Postgresql crash (signal 11). keywords: distinct, subselect, union

"Magnus Naeslund(f)" <mag@fbab.net> writes:

SELECT DISTINCT
*
FROM
(
SELECT
vtgm.snicker_id
FROM snicker_group_mapping vtgm
WHERE exists
(
SELECT
*
FROM snicker_group vtg
WHERE vtgm.snicker_group_id = vtg.id
AND lower(vtg.title) ~* 'test'
)
UNION
SELECT
snicker.id
FROM snicker
WHERE lower(snicker.name_singular) ~* 'test'
OR lower(snicker.name_plural) ~* 'test'
) AS vt_id
WHERE vt_id is not null;

While the crash is certainly a bug, the answer is going to be "don't do
that". Testing a whole record for null-ness is not meaningful.

regards, tom lane

Tom Lane wrote:

"Magnus Naeslund(f)" <mag@fbab.net> writes:

SELECT DISTINCT
*
FROM
(
SELECT
vtgm.snicker_id
FROM snicker_group_mapping vtgm
WHERE exists
(
SELECT
*
FROM snicker_group vtg
WHERE vtgm.snicker_group_id = vtg.id
AND lower(vtg.title) ~* 'test'
)
UNION
SELECT
snicker.id
FROM snicker
WHERE lower(snicker.name_singular) ~* 'test'
OR lower(snicker.name_plural) ~* 'test'
) AS vt_id
WHERE vt_id is not null;

While the crash is certainly a bug, the answer is going to be "don't do
that". Testing a whole record for null-ness is not meaningful.

Yep, my "workaround" (or bugfix) was to push that null test infront of
the exists. Also I might not need the surrounding distinct either,
doesn't union make the result distinct?

So if I would like to do the test after the union, I should add "AS xxx"
on both union queries and then "vt_id.xxx is not null", right ?

Regards,
Magnus

"Magnus Naeslund(f)" <mag@fbab.net> writes:

I just wanted to check if this has been fixed in any recent v8.1.x
release, since I'm using v8.1.0 now.

Here's the fix if you need it.

regards, tom lane

Index: allpaths.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/optimizer/path/allpaths.c,v
retrieving revision 1.137.2.1
diff -c -r1.137.2.1 allpaths.c
*** allpaths.c	22 Nov 2005 18:23:10 -0000	1.137.2.1
--- allpaths.c	13 Feb 2006 16:07:30 -0000
***************
*** 793,803 ****
   * it will work correctly: sublinks will already have been transformed into
   * subplans in the qual, but not in the subquery).
   *
!  * 2. The qual must not refer to any subquery output columns that were
   * found to have inconsistent types across a set operation tree by
   * subquery_is_pushdown_safe().
   *
!  * 3. If the subquery uses DISTINCT ON, we must not push down any quals that
   * refer to non-DISTINCT output columns, because that could change the set
   * of rows returned.  This condition is vacuous for DISTINCT, because then
   * there are no non-DISTINCT output columns, but unfortunately it's fairly
--- 793,806 ----
   * it will work correctly: sublinks will already have been transformed into
   * subplans in the qual, but not in the subquery).
   *
!  * 2. The qual must not refer to the whole-row output of the subquery
!  * (since there is no easy way to name that within the subquery itself).
!  *
!  * 3. The qual must not refer to any subquery output columns that were
   * found to have inconsistent types across a set operation tree by
   * subquery_is_pushdown_safe().
   *
!  * 4. If the subquery uses DISTINCT ON, we must not push down any quals that
   * refer to non-DISTINCT output columns, because that could change the set
   * of rows returned.  This condition is vacuous for DISTINCT, because then
   * there are no non-DISTINCT output columns, but unfortunately it's fairly
***************
*** 805,811 ****
   * parsetree representation.  It's cheaper to just make sure all the Vars
   * in the qual refer to DISTINCT columns.
   *
!  * 4. We must not push down any quals that refer to subselect outputs that
   * return sets, else we'd introduce functions-returning-sets into the
   * subquery's WHERE/HAVING quals.
   */
--- 808,814 ----
   * parsetree representation.  It's cheaper to just make sure all the Vars
   * in the qual refer to DISTINCT columns.
   *
!  * 5. We must not push down any quals that refer to subselect outputs that
   * return sets, else we'd introduce functions-returning-sets into the
   * subquery's WHERE/HAVING quals.
   */
***************
*** 834,839 ****
--- 837,849 ----

Assert(var->varno == rti);

+ 		/* Check point 2 */
+ 		if (var->varattno == 0)
+ 		{
+ 			safe = false;
+ 			break;
+ 		}
+ 
  		/*
  		 * We use a bitmapset to avoid testing the same attno more than once.
  		 * (NB: this only works because subquery outputs can't have negative
***************
*** 843,849 ****
  			continue;
  		tested = bms_add_member(tested, var->varattno);

! 		/* Check point 2 */
  		if (differentTypes[var->varattno])
  		{
  			safe = false;
--- 853,859 ----
  			continue;
  		tested = bms_add_member(tested, var->varattno);

! /* Check point 3 */
if (differentTypes[var->varattno])
{
safe = false;
***************
*** 855,861 ****
Assert(tle != NULL);
Assert(!tle->resjunk);

! 		/* If subquery uses DISTINCT or DISTINCT ON, check point 3 */
  		if (subquery->distinctClause != NIL &&
  			!targetIsInSortList(tle, subquery->distinctClause))
  		{
--- 865,871 ----
  		Assert(tle != NULL);
  		Assert(!tle->resjunk);

! /* If subquery uses DISTINCT or DISTINCT ON, check point 4 */
if (subquery->distinctClause != NIL &&
!targetIsInSortList(tle, subquery->distinctClause))
{
***************
*** 864,870 ****
break;
}

! 		/* Refuse functions returning sets (point 4) */
  		if (expression_returns_set((Node *) tle->expr))
  		{
  			safe = false;
--- 874,880 ----
  			break;
  		}

! /* Refuse functions returning sets (point 5) */
if (expression_returns_set((Node *) tle->expr))
{
safe = false;