Consider index-only scans even when there is no matching qual or ORDER BY.

By popular demand.

diff --git a/src/backend/optimizer/path/indxpath.c b/src/backend/optimizer/path/indxpath.c
index 9ab146a1f74b72a24ee45611b68b327c47b49013..ece326d88507bb202ce9d049d1023ef531f7a829 100644 (file)
--- a/src/backend/optimizer/path/indxpath.c
+++ b/src/backend/optimizer/path/indxpath.c
@@ -317,8 +317,7 @@ find_usable_indexes(PlannerInfo *root, RelOptInfo *rel,
                bool            useful_predicate;
                bool            found_clause;
                bool            index_is_ordered;
-               bool            index_only_scan = false;
-               bool            checked_index_only = false;
+               bool            index_only_scan;
 
                /*
                 * Check that index supports the desired scan type(s)
@@ -436,17 +435,20 @@ find_usable_indexes(PlannerInfo *root, RelOptInfo *rel,
                }
 
                /*
-                * 3. Generate an indexscan path if there are relevant restriction
+                * 3. Check if an index-only scan is possible.
+                */
+               index_only_scan = check_index_only(rel, index);
+
+               /*
+                * 4. Generate an indexscan path if there are relevant restriction
                 * clauses in the current clauses, OR the index ordering is
                 * potentially useful for later merging or final output ordering, OR
-                * the index has a predicate that was proven by the current clauses.
+                * the index has a predicate that was proven by the current clauses,
+                * OR an index-only scan is possible.
                 */
-               if (found_clause || useful_pathkeys != NIL || useful_predicate)
+               if (found_clause || useful_pathkeys != NIL || useful_predicate ||
+                       index_only_scan)
                {
-                       /* First, detect whether index-only scan is possible */
-                       index_only_scan = check_index_only(rel, index);
-                       checked_index_only = true;
-
                        ipath = create_index_path(root, index,
                                                                          restrictclauses,
                                                                          orderbyclauses,
@@ -460,7 +462,7 @@ find_usable_indexes(PlannerInfo *root, RelOptInfo *rel,
                }
 
                /*
-                * 4. If the index is ordered, a backwards scan might be interesting.
+                * 5. If the index is ordered, a backwards scan might be interesting.
                 * Again, this is only interesting at top level.
                 */
                if (index_is_ordered && possibly_useful_pathkeys &&
@@ -472,9 +474,6 @@ find_usable_indexes(PlannerInfo *root, RelOptInfo *rel,
                                                                                                                index_pathkeys);
                        if (useful_pathkeys != NIL)
                        {
-                               if (!checked_index_only)
-                                       index_only_scan = check_index_only(rel, index);
-
                                ipath = create_index_path(root, index,
                                                                                  restrictclauses,
                                                                                  NIL,