postgres.git / summary / log / commit / refs

commit    c1da1f45a1033fecd888c56568db5ca9b1c050d2
Author:   Tom Lane <tgl@sss.pgh.pa.us>
Date:     Sun Jan 30 22:04:31 2011 +0000
First released in: 9.1.0, 8.4.8, 9.0.4

    Make reduce_outer_joins() smarter about semijoins.
    
    reduce_outer_joins() mistakenly treated a semijoin like a left join for
    purposes of deciding whether not-null constraints created by the join's
    quals could be passed down into the join's left-hand side (possibly
    resulting in outer-join simplification there).  Actually, semijoin works
    like inner join for this purpose, ie, we do not need to see any rows that
    can't possibly satisfy the quals.  Hence, two-line fix to treat semi and
    inner joins alike.  Per observation by Andres Freund about a performance
    gripe from Yazan Suleiman.
    
    Back-patch to 8.4, since this oversight has been there since the current
    handling of semijoins was implemented.


 src/backend/optimizer/prep/prepjointree.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/src/backend/optimizer/prep/prepjointree.c b/src/backend/optimizer/prep/prepjointree.c
index cd99752ee70..5e4b11782b9 100644
--- a/src/backend/optimizer/prep/prepjointree.c
+++ b/src/backend/optimizer/prep/prepjointree.c
@@ -1738,6 +1738,11 @@ reduce_outer_joins_pass2(Node *jtnode,
 			 * is that we pass either the local or the upper constraints,
 			 * never both, to the children of an outer join.
 			 *
+			 * Note that a SEMI join works like an inner join here: it's okay
+			 * to pass down both local and upper constraints.  (There can't
+			 * be any upper constraints affecting its inner side, but it's
+			 * not worth having a separate code path to avoid passing them.)
+			 *
 			 * At a FULL join we just punt and pass nothing down --- is it
 			 * possible to be smarter?
 			 */
@@ -1747,7 +1752,7 @@ reduce_outer_joins_pass2(Node *jtnode,
 				if (!computed_local_nonnullable_vars)
 					local_nonnullable_vars = find_nonnullable_vars(j->quals);
 				local_forced_null_vars = find_forced_null_vars(j->quals);
-				if (jointype == JOIN_INNER)
+				if (jointype == JOIN_INNER || jointype == JOIN_SEMI)
 				{
 					/* OK to merge upper and local constraints */
 					local_nonnullable_rels = bms_add_members(local_nonnullable_rels,
@@ -1767,14 +1772,14 @@ reduce_outer_joins_pass2(Node *jtnode,
 
 			if (left_state->contains_outer)
 			{
-				if (jointype == JOIN_INNER)
+				if (jointype == JOIN_INNER || jointype == JOIN_SEMI)
 				{
 					/* pass union of local and upper constraints */
 					pass_nonnullable_rels = local_nonnullable_rels;
 					pass_nonnullable_vars = local_nonnullable_vars;
 					pass_forced_null_vars = local_forced_null_vars;
 				}
-				else if (jointype != JOIN_FULL) /* ie, LEFT/SEMI/ANTI */
+				else if (jointype != JOIN_FULL)		/* ie, LEFT or ANTI */
 				{
 					/* can't pass local constraints to non-nullable side */
 					pass_nonnullable_rels = nonnullable_rels;


[parent: 7df910c7d128]