Bug #21019: First result of SELECT COUNT(*) different than consecutive runs

 When optimizing conditions like 'a = <some_val> OR a IS NULL' so that they're
 united into a single condition on the key and checked together the server must 
 check which value is the NULL value in a correct way : not only using ->is_null 
 but also check if the expression doesn't depend on any tables referenced in the 
 current statement. 
 This additional check must be performed because that optimization takes place 
 before the actual execution of the statement, so if the field was initialized 
 to NULL from a previous statement the optimization would be applied incorrectly.

mysql-test/r/select.result

@@ -2744,3 +2744,29 @@ SELECT i='1e+01',i=1e+01, i in (1e+01), i in ('1e+01') FROM t1;
 i='1e+01'	i=1e+01	i in (1e+01)	i in ('1e+01')
 0	1	1	1
 DROP TABLE t1;
+CREATE TABLE t1 (a int, b int);
+INSERT INTO t1 VALUES (1,1), (2,1), (4,10);
+CREATE TABLE t2 (a int PRIMARY KEY, b int, KEY b (b));
+INSERT INTO t2 VALUES (1,NULL), (2,10);
+ALTER TABLE t1 ENABLE KEYS;
+EXPLAIN SELECT STRAIGHT_JOIN SQL_NO_CACHE COUNT(*) FROM t2, t1 WHERE t1.b = t2.b OR t2.b IS NULL;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t2	index	b	b	5	NULL	2	Using index
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	3	Using where
+SELECT STRAIGHT_JOIN SQL_NO_CACHE * FROM t2, t1 WHERE t1.b = t2.b OR t2.b IS NULL;
+a	b	a	b
+1	NULL	1	1
+1	NULL	2	1
+1	NULL	4	10
+2	10	4	10
+EXPLAIN SELECT STRAIGHT_JOIN SQL_NO_CACHE COUNT(*) FROM t2, t1 WHERE t1.b = t2.b OR t2.b IS NULL;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t2	index	b	b	5	NULL	2	Using index
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	3	Using where
+SELECT STRAIGHT_JOIN SQL_NO_CACHE * FROM t2, t1 WHERE t1.b = t2.b OR t2.b IS NULL;
+a	b	a	b
+1	NULL	1	1
+1	NULL	2	1
+1	NULL	4	10
+2	10	4	10
+DROP TABLE IF EXISTS t1,t2;

mysql-test/t/select.test

@@ -2297,4 +2297,19 @@ INSERT INTO t1 VALUES (10);
 SELECT i='1e+01',i=1e+01, i in (1e+01), i in ('1e+01') FROM t1;
 DROP TABLE t1;
 
+#
+# Bug #21019: First result of SELECT COUNT(*) different than consecutive runs
+#
+CREATE TABLE t1 (a int, b int);
+INSERT INTO t1 VALUES (1,1), (2,1), (4,10);
+
+CREATE TABLE t2 (a int PRIMARY KEY, b int, KEY b (b));
+INSERT INTO t2 VALUES (1,NULL), (2,10);
+ALTER TABLE t1 ENABLE KEYS;
+
+EXPLAIN SELECT STRAIGHT_JOIN SQL_NO_CACHE COUNT(*) FROM t2, t1 WHERE t1.b = t2.b OR t2.b IS NULL;
+SELECT STRAIGHT_JOIN SQL_NO_CACHE * FROM t2, t1 WHERE t1.b = t2.b OR t2.b IS NULL;
+EXPLAIN SELECT STRAIGHT_JOIN SQL_NO_CACHE COUNT(*) FROM t2, t1 WHERE t1.b = t2.b OR t2.b IS NULL;
+SELECT STRAIGHT_JOIN SQL_NO_CACHE * FROM t2, t1 WHERE t1.b = t2.b OR t2.b IS NULL;
+DROP TABLE IF EXISTS t1,t2;
 # End of 4.1 tests

sql/sql_select.cc

@@ -2143,8 +2143,11 @@ merge_key_fields(KEY_FIELD *start,KEY_FIELD *new_fields,KEY_FIELD *end,
 	  /* field = expression OR field IS NULL */
 	  old->level= and_level;
 	  old->optimize= KEY_OPTIMIZE_REF_OR_NULL;
-	  /* Remember the NOT NULL value */
-	  if (old->val->is_null())
+	  /*
+            Remember the NOT NULL value unless the value does not depend
+            on other tables.
+          */
+	  if (!old->val->used_tables() && old->val->is_null())
 	    old->val= new_fields->val;
           /* The referred expression can be NULL: */ 
           old->null_rejecting= 0;