Improve comments about semijoin implementation strategy, per a question

from Robert Haas.

diff --git a/src/backend/optimizer/path/joinrels.c b/src/backend/optimizer/path/joinrels.c
index 1f1309771fabd878bfaebc827cc0e9f392428dd4..fdf322a386dd6d4229cbc1564e5f4c2877eedef1 100644 (file)
--- a/src/backend/optimizer/path/joinrels.c
+++ b/src/backend/optimizer/path/joinrels.c
@@ -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))