Don’t put a NULL in the IN clause in 5.1

Gary — Wed, 08 Apr 2009 07:36:08 +0000

There seems to be an optimizer problem in 5.1, if you put a NULL in the IN clause of a SELECT. For example, given the following table:

CREATE TABLE foo (
    a INT NOT NULL AUTO_INCREMENT,
    PRIMARY KEY (a)
);

Compare these two EXPLAINs:

mysql> EXPLAIN * FROM foo WHERE a IN (160000, 160001, 160002)\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: foo
         type: range
possible_keys: PRIMARY
          key: PRIMARY
      key_len: 4
          ref: NULL
         rows: 3
        Extra: Using where
1 row in set (0.06 sec)
 
mysql> EXPLAIN SELECT * FROM foo WHERE a IN (NULL, 160000, 160001, 160002)\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: foo
         type: ALL
possible_keys: PRIMARY
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 327680
        Extra: Using where
1 row in set (0.00 sec)

In the query with the NULL, it does a full table scan. So, if you’ve run into this problem under MySQL 5.1, the workaround is to remove the NULL. This doesn’t affect MySQL 4.x or 5.0.

You can also follow along with Bug #33139.

Replication with InnoDB and MyISAM Transactions

Gary — Fri, 13 Feb 2009 05:26:57 +0000

There’s a change of behaviour in MySQL 5.1.31 for Row Based Replication, if you have InnoDB transactions that also write to a MyISAM (or other non-transactional engine) table. It’s a side effect of fixing Bug #40116. Take this simple example:

TRANSACTION 1: INSERT INTO myisam_tbl (item, val) VALUES (1, 0);
TRANSACTION 1: INSERT INTO innodb_tbl (item, val) VALUES (1, -1), (2, -1);
TRANSACTION 1: START TRANSACTION;
TRANSACTION 1: UPDATE myisam_tbl SET val=val+1 WHERE item=1;
TRANSACTION 1: UPDATE innodb_tbl SET val=( SELECT val FROM myisam_tbl WHERE item=1 ) WHERE item=1;
TRANSACTION 2: START TRANSACTION;
TRANSACTION 2: UPDATE myisam_tbl SET val=val+1 WHERE item=1;
TRANSACTION 2: UPDATE innodb_tbl SET val=( SELECT val FROM myisam_tbl WHERE item=1 ) WHERE item=2;
TRANSACTION 2: COMMIT;
TRANSACTION 1: COMMIT;

After this, the Master innodb_tbl would look like this:

item	val
1	1
2	2

And the Master myisam_tbl will look like this:

item	val
1	2

In 5.1.30 and earlier, the Slave tables will be correct. However, in 5.1.31, the Slave myisam_tbl will be correct, but the innodb_tbl will look like this:

item	val
1	0
2	1

As a bonus, there’s no workaround. Statement Based Replication has never worked for this case. For an SBR Slave (In MySQL 5.0.x and 5.1.x), the Slave myisam_tbl will be correct, but the Slave innodb_tbl will look like this:

item	val
1	2
2	2

And so, we come to the moral of the story. Don’t use non-transactional tables in the middle of a transaction. Ever. You will only cause yourself more pain than you can possibly imagine. Instead, move the writes to the non-transactional tables outside of the transaction.

Gary Pendergast » regression

Don’t put a NULL in the IN clause in 5.1

Replication with InnoDB and MyISAM Transactions