BUG#38842 - Fix for 25951 seems incorrect

With fix for bug 25951 index hints are ignored for fulltext
searches, as handling of fulltext indexes is different from
handling regular indexes. Meaning it is not possible to
implement true index hints support for fulltext indexes within
the scope of current fulltext architecture.

The problem is that prior to fix for bug 25951, some useful
index hints still could be given for boolean mode searches.

This patch implements special index hints support for fulltext
indexes with the following characteristics:
- all index hints are still ignored for NLQ mode searches -
  it cannot work without an index;
- for 5.1 and up index hints FOR ORDER BY and FOR GROUP BY are
  still ignored for fulltext indexes;
- boolean mode searches honor USE/FORCE/IGNORE INDEX hints;
- as opposed to index hints for regular indexes, index hints
  for fulltext BOOLEAN mode searches affect the usage of the
  index for the whole query.

mysql-test/r/fulltext.result

@@ -469,3 +469,31 @@ SELECT * FROM t1 WHERE MATCH(a) AGAINST ('"aaaa"' IN BOOLEAN MODE);
 a
  aaaaa aaaa
 DROP TABLE t1;
+CREATE TABLE t1 (a VARCHAR(255), b INT, FULLTEXT(a), KEY(b));
+INSERT INTO t1 VALUES('test', 1),('test', 1),('test', 1),('test', 1),
+('test', 1),('test', 2),('test', 3),('test', 4);
+EXPLAIN SELECT * FROM t1
+WHERE MATCH(a) AGAINST('test' IN BOOLEAN MODE) AND b=1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	fulltext	b,a	a	0		1	Using where
+EXPLAIN SELECT * FROM t1 USE INDEX(a)
+WHERE MATCH(a) AGAINST('test' IN BOOLEAN MODE) AND b=1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	fulltext	a	a	0		1	Using where
+EXPLAIN SELECT * FROM t1 FORCE INDEX(a)
+WHERE MATCH(a) AGAINST('test' IN BOOLEAN MODE) AND b=1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	fulltext	a	a	0		1	Using where
+EXPLAIN SELECT * FROM t1 IGNORE INDEX(a)
+WHERE MATCH(a) AGAINST('test' IN BOOLEAN MODE) AND b=1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ref	b	b	5	const	4	Using where
+EXPLAIN SELECT * FROM t1 USE INDEX(b)
+WHERE MATCH(a) AGAINST('test' IN BOOLEAN MODE) AND b=1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ref	b	b	5	const	4	Using where
+EXPLAIN SELECT * FROM t1 FORCE INDEX(b)
+WHERE MATCH(a) AGAINST('test' IN BOOLEAN MODE) AND b=1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ref	b	b	5	const	4	Using where
+DROP TABLE t1;

mysql-test/t/fulltext.test

@@ -396,3 +396,30 @@ SELECT * FROM t1 WHERE MATCH(a) AGAINST ('"aaaa"' IN BOOLEAN MODE);
 DROP TABLE t1;
 
 # End of 4.1 tests
+
+#
+# BUG#38842 - Fix for 25951 seems incorrect
+#
+CREATE TABLE t1 (a VARCHAR(255), b INT, FULLTEXT(a), KEY(b));
+INSERT INTO t1 VALUES('test', 1),('test', 1),('test', 1),('test', 1),
+                     ('test', 1),('test', 2),('test', 3),('test', 4);
+
+EXPLAIN SELECT * FROM t1
+WHERE MATCH(a) AGAINST('test' IN BOOLEAN MODE) AND b=1;
+
+EXPLAIN SELECT * FROM t1 USE INDEX(a)
+WHERE MATCH(a) AGAINST('test' IN BOOLEAN MODE) AND b=1;
+
+EXPLAIN SELECT * FROM t1 FORCE INDEX(a)
+WHERE MATCH(a) AGAINST('test' IN BOOLEAN MODE) AND b=1;
+
+EXPLAIN SELECT * FROM t1 IGNORE INDEX(a)
+WHERE MATCH(a) AGAINST('test' IN BOOLEAN MODE) AND b=1;
+
+EXPLAIN SELECT * FROM t1 USE INDEX(b)
+WHERE MATCH(a) AGAINST('test' IN BOOLEAN MODE) AND b=1;
+
+EXPLAIN SELECT * FROM t1 FORCE INDEX(b)
+WHERE MATCH(a) AGAINST('test' IN BOOLEAN MODE) AND b=1;
+
+DROP TABLE t1;

sql/item_func.cc

@@ -5022,7 +5022,9 @@ bool Item_func_match::fix_index()
   for (keynr=0 ; keynr < table->s->keys ; keynr++)
   {
     if ((table->key_info[keynr].flags & HA_FULLTEXT) &&
-        (table->s->keys_in_use.is_set(keynr)))
+        (flags & FT_BOOL ? table->keys_in_use_for_query.is_set(keynr) :
+                           table->s->keys_in_use.is_set(keynr)))
+
     {
       ft_to_key[fts]=keynr;
       ft_cnt[fts]=0;