intentionally rename the macros to force
people to take another look at their logic.  Or is that going too far?
Comments anyone?
        regards, tom lane

On 8 October 2010 16:50, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Bernd Helmle <mailings@oopsware.de> writes:
>> I would like to do some more tests/review, but going to mark this patch as
>> "Ready for Committer", so that someone more qualified on the executor part
>> can have a look on it during this commitfest, if that's okay for us?
>
> I've started looking at this patch now.  I think it would have been best
> submitted as two patches: one to add the SQL-spec "INSTEAD OF" trigger
> functionality, and a follow-on to extend INSTEAD OF triggers to views.

SQL-spec "INSTEAD OF" triggers *are* view triggers. I don't see how
you can separate the two.


> I'm thinking of breaking it apart into two separate commits along that
> line, mainly because I think INSTEAD OF is probably committable but
> I'm much less sure about the other part.
>
> In the INSTEAD OF part, the main gripe I've got is the data
> representation choice:
>
> ***************
> *** 1609,1614 ****
> --- 1609,1615 ----
>      List       *funcname;       /* qual. name of function to call */
>      List       *args;           /* list of (T_String) Values or NIL */
>      bool        before;         /* BEFORE/AFTER */
> +     bool        instead;        /* INSTEAD OF (overrides BEFORE/AFTER) */
>      bool        row;            /* ROW/STATEMENT */
>      /* events uses the TRIGGER_TYPE bits defined in catalog/pg_trigger.h */
>      int16       events;         /* INSERT/UPDATE/DELETE/TRUNCATE */
>
> This pretty much sucks, because not only is this a rather confusing
> definition, it's not really backwards compatible: any code that thinks
> "!before" must mean "after" is going to be silently broken.  Usually,
> when you do something that necessarily breaks old code, what you want
> is to make sure the breakage is obvious at compile time.  I think the
> right thing here is to replace "before" with a three-valued enum,
> perhaps called "timing", so as to force people to take another look
> at any code that touches the field directly.
>
> Although we already have macros TRIGGER_FIRED_AFTER/TRIGGER_FIRED_BEFORE
> that seem to mask the details here, the changes you had to make in
> contrib illustrate that the macros' callers could still be embedding this
> basic mistake of testing "!before" when they mean "after" or vice versa.
> I wonder whether we should intentionally rename the macros to force
> people to take another look at their logic.  Or is that going too far?
> Comments anyone?
>

I think that you're confusing 2 different parts of code here. The
TRIGGER_FIRED_AFTER/TRIGGER_FIRED_BEFORE macros operate on the bits
from the tg_event field of the TriggerData structure. This has a
completely different representation for the trigger timing compared to
the code you mention above, which is from the CreateTrigStmt
structure. That structure is only used internally in a couple of
places, by the parser and CreateTrigger().

I agree that perhaps it would be neater to represent it as an enum,
but I think the scope for code breakage is far smaller than you claim.
This structure is not being exposed to trigger code.

The changes I made in contrib are unrelated to the representation used
in CreateTrigStmt. It's just a matter of tidying up code in before
triggers to say "if (!fired_before) error" rather than "if
(fired_after) error". That's neater anyway, even in the case where
they're mutually exclusive (as they are on tables). The scope for user
code being broken is limited beause they'd have to put one of these
trigger functions in an INSTEAD OF trigger on a view.

Regards,
Dean



>                        regards, tom lane
>

Dean Rasheed <dean.a.rasheed@gmail.com> writes:
> On 8 October 2010 16:50, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> I've started looking at this patch now. �I think it would have been best
>> submitted as two patches: one to add the SQL-spec "INSTEAD OF" trigger
>> functionality, and a follow-on to extend INSTEAD OF triggers to views.

> SQL-spec "INSTEAD OF" triggers *are* view triggers. I don't see how
> you can separate the two.

Oh, they're not allowed on tables?  Why not?  AFAICS they'd be exactly
equivalent to a BEFORE trigger that always returns NULL.

> I think that you're confusing 2 different parts of code here. The
> TRIGGER_FIRED_AFTER/TRIGGER_FIRED_BEFORE macros operate on the bits
> from the tg_event field of the TriggerData structure.

Yeah, I'm aware of that.  My point is that all code that deals with
trigger firing times now has to consider three possible states where
before there were two; and it's entirely likely that some places are
testing for the wrong condition once a third state is admitted as a
possibility.

> The scope for user
> code being broken is limited beause they'd have to put one of these
> trigger functions in an INSTEAD OF trigger on a view.

Well, the only reason those tests are being made at all is as
self-defense against being called via an incorrect trigger definition.
So they're worth nothing if they fail to treat the INSTEAD OF case
correctly.
        regards, tom lane

On Fri, Oct 8, 2010 at 11:50 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> I think the
> right thing here is to replace "before" with a three-valued enum,
> perhaps called "timing", so as to force people to take another look
> at any code that touches the field directly.

+1.  That seems much nicer.

> Although we already have macros TRIGGER_FIRED_AFTER/TRIGGER_FIRED_BEFORE
> that seem to mask the details here, the changes you had to make in
> contrib illustrate that the macros' callers could still be embedding this
> basic mistake of testing "!before" when they mean "after" or vice versa.
> I wonder whether we should intentionally rename the macros to force
> people to take another look at their logic.  Or is that going too far?
> Comments anyone?

