Bug #28476: force index on a disabled myisam index gives error 124

When processing the USE/FORCE index hints
the optimizer was not checking if the indexes 
specified are enabled (see ALTER TABLE).
Fixed by:
 Backporting the fix for bug 20604 to 5.0


mysql-test/r/key.result:
  Test for BUG 20604.
  The important part of the test is the explain output that 
  tests what indexes are used.
mysql-test/r/myisam.result:
  Bug #28476: test cases
mysql-test/t/key.test:
  Bug 20604: 
  The minimal test case that reveals the bug. The optimizer for 
  aggregates relies on keys disabled with ALTER TABLE ... DISABLE KEYS
  not being in the set TABLE::keys_in_use_for_query.
  When the execution engine tries to use a disabled index, MyISAM
  returns an error.
mysql-test/t/myisam.test:
  Bug #28476: test cases
sql/sql_base.cc:
  Bug #28476: 
   - Ignore disabled indexes in USE/FORCE index
sql/sql_select.cc:
  Bug 20604 : The intersection operation between table->s->keys_in_use 
  and table->keys_in_use_for_query is no longer necessary.
  We can trust that the latter is a subset of the former.
sql/table.h:
  Bug 20604:
  Added comments to TABLE_SHARE::keys_in_use and
  TABLE::keys_in_use_for_query.

1 files changed, 21 insertions, 2 deletions

diff --git a/sql/table.h b/sql/table.h
index 61232a39f0e..ebcafbd4dc7 100644
--- a/sql/table.h
+++ b/sql/table.h
@@ -137,7 +137,12 @@ typedef struct st_table_share
   const char *table_name;               /* Table name (for open) */
   const char *path;                     /* Path to .frm file (from datadir) */
   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;
   ulong   avg_row_length;		/* create information */
   ulong   raid_chunksize;
@@ -206,7 +211,21 @@ struct st_table {
 
   byte *record[2];			/* Pointer to records */
   byte *insert_values;                  /* used by INSERT ... UPDATE */
-  key_map quick_keys, used_keys, keys_in_use_for_query;
+  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 */