BUG#39185 - Cardinality for merge tables calculated incorrectly.

Every subsequent query to a merge table with indexes was lowering
down cardinality.

The problem was that key statistics was not cleared when merge
children were detached. Causing next attach children perform
incremental key statistics calculation.

Fixed by clearing key statistics when attaching first child.

mysql-test/r/merge.result:
  A test case for BUG#39185.
mysql-test/t/merge.test:
  A test case for BUG#39185.
storage/myisammrg/myrg_open.c:
  Clear key statistics when we're attaching first child, even
  if it's buffer was allocated before. This is needed because
  detach_children() doesn't clear statistics, causing incremental
  statistics calculation.

mysql-test/r/merge.result

@@ -2041,4 +2041,23 @@ EXPLAIN SELECT COUNT(*) FROM t4;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
 DROP TABLE t1, t2, t3, t4;
+CREATE TABLE t1(a INT, KEY(a));
+INSERT INTO t1 VALUES(0),(1),(2),(3),(4);
+ANALYZE TABLE t1;
+Table	Op	Msg_type	Msg_text
+test.t1	analyze	status	OK
+CREATE TABLE m1(a INT, KEY(a)) ENGINE=MERGE UNION=(t1);
+SELECT CARDINALITY FROM INFORMATION_SCHEMA.STATISTICS WHERE TABLE_SCHEMA='test' AND TABLE_NAME='m1';
+CARDINALITY
+5
+SELECT CARDINALITY FROM INFORMATION_SCHEMA.STATISTICS WHERE TABLE_SCHEMA='test' AND TABLE_NAME='m1';
+CARDINALITY
+5
+SELECT CARDINALITY FROM INFORMATION_SCHEMA.STATISTICS WHERE TABLE_SCHEMA='test' AND TABLE_NAME='m1';
+CARDINALITY
+5
+SELECT CARDINALITY FROM INFORMATION_SCHEMA.STATISTICS WHERE TABLE_SCHEMA='test' AND TABLE_NAME='m1';
+CARDINALITY
+5
+DROP TABLE t1, m1;
 End of 5.1 tests

mysql-test/t/merge.test

@@ -1435,4 +1435,17 @@ EXPLAIN SELECT COUNT(*) FROM t1;
 EXPLAIN SELECT COUNT(*) FROM t4;
 DROP TABLE t1, t2, t3, t4;
 
+#
+# BUG#39185 - Cardinality for merge tables calculated incorrectly.
+#
+CREATE TABLE t1(a INT, KEY(a));
+INSERT INTO t1 VALUES(0),(1),(2),(3),(4);
+ANALYZE TABLE t1;
+CREATE TABLE m1(a INT, KEY(a)) ENGINE=MERGE UNION=(t1);
+SELECT CARDINALITY FROM INFORMATION_SCHEMA.STATISTICS WHERE TABLE_SCHEMA='test' AND TABLE_NAME='m1'; 
+SELECT CARDINALITY FROM INFORMATION_SCHEMA.STATISTICS WHERE TABLE_SCHEMA='test' AND TABLE_NAME='m1'; 
+SELECT CARDINALITY FROM INFORMATION_SCHEMA.STATISTICS WHERE TABLE_SCHEMA='test' AND TABLE_NAME='m1'; 
+SELECT CARDINALITY FROM INFORMATION_SCHEMA.STATISTICS WHERE TABLE_SCHEMA='test' AND TABLE_NAME='m1'; 
+DROP TABLE t1, m1;
+
 --echo End of 5.1 tests

storage/myisammrg/myrg_open.c

@@ -428,10 +428,11 @@ int myrg_attach_children(MYRG_INFO *m_info, int handle_locking,
       if (!m_info->rec_per_key_part)
       {
         if(!(m_info->rec_per_key_part= (ulong*)
-             my_malloc(key_parts * sizeof(long), MYF(MY_WME|MY_ZEROFILL))))
+             my_malloc(key_parts * sizeof(long), MYF(MY_WME))))
           goto err; /* purecov: inspected */
         errpos= 1;
       }
+      bzero((char*) m_info->rec_per_key_part, key_parts * sizeof(long));
     }
 
     /* Add MyISAM table info. */