I'm less sold on this one.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Robert Haas <robertmhaas@gmail.com> writes:
> On Fri, Oct 8, 2010 at 11:50 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Although we already have macros TRIGGER_FIRED_AFTER/TRIGGER_FIRED_BEFORE
>> that seem to mask the details here, the changes you had to make in
>> contrib illustrate that the macros' callers could still be embedding this
>> basic mistake of testing "!before" when they mean "after" or vice versa.
>> I wonder whether we should intentionally rename the macros to force
>> people to take another look at their logic. �Or is that going too far?
>> Comments anyone?

> I'm less sold on this one.

I'm not sold on it either, just wanted to run it up the flagpole to see
if anyone would salute.  For the moment I'm thinking that calling out
the point in the 9.1 release notes should be sufficient.  I made an
extra commit to make sure the issue is salient in the commit log.
        regards, tom lane

BTW, while I'm looking at this: it seems like the "index" arrays in
struct TrigDesc are really a lot more complication than they are worth.
It'd be far easier to dispense with them and instead iterate through
the main trigger array, skipping any triggers whose tgtype doesn't match
what we need.  If you had a really huge number of triggers on a table,
it's possible that could be marginally slower, but I'm having a hard
time envisioning practical situations where anybody could tell the
difference.
        regards, tom lane

Bernd Helmle <mailings@oopsware.de> writes:
> --On 8. September 2010 09:00:33 +0100 Dean Rasheed 
> <dean.a.rasheed@gmail.com> wrote:
>> Here's an updated version with improved formatting and a few minor
>> wording changes to the triggers chapter.

> This version doesn't apply clean anymore due to some rejects in 
> plainmain.c. Corrected version attached.

Applied with revisions.  A couple of points worth remarking:

* I took out this change in planmain.c: 
+       /*
+       * If the query target is a VIEW, it won't be in the jointree, but we
+       * need a dummy RelOptInfo node for it. This need not have any stats in
+       * it because it always just goes at the top of the plan tree.
+       */
+      if (parse->resultRelation &&
+          root->simple_rel_array[parse->resultRelation] == NULL)
+          build_simple_rel(root, parse->resultRelation, RELOPT_OTHER_MEMBER_REL);

AFAICT that's just dead code: the only reason to build such a rel would
be if there were Vars referencing it in the main part of the plan tree.
But there aren't.  Perhaps this was left over from some early iteration
of the patch before you had the Var numbering done right?  Do you know
of any cases where it's still needed?

* I also took out the changes in preprocess_targetlist() that tried to
prevent equivalent wholerow vars from getting entered in the targetlist.
This would not work as intended since the executor has specific
expectations for the names of those junk TLEs; it'd fail if it ever
actually tried to do an EvalPlanQual recheck that needed those TLEs.
Now I believe that an EPQ recheck is impossible so far as the update or
delete itself is concerned, when the target is a view.  So if you were
really concerned about the extra vars, the non-kluge route to a solution
would be to avoid generating RowMarks in the first place.  You'd have to
think a bit about the possibility of SELECT FOR UPDATE in sub-selects
though; the query as a whole might need some rowmarks even if the top
level Modify node doesn't.  On the whole I couldn't get excited about
this issue, so I just left it alone.
        regards, tom lane

On 10 October 2010 19:06, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Applied with revisions.

Brilliant! Thank you very much.

> * I took out this change in planmain.c:
>
> +       /*
> +        * If the query target is a VIEW, it won't be in the jointree, but we
> +        * need a dummy RelOptInfo node for it. This need not have any stats in
> +        * it because it always just goes at the top of the plan tree.
> +        */
> +       if (parse->resultRelation &&
> +               root->simple_rel_array[parse->resultRelation] == NULL)
> +               build_simple_rel(root, parse->resultRelation, RELOPT_OTHER_MEMBER_REL);
>
> AFAICT that's just dead code: the only reason to build such a rel would
> be if there were Vars referencing it in the main part of the plan tree.
> But there aren't.  Perhaps this was left over from some early iteration
> of the patch before you had the Var numbering done right?  Do you know
> of any cases where it's still needed?

No, I think you're right. It was just the leftovers of an early
attempt to get the rewriter changes right.

> * I also took out the changes in preprocess_targetlist() that tried to
> prevent equivalent wholerow vars from getting entered in the targetlist.
> This would not work as intended since the executor has specific
> expectations for the names of those junk TLEs; it'd fail if it ever
> actually tried to do an EvalPlanQual recheck that needed those TLEs.

Ah yes, I missed that. I still don't see where it uses those TLEs by
name though. It looks as though it's using wholeAttNo, so perhaps my
code would have worked if I had set wholeAttNo on the RowMark? Anyway,
I don't think it's likely that this extra Var is going to be present
often in practice, so I don't think it's a problem worth worrying
about.

Thanks very much for looking at this.

Regards,
Dean


> Now I believe that an EPQ recheck is impossible so far as the update or
> delete itself is concerned, when the target is a view.  So if you were
> really concerned about the extra vars, the non-kluge route to a solution
> would be to avoid generating RowMarks in the first place.  You'd have to
> think a bit about the possibility of SELECT FOR UPDATE in sub-selects
> though; the query as a whole might need some rowmarks even if the top
> level Modify node doesn't.  On the whole I couldn't get excited about
> this issue, so I just left it alone.
>
>                        regards, tom lane
>