Re: [HACKERS] [BUGS] BUG #2873: Function that returns an empty set with a 'not null' domain errors in 8.2 but not 8.1

2007-01-09 Thread Jim C. Nasby
On Mon, Jan 08, 2007 at 10:27:15AM -0500, Tom Lane wrote:
 Jonathan Hull [EMAIL PROTECTED] writes:
  The key feature for the error is that when a result structure (eg : pg_foo)
  is defined with a domain type that is not null, only PG 8.2 errors if the
  result is an empty set.
 
 The problem is explained well enough by this comment in plpgsql's code
 for FOR-over-query:
 
 /*
  * If the query didn't return any rows, set the target to NULL and return
  * with FOUND = false.
  */
 
 At the time this code was written, there weren't any potential negative
 side-effects of trying to set a row value to all NULLs, but now it's
 possible that that fails because of domain constraints.
 
 I think the idea was to ensure that a record variable would have the
 correct structure (matching the query output) post-loop, even if the
 query produced zero rows.  But it's not clear that that is really
 useful for anything, given plpgsql's dearth of introspection facilities.
 So we could make Jonathan's problem go away if we just take out the
 assignment of nulls, and say that FOR over no rows leaves the record
 variable unchanged.  The documentation doesn't specify the current
 behavior.
 
 Looking through the code, I see another place that does the same thing:
 FETCH from a cursor, when the cursor has no more rows to return.  It's
 a bit harder to argue that it's sane to leave the variable unchanged
 in this case.  However, the documentation doesn't actually promise that
 the target gets set to null in this case either.

I think it would be safer to set the record variable to something that
wipes any old data. I can't think of any examples of good code that
would fall prey to this, but I can certainly think of some nasty bugs
that users could inadvertently create. I know I'd personally like to
have the safety net...

Perhaps a means to mark the record as being null, other than setting all
the fields to null? That might also allow for a means for users to set
records to null, which I think would be useful in some cases.

BTW, are row variables also affected by this bug or is it just record
variables?
-- 
Jim Nasby[EMAIL PROTECTED]
EnterpriseDB  http://enterprisedb.com  512.569.9461 (cell)

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Re: [HACKERS] [BUGS] BUG #2873: Function that returns an empty set

2007-01-09 Thread Andrew Dunstan
Tom Lane wrote:

 This is closely related to the discussion a couple weeks ago about how
 a LEFT JOIN could produce nulls in an output column that was labeled as
 having a non-null-domain type.  We haven't figured out what is a sane
 behavior for that case, either.  I'm beginning to think that domains
 constrained not null are just fundamentally a bad idea.


I think we just expect left joins to produce nulls regardless of
constraints on the underlying cols, don't we? Concluding that not null in
domains is bad seems a bit drastic.

cheers

andrew


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Re: [HACKERS] [BUGS] BUG #2873: Function that returns an empty set with a 'not null' domain errors in 8.2 but not 8.1

2007-01-08 Thread Tom Lane
Jonathan Hull [EMAIL PROTECTED] writes:
 The key feature for the error is that when a result structure (eg : pg_foo)
 is defined with a domain type that is not null, only PG 8.2 errors if the
 result is an empty set.

The problem is explained well enough by this comment in plpgsql's code
for FOR-over-query:

/*
 * If the query didn't return any rows, set the target to NULL and return
 * with FOUND = false.
 */

At the time this code was written, there weren't any potential negative
side-effects of trying to set a row value to all NULLs, but now it's
possible that that fails because of domain constraints.

I think the idea was to ensure that a record variable would have the
correct structure (matching the query output) post-loop, even if the
query produced zero rows.  But it's not clear that that is really
useful for anything, given plpgsql's dearth of introspection facilities.
So we could make Jonathan's problem go away if we just take out the
assignment of nulls, and say that FOR over no rows leaves the record
variable unchanged.  The documentation doesn't specify the current
behavior.

Looking through the code, I see another place that does the same thing:
FETCH from a cursor, when the cursor has no more rows to return.  It's
a bit harder to argue that it's sane to leave the variable unchanged
in this case.  However, the documentation doesn't actually promise that
the target gets set to null in this case either.

Thoughts?

regards, tom lane

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