Fix assertion failure when a SELECT DISTINCT ON expression is volatile.

In this case we generate two PathKey references to the expression (one for
DISTINCT and one for ORDER BY) and they really need to refer to the same
EquivalenceClass.  However get_eclass_for_sort_expr was being overly paranoid
and creating two different EC's.  Correct behavior is to use the SortGroupRef
index to decide whether two references to volatile expressions that are
equal() (ie textually equivalent) should be considered the same.

Backpatch to 8.4.  Possibly this should be changed in 8.3 as well, but
I'll refrain in the absence of evidence of a visible failure in that branch.

Per bug #5049.

src/backend/optimizer/path/equivclass.c

@@ -10,7 +10,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/equivclass.c,v 1.19 2009/06/11 14:48:58 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/equivclass.c,v 1.20 2009/09/12 00:04:58 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -357,7 +357,7 @@ add_eq_member(EquivalenceClass *ec, Expr *expr, Relids relids,
  *	  EquivalenceClass for it.
  *
  * sortref is the SortGroupRef of the originating SortGroupClause, if any,
- * or zero if not.
+ * or zero if not.  (It should never be zero if the expression is volatile!)
  *
  * This can be used safely both before and after EquivalenceClass merging;
  * since it never causes merging it does not invalidate any existing ECs
@@ -388,8 +388,12 @@ get_eclass_for_sort_expr(PlannerInfo *root,
 		EquivalenceClass *cur_ec = (EquivalenceClass *) lfirst(lc1);
 		ListCell   *lc2;
 
-		/* Never match to a volatile EC */
-		if (cur_ec->ec_has_volatile)
+		/*
+		 * Never match to a volatile EC, except when we are looking at another
+		 * reference to the same volatile SortGroupClause.
+		 */
+		if (cur_ec->ec_has_volatile &&
+			(sortref == 0 || sortref != cur_ec->ec_sortref))
 			continue;
 
 		if (!equal(opfamilies, cur_ec->ec_opfamilies))
@@ -433,6 +437,10 @@ get_eclass_for_sort_expr(PlannerInfo *root,
 	newec->ec_broken = false;
 	newec->ec_sortref = sortref;
 	newec->ec_merged = NULL;
+
+	if (newec->ec_has_volatile && sortref == 0)		/* should not happen */
+		elog(ERROR, "volatile EquivalenceClass has no sortref");
+
 	newem = add_eq_member(newec, expr, pull_varnos((Node *) expr),
 						  false, expr_datatype);
 

src/backend/optimizer/path/pathkeys.c

@@ -11,7 +11,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/pathkeys.c,v 1.98 2009/07/17 23:19:34 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/pathkeys.c,v 1.99 2009/09/12 00:04:59 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -635,6 +635,15 @@ convert_subquery_pathkeys(PlannerInfo *root, RelOptInfo *rel,
 							exprType((Node *) tle->expr),
 							exprTypmod((Node *) tle->expr),
 							0);
+
+				/*
+				 * Note: it might look funny to be setting sortref = 0 for
+				 * a reference to a volatile sub_eclass.  However, the
+				 * expression is *not* volatile in the outer query: it's
+				 * just a Var referencing whatever the subquery emitted.
+				 * (IOW, the outer query isn't going to re-execute the
+				 * volatile expression itself.)  So this is okay.
+				 */
 				outer_ec =
 					get_eclass_for_sort_expr(root,
 											 outer_expr,

src/test/regress/expected/select_distinct_on.out

@@ -66,3 +66,10 @@ SELECT DISTINCT ON (string4, ten) string4, ten, two
  VVVVxx  |   0 |   0
 (40 rows)
 
+-- bug #5049: early 8.4.x chokes on volatile DISTINCT ON clauses
+select distinct on (1) floor(random()) as r, f1 from int4_tbl order by 1,2;
+ r |     f1      
+---+-------------
+ 0 | -2147483647
+(1 row)
+

src/test/regress/sql/select_distinct_on.sql

@@ -14,3 +14,6 @@ SELECT DISTINCT ON (string4, ten) string4, two, ten
 SELECT DISTINCT ON (string4, ten) string4, ten, two
    FROM tmp
    ORDER BY string4 using <, ten using >, two using <;
+
+-- bug #5049: early 8.4.x chokes on volatile DISTINCT ON clauses
+select distinct on (1) floor(random()) as r, f1 from int4_tbl order by 1,2;