diff options
Diffstat (limited to 'src')
| -rw-r--r-- | src/backend/optimizer/path/joinrels.c | 28 |
1 files changed, 24 insertions, 4 deletions
diff --git a/src/backend/optimizer/path/joinrels.c b/src/backend/optimizer/path/joinrels.c index f8c88f654d2..d3287b687c9 100644 --- a/src/backend/optimizer/path/joinrels.c +++ b/src/backend/optimizer/path/joinrels.c @@ -8,7 +8,7 @@ * * * IDENTIFICATION - * $PostgreSQL: pgsql/src/backend/optimizer/path/joinrels.c,v 1.97 2009/01/01 17:23:44 momjian Exp $ + * $PostgreSQL: pgsql/src/backend/optimizer/path/joinrels.c,v 1.98 2009/02/19 20:32:45 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -424,9 +424,27 @@ join_is_legal(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2, create_unique_path(root, rel2, rel2->cheapest_total_path, sjinfo) != NULL) { - /* + /*---------- * For a semijoin, we can join the RHS to anything else by * unique-ifying the RHS (if the RHS can be unique-ified). + * We will only get here if we have the full RHS but less + * than min_lefthand on the LHS. + * + * The reason to consider such a join path is exemplified by + * SELECT ... FROM a,b WHERE (a.x,b.y) IN (SELECT c1,c2 FROM c) + * If we insist on doing this as a semijoin we will first have + * to form the cartesian product of A*B. But if we unique-ify + * C then the semijoin becomes a plain innerjoin and we can join + * in any order, eg C to A and then to B. When C is much smaller + * than A and B this can be a huge win. So we allow C to be + * joined to just A or just B here, and then make_join_rel has + * to handle the case properly. + * + * Note that actually we'll allow unique-ified C to be joined to + * some other relation D here, too. That is legal, if usually not + * very sane, and this routine is only concerned with legality not + * with whether the join is good strategy. + *---------- */ if (match_sjinfo) return false; /* invalid join path */ @@ -648,8 +666,10 @@ make_join_rel(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2) break; case JOIN_SEMI: /* - * Do these steps only if we actually have a regular semijoin, - * as opposed to a case where we should unique-ify the RHS. + * We might have a normal semijoin, or a case where we don't have + * enough rels to do the semijoin but can unique-ify the RHS and + * then do an innerjoin (see comments in join_is_legal). In the + * latter case we can't apply JOIN_SEMI joining. */ if (bms_is_subset(sjinfo->min_lefthand, rel1->relids) && bms_is_subset(sjinfo->min_righthand, rel2->relids)) |