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Improve comments about semijoin implementation strategy, per a question

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from Robert Haas.
This commit is contained in:
Tom Lane 2009-02-19 20:32:45 +00:00
parent 5717f3a3e6
commit 233b8a99ad
1 changed files with 24 additions and 4 deletions
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28
src/backend/optimizer/path/joinrels.c
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@ -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))