Teach convert_subquery_pathkeys() to handle the case where the

subquery's pathkey is a RelabelType applied to something that appears
in the subquery's output; for example where the subquery returns a
varchar Var and the sort order is shown as that Var coerced to text.
This comes up because varchar doesn't have its own sort operator.
Per example from Peter Hardman.

diff --git a/src/backend/optimizer/path/pathkeys.c b/src/backend/optimizer/path/pathkeys.c
index a79ffb55daa89b3aae7ded53d18334464462ce57..84981d0b02f60906fb9b419a5d1037519fe0b5ec 100644 (file)
--- a/src/backend/optimizer/path/pathkeys.c
+++ b/src/backend/optimizer/path/pathkeys.c
@@ -1059,39 +1059,73 @@ convert_subquery_pathkeys(PlannerInfo *root, RelOptInfo *rel,
                {
                        PathKeyItem *sub_item = (PathKeyItem *) lfirst(j);
                        Node       *sub_key = sub_item->key;
+                       Expr       *rtarg;
                        ListCell   *k;
 
+                       /*
+                        * We handle two cases: the sub_pathkey key can be either an exact
+                        * match for a targetlist entry, or a RelabelType of a targetlist
+                        * entry.  (The latter case is worth extra code because it arises
+                        * frequently in connection with varchar fields.)
+                        */
+                       if (IsA(sub_key, RelabelType))
+                               rtarg = ((RelabelType *) sub_key)->arg;
+                       else
+                               rtarg = NULL;
+
                        foreach(k, sub_tlist)
                        {
                                TargetEntry *tle = (TargetEntry *) lfirst(k);
+                               Node       *outer_expr;
+                               PathKeyItem *outer_item;
+                               int                     score;
 
-                               if (!tle->resjunk &&
-                                       equal(tle->expr, sub_key))
+                               /* resjunk items aren't visible to outer query */
+                               if (tle->resjunk)
+                                       continue;
+
+                               if (equal(tle->expr, sub_key))
                                {
-                                       /* Found a representation for this sub_key */
-                                       Var                *outer_var;
-                                       PathKeyItem *outer_item;
-                                       int                     score;
-
-                                       outer_var = makeVar(rel->relid,
-                                                                               tle->resno,
-                                                                               exprType((Node *) tle->expr),
-                                                                               exprTypmod((Node *) tle->expr),
-                                                                               0);
-                                       outer_item = makePathKeyItem((Node *) outer_var,
-                                                                                                sub_item->sortop,
-                                                                                                true);
-                                       /* score = # of mergejoin peers */
-                                       score = count_canonical_peers(root, outer_item);
-                                       /* +1 if it matches the proper query_pathkeys item */
-                                       if (retvallen < outer_query_keys &&
-                                               list_member(list_nth(root->query_pathkeys, retvallen), outer_item))
-                                               score++;
-                                       if (score > best_score)
-                                       {
-                                               best_item = outer_item;
-                                               best_score = score;
-                                       }
+                                       /* Exact match */
+                                       outer_expr = (Node *)
+                                               makeVar(rel->relid,
+                                                               tle->resno,
+                                                               exprType((Node *) tle->expr),
+                                                               exprTypmod((Node *) tle->expr),
+                                                               0);
+                               }
+                               else if (rtarg && equal(tle->expr, rtarg))
+                               {
+                                       /* Match after discarding RelabelType */
+                                       outer_expr = (Node *)
+                                               makeVar(rel->relid,
+                                                               tle->resno,
+                                                               exprType((Node *) tle->expr),
+                                                               exprTypmod((Node *) tle->expr),
+                                                               0);
+                                       outer_expr = (Node *)
+                                               makeRelabelType((Expr *) outer_expr,
+                                                                               ((RelabelType *) sub_key)->resulttype,
+                                                                               ((RelabelType *) sub_key)->resulttypmod,
+                                                                               ((RelabelType *) sub_key)->relabelformat);
+                               }
+                               else
+                                       continue;
+
+                               /* Found a representation for this sub_key */
+                               outer_item = makePathKeyItem(outer_expr,
+                                                                                        sub_item->sortop,
+                                                                                        true);
+                               /* score = # of mergejoin peers */
+                               score = count_canonical_peers(root, outer_item);
+                               /* +1 if it matches the proper query_pathkeys item */
+                               if (retvallen < outer_query_keys &&
+                                       list_member(list_nth(root->query_pathkeys, retvallen), outer_item))
+                                       score++;
+                               if (score > best_score)
+                               {
+                                       best_item = outer_item;
+                                       best_score = score;
                                }
                        }
                }