Bug#14980 - COUNT(*) incorrect on MyISAM table with certain INDEX

For "count(*) while index_column = value" an index read
is done. It consists of an index scan and retrieval of
each key.

For efficiency reasons the index scan stores the key in
the special buffer 'lastkey2' once only. At the first 
iteration it notes this fact with the flag 
HA_STATE_RNEXT_SAME in 'info->update'.

For efficiency reasons, the key retrieval for blobs
does not allocate a new buffer, but uses 'lastkey2'...

Now I clear the HA_STATE_RNEXT_SAME flag whenever the 
buffer has been polluted. In this case, the index scan
copies the key value again (and sets the flag again).

include/my_base.h

@@ -340,7 +340,7 @@ enum ha_base_keytype {
 #define HA_STATE_BUFF_SAVED	512	/* If current keybuff is info->buff */
 #define HA_STATE_ROW_CHANGED	1024	/* To invalide ROW cache */
 #define HA_STATE_EXTEND_BLOCK	2048
-#define HA_STATE_RNEXT_SAME	4096	/* rnext_same was called */
+#define HA_STATE_RNEXT_SAME	4096	/* rnext_same occupied lastkey2 */
 
 enum en_fieldtype {
   FIELD_LAST=-1,FIELD_NORMAL,FIELD_SKIP_ENDSPACE,FIELD_SKIP_PRESPACE,

myisam/mi_delete.c

@@ -82,6 +82,8 @@ int mi_delete(MI_INFO *info,const byte *record)
                 _mi_make_key(info,i,old_key,record,info->lastpos)))
           goto err;
       }
+      /* The above changed info->lastkey2. Inform mi_rnext_same(). */
+      info->update&= ~HA_STATE_RNEXT_SAME;
     }
   }
 

myisam/mi_key.c

@@ -393,6 +393,10 @@ static int _mi_put_key_in_record(register MI_INFO *info, uint keynr,
 	     (char*) &blob_ptr,sizeof(char*));
       memcpy(blob_ptr,key,length);
       blob_ptr+=length;
+
+      /* The above changed info->lastkey2. Inform mi_rnext_same(). */
+      info->update&= ~HA_STATE_RNEXT_SAME;
+
       _my_store_blob_length(record+keyseg->start,
 			    (uint) keyseg->bit_start,length);
       key+=length;

myisam/mi_rnext_same.c

@@ -40,7 +40,7 @@ int mi_rnext_same(MI_INFO *info, byte *buf)
 
   if (info->s->concurrent_insert)
     rw_rdlock(&info->s->key_root_lock[inx]);
-    
+
   switch (keyinfo->key_alg)
   {
 #ifdef HAVE_RTREE_KEYS
@@ -102,4 +102,4 @@ int mi_rnext_same(MI_INFO *info, byte *buf)
     DBUG_RETURN(0);
   }
   DBUG_RETURN(my_errno);
-} /* mi_rnext */
+} /* mi_rnext_same */

myisam/mi_unique.c

@@ -30,6 +30,9 @@ my_bool mi_check_unique(MI_INFO *info, MI_UNIQUEDEF *def, byte *record,
   mi_unique_store(record+key->seg->start, unique_hash);
   _mi_make_key(info,def->key,key_buff,record,0);
 
+  /* The above changed info->lastkey2. Inform mi_rnext_same(). */
+  info->update&= ~HA_STATE_RNEXT_SAME;
+
   if (_mi_search(info,info->s->keyinfo+def->key,key_buff,MI_UNIQUE_HASH_LENGTH,
 		 SEARCH_FIND,info->s->state.key_root[def->key]))
   {

myisam/mi_update.c

@@ -108,6 +108,10 @@ int mi_update(register MI_INFO *info, const byte *oldrec, byte *newrec)
       {
 	uint new_length=_mi_make_key(info,i,new_key,newrec,pos);
 	uint old_length=_mi_make_key(info,i,old_key,oldrec,pos);
+
+        /* The above changed info->lastkey2. Inform mi_rnext_same(). */
+        info->update&= ~HA_STATE_RNEXT_SAME;
+
 	if (new_length != old_length ||
 	    memcmp((byte*) old_key,(byte*) new_key,new_length))
 	{

myisam/mi_write.c

@@ -128,6 +128,10 @@ int mi_write(MI_INFO *info, byte *record)
           goto err;
         }
       }
+
+      /* The above changed info->lastkey2. Inform mi_rnext_same(). */
+      info->update&= ~HA_STATE_RNEXT_SAME;
+
       if (local_lock_tree)
         rw_unlock(&share->key_root_lock[i]);
     }

mysql-test/r/myisam.result

@@ -730,3 +730,21 @@ select * from t1 where bob is null and cip=1;
 cip	time	score	bob
 1	00:01:00	0	NULL
 drop table t1;
+create table t1 (
+id1 int not null auto_increment,
+id2 int not null default '0',
+t text not null,
+primary key  (id1),
+key x (id2, t(32))
+) engine=myisam;
+insert into t1 (id2, t) values
+(10, 'abc'), (10, 'abc'), (10, 'abc'),
+(20, 'abc'), (20, 'abc'), (20, 'def'),
+(10, 'abc'), (10, 'abc');
+select count(*)   from t1 where id2 = 10;
+count(*)
+5
+select count(id1) from t1 where id2 = 10;
+count(id1)
+5
+drop table t1;

mysql-test/t/myisam.test

@@ -686,4 +686,23 @@ select * from t1 where bob is null and cip=1;
 create index bug on t1 (bob(22), cip, time);
 select * from t1 where bob is null and cip=1;
 drop table t1;
+
+#
+# Bug#14980 - COUNT(*) incorrect on MyISAM table with certain INDEX
+#
+create table t1 (
+  id1 int not null auto_increment,
+  id2 int not null default '0',
+  t text not null,
+  primary key  (id1),
+  key x (id2, t(32))
+) engine=myisam;
+insert into t1 (id2, t) values
+(10, 'abc'), (10, 'abc'), (10, 'abc'),
+(20, 'abc'), (20, 'abc'), (20, 'def'),
+(10, 'abc'), (10, 'abc');
+select count(*)   from t1 where id2 = 10;
+select count(id1) from t1 where id2 = 10;
+drop table t1;
+
 # End of 4.1 tests