Bug #43029: FORCE INDEX FOR ORDER BY is ignored when join

buffering is used

FORCE INDEX FOR ORDER BY now prevents the optimizer from 
using join buffering. As a result the optimizer can use
indexed access on the first table and doesn't need to 
sort the complete resultset at the end of the statement.

1 files changed, 20 insertions, 5 deletions

diff --git a/sql/table.cc b/sql/table.cc
index 04f2a3fbcf8..d2538eb4d59 100644
--- a/sql/table.cc
+++ b/sql/table.cc
@@ -4637,7 +4637,8 @@ Item_subselect *TABLE_LIST::containing_subselect()
     (TABLE_LIST::index_hints). Using the information in this tagged list
     this function sets the members st_table::keys_in_use_for_query, 
     st_table::keys_in_use_for_group_by, st_table::keys_in_use_for_order_by,
-    st_table::force_index and st_table::covering_keys.
+    st_table::force_index, st_table::force_index_order, 
+    st_table::force_index_group and st_table::covering_keys.
 
     Current implementation of the runtime does not allow mixing FORCE INDEX
     and USE INDEX, so this is checked here. Then the FORCE INDEX list 
@@ -4765,14 +4766,28 @@ bool TABLE_LIST::process_index_hints(TABLE *tbl)
     }
 
     /* process FORCE INDEX as USE INDEX with a flag */
+    if (!index_order[INDEX_HINT_FORCE].is_clear_all())
+    {
+      tbl->force_index_order= TRUE;
+      index_order[INDEX_HINT_USE].merge(index_order[INDEX_HINT_FORCE]);
+    }
+
+    if (!index_group[INDEX_HINT_FORCE].is_clear_all())
+    {
+      tbl->force_index_group= TRUE;
+      index_group[INDEX_HINT_USE].merge(index_group[INDEX_HINT_FORCE]);
+    }
+
+    /*
+      TODO: get rid of tbl->force_index (on if any FORCE INDEX is specified) and
+      create tbl->force_index_join instead.
+      Then use the correct force_index_XX instead of the global one.
+    */
     if (!index_join[INDEX_HINT_FORCE].is_clear_all() ||
-        !index_order[INDEX_HINT_FORCE].is_clear_all() ||
-        !index_group[INDEX_HINT_FORCE].is_clear_all())
+        tbl->force_index_group || tbl->force_index_order)
     {
       tbl->force_index= TRUE;
       index_join[INDEX_HINT_USE].merge(index_join[INDEX_HINT_FORCE]);
-      index_order[INDEX_HINT_USE].merge(index_order[INDEX_HINT_FORCE]);
-      index_group[INDEX_HINT_USE].merge(index_group[INDEX_HINT_FORCE]);
     }
 
     /* apply USE INDEX */