From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Fri, 15 Nov 2002 02:37:08 +0000 (+0000)
Subject: Push qual clauses containing subplans to the back of the qual list
X-Git-Url: http://git.postgresql.org/gitweb/static/gitweb.js?a=commitdiff_plain;h=18cf40dac6d618e8e284e81e1f248e50de1393ee;p=users%2Fbernd%2Fpostgres.git

Push qual clauses containing subplans to the back of the qual list
at each plan node.  Per gripe from Ross Reedstrom.
---

diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index 8723e80627..b0584b8480 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -69,6 +69,7 @@ static Node *fix_indxqual_operand(Node *node, int baserelid,
 					 IndexOptInfo *index,
 					 Oid *opclass);
 static List *switch_outer(List *clauses);
+static List *order_qual_clauses(Query *root, List *clauses);
 static void copy_path_costsize(Plan *dest, Path *src);
 static void copy_plan_costsize(Plan *dest, Plan *src);
 static SeqScan *make_seqscan(List *qptlist, List *qpqual, Index scanrelid);
@@ -177,6 +178,9 @@ create_scan_plan(Query *root, Path *best_path)
 	 */
 	scan_clauses = get_actual_clauses(best_path->parent->baserestrictinfo);
 
+	/* Sort clauses into best execution order */
+	scan_clauses = order_qual_clauses(root, scan_clauses);
+
 	switch (best_path->pathtype)
 	{
 		case T_SeqScan:
@@ -1178,6 +1182,43 @@ switch_outer(List *clauses)
 	return t_list;
 }
 
+/*
+ * order_qual_clauses
+ *		Given a list of qual clauses that will all be evaluated at the same
+ *		plan node, sort the list into the order we want to check the quals
+ *		in at runtime.
+ *
+ * Ideally the order should be driven by a combination of execution cost and
+ * selectivity, but unfortunately we have so little information about
+ * execution cost of operators that it's really hard to do anything smart.
+ * For now, we just move any quals that contain SubPlan references (but not
+ * InitPlan references) to the end of the list.
+ */
+static List *
+order_qual_clauses(Query *root, List *clauses)
+{
+	List	   *nosubplans;
+	List	   *withsubplans;
+	List	   *l;
+
+	/* No need to work hard if the query is subselect-free */
+	if (!root->hasSubLinks)
+		return clauses;
+
+	nosubplans = withsubplans = NIL;
+	foreach(l, clauses)
+	{
+		Node   *clause = lfirst(l);
+
+		if (contain_subplans(clause))
+			withsubplans = lappend(withsubplans, clause);
+		else
+			nosubplans = lappend(nosubplans, clause);
+	}
+
+	return nconc(nosubplans, withsubplans);
+}
+
 /*
  * Copy cost and size info from a Path node to the Plan node created from it.
  * The executor won't use this info, but it's needed by EXPLAIN.