From 9688c4e6f1516d2fc0db5d200112c4d91538878d Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Sun, 30 Jan 2011 17:04:31 -0500
Subject: [PATCH] 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 c386586f40d..f92bcd41b1a 100644
--- a/src/backend/optimizer/prep/prepjointree.c
+++ b/src/backend/optimizer/prep/prepjointree.c
@@ -1840,6 +1840,11 @@ 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?
*/
@@ -1849,7 +1854,7 @@ 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,
@@ -1869,14 +1874,14 @@ 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;
--
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