In find_mergeclauses_for_pathkeys, it's okay to return multiple merge

clauses per path key.  Indeed, we *must* do so or we will be unable to
form a valid plan for FULL JOIN with overlapping join conditions, eg
select * from a full join b on
a.v1 = b.v1 and a.v2 = b.v2 and a.v1 = b.v2.

diff --git a/src/backend/optimizer/path/pathkeys.c b/src/backend/optimizer/path/pathkeys.c
index 6d68858..f91e25c 100644 (file)
--- a/src/backend/optimizer/path/pathkeys.c
+++ b/src/backend/optimizer/path/pathkeys.c
@@ -11,7 +11,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *       $Header: /cvsroot/pgsql/src/backend/optimizer/path/pathkeys.c,v 1.35 2001/10/28 06:25:44 momjian Exp $
+ *       $Header: /cvsroot/pgsql/src/backend/optimizer/path/pathkeys.c,v 1.36 2001/11/11 20:33:53 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -752,25 +752,43 @@ find_mergeclauses_for_pathkeys(Query *root,
        List       *mergeclauses = NIL;
        List       *i;
 
+       /* make sure we have pathkeys cached in the clauses */
+       foreach(i, restrictinfos)
+       {
+               RestrictInfo *restrictinfo = lfirst(i);
+
+               cache_mergeclause_pathkeys(root, restrictinfo);
+       }
+
        foreach(i, pathkeys)
        {
                List       *pathkey = lfirst(i);
-               RestrictInfo *matched_restrictinfo = NULL;
+               List       *matched_restrictinfos = NIL;
                List       *j;
 
                /*
                 * We can match a pathkey against either left or right side of any
-                * mergejoin clause we haven't used yet.  For the moment we use a
-                * dumb "greedy" algorithm with no backtracking.  Is it worth
-                * being any smarter to make a longer list of usable mergeclauses?
-                * Probably not.
+                * mergejoin clause.  (We examine both sides since we aren't told if
+                * the given pathkeys are for inner or outer input path; no confusion
+                * is possible.)  Furthermore, if there are multiple matching
+                * clauses, take them all.  In plain inner-join scenarios we expect
+                * only one match, because redundant-mergeclause elimination will
+                * have removed any redundant mergeclauses from the input list.
+                * However, in outer-join scenarios there might be multiple matches.
+                * An example is
+                *
+                *              select * from a full join b on
+                *              a.v1 = b.v1 and a.v2 = b.v2 and a.v1 = b.v2;
+                *
+                * Given the pathkeys ((a.v1), (a.v2)) it is okay to return all
+                * three clauses (in the order a.v1=b.v1, a.v1=b.v2, a.v2=b.v2)
+                * and indeed we *must* do so or we will be unable to form a
+                * valid plan.
                 */
                foreach(j, restrictinfos)
                {
                        RestrictInfo *restrictinfo = lfirst(j);
 
-                       cache_mergeclause_pathkeys(root, restrictinfo);
-
                        /*
                         * We can compare canonical pathkey sublists by simple pointer
                         * equality; see compare_pathkeys.
@@ -779,8 +797,8 @@ find_mergeclauses_for_pathkeys(Query *root,
                                 pathkey == restrictinfo->right_pathkey) &&
                                !ptrMember(restrictinfo, mergeclauses))
                        {
-                               matched_restrictinfo = restrictinfo;
-                               break;
+                               matched_restrictinfos = lappend(matched_restrictinfos,
+                                                                                               restrictinfo);
                        }
                }
 
@@ -789,14 +807,14 @@ find_mergeclauses_for_pathkeys(Query *root,
                 * sort-key positions in the pathkeys are useless.      (But we can
                 * still mergejoin if we found at least one mergeclause.)
                 */
-               if (!matched_restrictinfo)
+               if (matched_restrictinfos == NIL)
                        break;
 
                /*
-                * If we did find a usable mergeclause for this sort-key position,
-                * add it to result list.
+                * If we did find usable mergeclause(s) for this sort-key position,
+                * add them to result list.
                 */
-               mergeclauses = lappend(mergeclauses, matched_restrictinfo);
+               mergeclauses = nconc(mergeclauses, matched_restrictinfos);
        }
 
        return mergeclauses;