BUG#20604: FORCE INDEX uses keys disabled by ALTER TABLE

 
The function that checks whether we can use keys for aggregates,
find_key_for_maxmin(), assumes that keys disabled by ALTER TABLE
... DISABLE KEYS are not in the set table->keys_in_use_for_query.
I.E., if a key is in this set, the optimizer assumes it is free to 
use it.
  
The bug is that keys disabled with ALTER TABLE ... DISABLE KEYS still
appear in table->keys_in_use_for_query When the TABLE object has been 
initialized with setup_tables(). Before setup_tables is called, however, 
keys that are disabled in the aforementioned way are not included in
TABLE::keys_in_use_for_query. 
  
The provided patch changes the code that updates keys_is_use_for_query so 
that it assumes that keys_is_use_for_query already takes into account all 
disabled keys, and generally all keys that should be used by the query.

mysql-test/r/key.result

@@ -482,3 +482,10 @@ alter table t1 drop index i3, drop index i2, drop index i1;
 alter table t1 add index i3 (c3), add index i2 (c2), add unique index i1 (c1);
 ERROR 23000: Duplicate entry '1' for key 'i1'
 drop table t1;
+CREATE TABLE t1( a TINYINT, KEY(a) ) ENGINE=MyISAM;
+INSERT INTO t1 VALUES( 1 );
+ALTER TABLE t1 DISABLE KEYS;
+EXPLAIN SELECT MAX(a) FROM t1 FORCE INDEX(a);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	1	
+drop table t1;

mysql-test/t/key.test

@@ -442,3 +442,14 @@ alter table t1 drop index i3, drop index i2, drop index i1;
 alter table t1 add index i3 (c3), add index i2 (c2), add unique index i1 (c1);
 drop table t1;
 
+
+#
+# Bug #20604: Test for disabled keys with aggregate functions and FORCE INDEX.
+#
+
+CREATE TABLE t1( a TINYINT, KEY(a) ) ENGINE=MyISAM;
+INSERT INTO t1 VALUES( 1 );
+ALTER TABLE t1 DISABLE KEYS;
+EXPLAIN SELECT MAX(a) FROM t1 FORCE INDEX(a);
+
+drop table t1;

sql/sql_base.cc

@@ -5462,7 +5462,12 @@ bool setup_tables(THD *thd, Name_resolution_context *context,
       get_key_map_from_key_list(&map, table, table_list->use_index);
       if (map.is_set_all())
 	DBUG_RETURN(1);
-      table->keys_in_use_for_query=map;
+      /* 
+	 Don't introduce keys in keys_in_use_for_query that weren't there 
+	 before. FORCE/USE INDEX should not add keys, it should only remove
+	 all keys except the key(s) specified in the hint.
+      */
+      table->keys_in_use_for_query.intersect(map);
     }
     if (table_list->ignore_index)
     {

sql/sql_select.cc

@@ -12262,13 +12262,14 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
   DBUG_ENTER("test_if_skip_sort_order");
   LINT_INIT(ref_key_parts);
 
+  /* Check which keys can be used to resolve ORDER BY. */
+  usable_keys= table->keys_in_use_for_query;
+
   /*
-    Check which keys can be used to resolve ORDER BY.
-    We must not try to use disabled keys.
+    Keys disabled by ALTER TABLE ... DISABLE KEYS should have already
+    been taken into account.
   */
-  usable_keys= table->s->keys_in_use;
-  /* we must not consider keys that are disabled by IGNORE INDEX */
-  usable_keys.intersect(table->keys_in_use_for_query);
+  DBUG_ASSERT(usable_keys.is_subset(table->s->keys_in_use));
 
   for (ORDER *tmp_order=order; tmp_order ; tmp_order=tmp_order->next)
   {

sql/table.h

@@ -158,7 +158,12 @@ typedef struct st_table_share
   LEX_STRING path;                	/* Path to .frm file (from datadir) */
   LEX_STRING normalized_path;		/* unpack_filename(path) */
   LEX_STRING connect_string;
-  key_map keys_in_use;                  /* Keys in use for table */
+
+  /* 
+     Set of keys in use, implemented as a Bitmap.
+     Excludes keys disabled by ALTER TABLE ... DISABLE KEYS.
+  */
+  key_map keys_in_use;
   key_map keys_for_keyread;
   ha_rows min_rows, max_rows;		/* create information */
   ulong   avg_row_length;		/* create information */
@@ -313,7 +318,21 @@ 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;
+  key_map quick_keys, used_keys;
+
+  /*
+    A set of keys that can be used in the query that references this
+    table 
+
+    All indexes disabled on the table's TABLE_SHARE (see TABLE::s) will be 
+    subtracted from this set upon instantiation. Thus for any TABLE t it holds
+    that t.keys_in_use_for_query is a subset of t.s.keys_in_use. Generally we 
+    must not introduce any new keys here (see setup_tables).
+
+    The set is implemented as a bitmap.
+  */
+  key_map keys_in_use_for_query;
+  key_map merge_keys;
   KEY  *key_info;			/* data of keys in database */
 
   Field *next_number_field;		/* Set if next_number is activated */