diff options
Diffstat (limited to 'src/backend/optimizer/path/equivclass.c')
| -rw-r--r-- | src/backend/optimizer/path/equivclass.c | 26 |
1 files changed, 18 insertions, 8 deletions
diff --git a/src/backend/optimizer/path/equivclass.c b/src/backend/optimizer/path/equivclass.c index 13ed31a5c96..56e93c2c41c 100644 --- a/src/backend/optimizer/path/equivclass.c +++ b/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 1.9.2.2 2008/12/01 21:06:20 tgl Exp $ + * $PostgreSQL: pgsql/src/backend/optimizer/path/equivclass.c,v 1.9.2.3 2009/09/29 01:21:02 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -114,6 +114,19 @@ process_equivalence(PlannerInfo *root, RestrictInfo *restrictinfo, item2_relids = restrictinfo->right_relids; /* + * Reject clauses of the form X=X. These are not as redundant as they + * might seem at first glance: assuming the operator is strict, this is + * really an expensive way to write X IS NOT NULL. So we must not risk + * just losing the clause, which would be possible if there is already + * a single-element EquivalenceClass containing X. The case is not + * common enough to be worth contorting the EC machinery for, so just + * reject the clause and let it be processed as a normal restriction + * clause. + */ + if (equal(item1, item2)) + return false; /* X=X is not a useful equivalence */ + + /* * If below outer join, check for strictness, else reject. */ if (below_outer_join) @@ -151,13 +164,10 @@ process_equivalence(PlannerInfo *root, RestrictInfo *restrictinfo, * * 4. We find neither. Make a new, two-entry EC. * - * Note: since all ECs are built through this process, it's impossible - * that we'd match an item in more than one existing EC. It is possible - * to match more than once within an EC, if someone fed us something silly - * like "WHERE X=X". (However, we can't simply discard such clauses, - * since they should fail when X is null; so we will build a 2-member EC - * to ensure the correct restriction clause gets generated. Hence there - * is no shortcut here for item1 and item2 equal.) + * Note: since all ECs are built through this process or the similar + * search in get_eclass_for_sort_expr(), it's impossible that we'd match + * an item in more than one existing nonvolatile EC. So it's okay to stop + * at the first match. */ ec1 = ec2 = NULL; em1 = em2 = NULL; |