Adjust remove_redundant_join_clauses() so that when it has a choice

of which redundant clause to remove, it removes the more expensive one.
In simple scenarios the clauses will be like 'var = var' and there's
no difference, but we are now capable of considering cases where there
are sub-selects in the clauses, and it makes a BIG difference.

diff --git a/src/backend/optimizer/util/restrictinfo.c b/src/backend/optimizer/util/restrictinfo.c
index e2cc53cc692350244cf8c0ea878028b63b8bffae..ddf965fdf2bde3569932dbf91b88dc8ef3e5bdf7 100644 (file)
--- a/src/backend/optimizer/util/restrictinfo.c
+++ b/src/backend/optimizer/util/restrictinfo.c
@@ -8,13 +8,14 @@
  *
  *
  * IDENTIFICATION
- *   $PostgreSQL: pgsql/src/backend/optimizer/util/restrictinfo.c,v 1.25 2004/01/05 23:39:54 tgl Exp $
+ *   $PostgreSQL: pgsql/src/backend/optimizer/util/restrictinfo.c,v 1.26 2004/02/27 21:48:04 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
 #include "postgres.h"
 
 #include "optimizer/clauses.h"
+#include "optimizer/cost.h"
 #include "optimizer/paths.h"
 #include "optimizer/restrictinfo.h"
 #include "optimizer/var.h"
@@ -27,7 +28,7 @@ static RestrictInfo *make_restrictinfo_internal(Expr *clause,
 static Expr *make_sub_restrictinfos(Expr *clause,
                                    bool is_pushed_down,
                                    bool valid_everywhere);
-static bool join_clause_is_redundant(Query *root,
+static RestrictInfo *join_clause_is_redundant(Query *root,
                         RestrictInfo *rinfo,
                         List *reference_list,
                         JoinType jointype);
@@ -317,17 +318,42 @@ remove_redundant_join_clauses(Query *root, List *restrictinfo_list,
 {
    List       *result = NIL;
    List       *item;
+   QualCost    cost;
+
+   /*
+    * If there are any redundant clauses, we want to eliminate the ones
+    * that are more expensive in favor of the ones that are less so.
+    * Run cost_qual_eval() to ensure the eval_cost fields are set up.
+    */
+   cost_qual_eval(&cost, restrictinfo_list);
+
+   /*
+    * We don't have enough knowledge yet to be able to estimate the number
+    * of times a clause might be evaluated, so it's hard to weight the
+    * startup and per-tuple costs appropriately.  For now just weight 'em
+    * the same.
+    */
+#define CLAUSECOST(r)  ((r)->eval_cost.startup + (r)->eval_cost.per_tuple)
 
    foreach(item, restrictinfo_list)
    {
        RestrictInfo *rinfo = (RestrictInfo *) lfirst(item);
+       RestrictInfo *prevrinfo;
 
-       /* drop it if redundant with any prior clause */
-       if (join_clause_is_redundant(root, rinfo, result, jointype))
-           continue;
-
-       /* otherwise, add it to result list */
-       result = lappend(result, rinfo);
+       /* is it redundant with any prior clause? */
+       prevrinfo = join_clause_is_redundant(root, rinfo, result, jointype);
+       if (prevrinfo == NULL)
+       {
+           /* no, so add it to result list */
+           result = lappend(result, rinfo);
+       }
+       else if (CLAUSECOST(rinfo) < CLAUSECOST(prevrinfo))
+       {
+           /* keep this one, drop the previous one */
+           result = lremove(prevrinfo, result);
+           result = lappend(result, rinfo);
+       }
+       /* else, drop this one */
    }
 
    return result;
@@ -361,7 +387,7 @@ select_nonredundant_join_clauses(Query *root,
        RestrictInfo *rinfo = (RestrictInfo *) lfirst(item);
 
        /* drop it if redundant with any reference clause */
-       if (join_clause_is_redundant(root, rinfo, reference_list, jointype))
+       if (join_clause_is_redundant(root, rinfo, reference_list, jointype) != NULL)
            continue;
 
        /* otherwise, add it to result list */
@@ -373,7 +399,8 @@ select_nonredundant_join_clauses(Query *root,
 
 /*
  * join_clause_is_redundant
- *     Returns true if rinfo is redundant with any clause in reference_list.
+ *     If rinfo is redundant with any clause in reference_list,
+ *     return one such clause; otherwise return NULL.
  *
  * This is the guts of both remove_redundant_join_clauses and
  * select_nonredundant_join_clauses.  See the docs above for motivation.
@@ -398,27 +425,30 @@ select_nonredundant_join_clauses(Query *root,
  * then they're not really redundant, because one constrains the
  * joined rows after addition of null fill rows, and the other doesn't.
  */
-static bool
+static RestrictInfo *
 join_clause_is_redundant(Query *root,
                         RestrictInfo *rinfo,
                         List *reference_list,
                         JoinType jointype)
 {
+   List       *refitem;
+
    /* always consider exact duplicates redundant */
-   /* XXX would it be sufficient to use ptrMember here? */
-   if (member(rinfo, reference_list))
-       return true;
+   foreach(refitem, reference_list)
+   {
+       RestrictInfo *refrinfo = (RestrictInfo *) lfirst(refitem);
+
+       if (equal(rinfo, refrinfo))
+           return refrinfo;
+   }
 
    /* check for redundant merge clauses */
    if (rinfo->mergejoinoperator != InvalidOid)
    {
-       bool        redundant = false;
-       List       *refitem;
-
        /* do the cheap test first: is it a "var = const" clause? */
        if (bms_is_empty(rinfo->left_relids) ||
            bms_is_empty(rinfo->right_relids))
-           return false;       /* var = const, so not redundant */
+           return NULL;        /* var = const, so not redundant */
 
        cache_mergeclause_pathkeys(root, rinfo);
 
@@ -426,21 +456,22 @@ join_clause_is_redundant(Query *root,
        {
            RestrictInfo *refrinfo = (RestrictInfo *) lfirst(refitem);
 
-           if (refrinfo->mergejoinoperator != InvalidOid &&
-               rinfo->left_pathkey == refrinfo->left_pathkey &&
-               rinfo->right_pathkey == refrinfo->right_pathkey &&
-               (rinfo->is_pushed_down == refrinfo->is_pushed_down ||
-                !IS_OUTER_JOIN(jointype)))
+           if (refrinfo->mergejoinoperator != InvalidOid)
            {
-               redundant = true;
-               break;
+               cache_mergeclause_pathkeys(root, refrinfo);
+
+               if (rinfo->left_pathkey == refrinfo->left_pathkey &&
+                   rinfo->right_pathkey == refrinfo->right_pathkey &&
+                   (rinfo->is_pushed_down == refrinfo->is_pushed_down ||
+                    !IS_OUTER_JOIN(jointype)))
+               {
+                   /* Yup, it's redundant */
+                   return refrinfo;
+               }
            }
        }
-
-       if (redundant)
-           return true;        /* var = var, so redundant */
    }
 
    /* otherwise, not redundant */
-   return false;
+   return NULL;
 }