WL#3527: Extend IGNORE INDEX so places where index is ignored

         can be specified
Currently MySQL allows one to specify what indexes to ignore during
join optimization. The scope of the current USE/FORCE/IGNORE INDEX 
statement is only the FROM clause, while all other clauses are not 
affected.

However, in certain cases, the optimizer
may incorrectly choose an index for sorting and/or grouping, and
produce an inefficient query plan.

This task provides the means to specify what indexes are
ignored/used for what operation in a more fine-grained manner, thus
making it possible to manually force a better plan. We do this
by extending the current IGNORE/USE/FORCE INDEX syntax to:

IGNORE/USE/FORCE INDEX [FOR {JOIN | ORDER | GROUP BY}]

so that:
- if no FOR is specified, the index hint will apply everywhere.
- if MySQL is started with the compatibility option --old_mode then
  an index hint without a FOR clause works as in 5.0 (i.e, the 
  index will only be ignored for JOINs, but can still be used to
  compute ORDER BY).

See the WL#3527 for further details.

1 files changed, 27 insertions, 2 deletions

diff --git a/sql/table.h b/sql/table.h
index 13666c82f4b..0dfca83d454 100644
--- a/sql/table.h
+++ b/sql/table.h
@@ -295,6 +295,12 @@ typedef struct st_table_share
 
 
 /* Information for one open table */
+enum index_hint_type
+{
+  INDEX_HINT_IGNORE,
+  INDEX_HINT_USE,
+  INDEX_HINT_FORCE
+};
 
 struct st_table {
   st_table() {}                               /* Remove gcc warning */
@@ -314,7 +320,18 @@ struct st_table {
   byte *write_row_record;		/* Used as optimisation in
 					   THD::write_row */
   byte *insert_values;                  /* used by INSERT ... UPDATE */
-  key_map quick_keys, used_keys, keys_in_use_for_query, merge_keys;
+  /* 
+    Map of keys that can be used to retrieve all data from this table 
+    needed by the query without reading the row.
+  */
+  key_map covering_keys;
+  key_map quick_keys, merge_keys;
+  /* Map of keys that can be used to access the table */
+  key_map keys_in_use_for_query;
+  /* Map of keys that can be used to calculate GROUP BY without sorting */
+  key_map keys_in_use_for_group_by;
+  /* Map of keys that can be used to calculate ORDER BY without sorting */
+  key_map keys_in_use_for_order_by;
   KEY  *key_info;			/* data of keys in database */
 
   Field *next_number_field;		/* Set if next_number is activated */
@@ -636,6 +653,7 @@ public:
          (TABLE_LIST::join_using_fields != NULL)
 */
 
+class index_hint;
 typedef struct st_table_list
 {
   st_table_list() {}                          /* Remove gcc warning */
@@ -692,7 +710,7 @@ typedef struct st_table_list
   */
   struct st_table_list *next_name_resolution_table;
   /* Index names in a "... JOIN ... USE/IGNORE INDEX ..." clause. */
-  List<String> *use_index, *ignore_index;
+  List<index_hint> *index_hints;
   TABLE        *table;                          /* opened table */
   uint          table_id; /* table id (from binlog) for opened table */
   /*
@@ -866,6 +884,13 @@ typedef struct st_table_list
   void reinit_before_use(THD *thd);
   Item_subselect *containing_subselect();
 
+  /* 
+    Compiles the tagged hints list and fills up 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.
+  */
+  bool process_index_hints(TABLE *table);
+
 private:
   bool prep_check_option(THD *thd, uint8 check_opt_type);
   bool prep_where(THD *thd, Item **conds, bool no_where_clause);