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 6d68858668429c43e0e1943e5e1cdcdb1b43a4ae..f91e25cdb4fc986056efa91f0e86c314d90e8f05 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;