diff options
Diffstat (limited to 'src/backend/optimizer/path/pathkeys.c')
| -rw-r--r-- | src/backend/optimizer/path/pathkeys.c | 23 |
1 files changed, 16 insertions, 7 deletions
diff --git a/src/backend/optimizer/path/pathkeys.c b/src/backend/optimizer/path/pathkeys.c index 469ff82db39..8f7e02c3b2e 100644 --- a/src/backend/optimizer/path/pathkeys.c +++ b/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 1.97 2009/02/28 03:51:05 tgl Exp $ + * $PostgreSQL: pgsql/src/backend/optimizer/path/pathkeys.c,v 1.97.2.1 2009/07/17 23:19:59 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -988,12 +988,21 @@ find_mergeclauses_for_pathkeys(PlannerInfo *root, * no two members have the same EC, so it's not possible for this * code to enter the same mergeclause into the result list twice. * - * XXX it's possible that multiple matching clauses might have - * different ECs on the other side, in which case the order we put - * them into our result makes a difference in the pathkeys required - * for the other input path. However this routine hasn't got any info - * about which order would be best, so for now we disregard that case - * (which is probably a corner case anyway). + * It's possible that multiple matching clauses might have different + * ECs on the other side, in which case the order we put them into our + * result makes a difference in the pathkeys required for the other + * input path. However this routine hasn't got any info about which + * order would be best, so we don't worry about that. + * + * It's also possible that the selected mergejoin clauses produce + * a noncanonical ordering of pathkeys for the other side, ie, we + * might select clauses that reference b.v1, b.v2, b.v1 in that + * order. This is not harmful in itself, though it suggests that + * the clauses are partially redundant. Since it happens only with + * redundant query conditions, we don't bother to eliminate it. + * make_inner_pathkeys_for_merge() has to delete duplicates when + * it constructs the canonical pathkeys list, and we also have to + * deal with the case in create_mergejoin_plan(). *---------- */ foreach(j, restrictinfos) |