postgres.git / summary / log / commit / refs

commit    7e765cb0688a9280a89f5054d3cb57fe892d73f5
Author:   Tom Lane <tgl@sss.pgh.pa.us>
Date:     Fri Oct 26 18:20:10 2012 +0000

    Prefer actual constants to pseudo-constants in equivalence class machinery.
    
    generate_base_implied_equalities_const() should prefer plain Consts over
    other em_is_const eclass members when choosing the "pivot" value that
    all the other members will be equated to.  This makes it more likely that
    the generated equalities will be useful in constraint-exclusion proofs.
    Per report from Rushabh Lathia.


 src/backend/optimizer/path/equivclass.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/backend/optimizer/path/equivclass.c b/src/backend/optimizer/path/equivclass.c
index c911bcda242..f40a46b0d7b 100644
--- a/src/backend/optimizer/path/equivclass.c
+++ b/src/backend/optimizer/path/equivclass.c
@@ -612,7 +612,12 @@ generate_base_implied_equalities_const(PlannerInfo *root,
 		}
 	}
 
-	/* Find the constant member to use */
+	/*
+	 * Find the constant member to use.  We prefer an actual constant to
+	 * pseudo-constants (such as Params), because the constraint exclusion
+	 * machinery might be able to exclude relations on the basis of generated
+	 * "var = const" equalities, but "var = param" won't work for that.
+	 */
 	foreach(lc, ec->ec_members)
 	{
 		EquivalenceMember *cur_em = (EquivalenceMember *) lfirst(lc);
@@ -620,7 +625,8 @@ generate_base_implied_equalities_const(PlannerInfo *root,
 		if (cur_em->em_is_const)
 		{
 			const_em = cur_em;
-			break;
+			if (IsA(cur_em->em_expr, Const))
+				break;
 		}
 	}
 	Assert(const_em != NULL);


[parent: 67efc03a81a4]