Consistent transactions between storage engines

You may not realize it, but in MariaDB 5.2 and earlier and in MySQL up to version 5.5, START TRANSACTION WITH CONSISTENT SNAPSHOT does not give any guarantees of consistency between different storage engines.

For example, suppose you have two transactions which run in parallel:

Transaction T1:

BEGIN;
    SET @t = NOW();
    UPDATE xtradb_table SET a= @t WHERE id = 5;
    UPDATE pbxt_table SET b= @t WHERE id = 5;
    COMMIT;

Transaction T2:

SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
    START TRANSACTION WITH CONSISTENT SNAPSHOT;
    SELECT t1.a, t2.b
      FROM xtradb_table t1 INNER JOIN pbxt_table t2 ON t1.id=t2.id
    WHERE t1.id = 5;

In the above case, it is possible, even with a "consistent" snapshot, to see the changes in a transaction only in InnoDB/XtraDB tables, and not in PBXT tables.

Naturally, it would be much better if the changes were visible no matter the storage engine used by the table. Thankfully this is possible, thanks to the introduction in MariaDB 5.3 of group commit and an enhanced storage engine API for COMMIT which allows storage engines to coordinate commit ordering and visibility with each other and with the binary log.

With these improvements, the START TRANSACTION WITH CONSISTENT SNAPSHOT statement in MariaDB 5.3 has been enhanced to ensure consistency in-between storage engines which support the new API. Currently the storage engines which support this are XtraDB (MariaDB's enhanced version of InnoDB) and PBXT. In addition, the binary log, while not a storage engine as such, also supports the new API and can provide a binlog position consistent with storage engine transaction snapshots.

This means that with transaction isolation level at least REPEATABLE READ, the START TRANSACTION WITH CONSISTENT SNAPSHOT statement can be used to ensure that queries will see a transaction-consistent view of the database between storage engines.

No longer is it possible for a query to see the changes from some transaction T in XtraDB tables without also seeing the changes T makes to PBXT tables.

More information on this can be found in the AskMonty Knowledgebase.