MySQL Transactions, Part III - BDB Tables, Table locking and Savepoints

BDB Transactions

The previous two columns in this series have covered general transactions with InnoDB tables and transaction isolation levels. This month, we look at transactions with other table types, specifically BDB tables, and all other table types, including the default MyISAM table type.

BDB Tables are not that commonly used. Most tables tend to be either the default MyISAM table type, or, for those specifically wanting ACID compliance, InnoDB. BDB (standing for BerkeleyDB) is the poor relation as far as MySQL goes, a table type that precedes InnoDB, and was included for its transactional capability, but which never achieved as much popularity. The BDB format itself is stable (it is supplied by a third party - Sleepycat Software), but the MySQL interface to it is not yet 'production'.

mysql>CREATE TABLE bdb_table(f INT) TYPE = BDB

First up, note a very important difference in the default behavior of BDB tables compared with InnoDB tables. From one connection, run the following:

mysql> BEGIN;
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO bdb_table VALUES(1);
Query OK, 1 row affected (0.00 sec)

From a second connection, see what the table contains:

mysql> SELECT * FROM bdb_table;

Note that the connection hangs - it is waiting for the other connection to COMMIT its transaction before returning a result. Remember that InnoDB tables returned a result based upon the data before any other uncommitted transactions had begun. Commit the transaction, and the result appears:

Connection 1:

mysql> COMMIT;
Query OK, 0 rows affected (0.00 sec)

Connection 2:

mysql> SELECT * FROM bdb_table;
+------+
| f    |
+------+
|    1 |
+------+
1 row in set (7.08 sec)

This default behavior could lead to performance problems in your applications, and is one of the reasons for InnoDB's greater popularity.

Autocommit

MySQL's default behavior is to automatically commit statements not explicitly wrapped in a transaction. As we saw here with InnoDB tables, this means any statement that does not have a BEGIN preceding it will be immediately carried out (as if you were using the non-transactional MyISAM table type), as demonstrated below.

From Connection 1:

mysql> INSERT INTO bdb_table VALUES(2);
Query OK, 1 row affected (0.00 sec)

Connection 2:

mysql> SELECT * FROM bdb_table;
+------+
| f    |
+------+
|    1 |
|    2 |
+------+
2 rows in set (0.00 sec)

You can circumvent this behavior by setting AUTOCOMMIT to 0, in which case all statements are treated as if a BEGIN statement precedes them, for example:

Connection 1:

mysql> SET AUTOCOMMIT=0;
Query OK, 0 rows affected (0.01 sec)
mysql> INSERT INTO bdb_table VALUES(3);
Query OK, 1 row affected (0.00 sec)

Connection 2:

mysql> SELECT * FROM bdb_table;

Again the thread hangs, waiting for a commit from the first connection.

Connection 1:

mysql> COMMIT;
Query OK, 0 rows affected (0.00 sec)

Connection 2:

mysql> SELECT * FROM bdb_table;
+------+
| f    |
+------+
|    1 |
|    2 |
|    3 |
+------+
3 rows in set (0.00 sec)

Of course, BDB tables also use the ROLLBACK statement to return the data back to the status it was in prior to the transaction:

Connection 1:

mysql> BEGIN;
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO bdb_table VALUES(4);
Query OK, 1 row affected (0.01 sec)

mysql> ROLLBACK;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT * FROM bdb_table;
+------+
| f    |
+------+
|    1 |
|    2 |
|    3 |
+------+
3 rows in set (0.00 sec)

Table locking for all table types

InnoDB and BDB tables both make use of row level locking, whereby only the row/s being manipulated are locked. This means that other rows can still be manipulated, reducing the risk of contention, but is also a less optimal process than table level locking if most of the queries are SELECT's. MyISAM tables do not have row-locking, which is why they are not a great choice if the application has a high ration of INSERT's or UPDATE's to SELECT's. However, all table types can make use of table-locking, using the LOCK TABLE statement.

Since this is the only kind of locking available to MyISAM tables, let's create a MyISAM table to test on:

mysql> CREATE TABLE myisam_table(f INT);
Query OK, 0 rows affected (0.01 sec)

There are two main kinds of locks - read locks and write locks. A read lock only allows reads on the table, not writes, while a write lock only allows that connection to read and write - all other connections are blocked. Let's see these in action:

Connection 1:

mysql> LOCK TABLE myisam_table READ;
Query OK, 0 rows affected (0.00 sec)

Connection 2:

mysql> INSERT INTO myisam_table VALUES(1);

The second connection hangs, waiting for the table to be unlocked.

Connection 1:

mysql> UNLOCK TABLES;
Query OK, 0 rows affected (0.00 sec)

Connection 2:

mysql> INSERT INTO myisam_table VALUES(1);
Query OK, 1 row affected (1 min 16.34 sec)

However, a read from the second connection is processed immediately:

Connection 1:

mysql> LOCK TABLE myisam_table READ;
Query OK, 0 rows affected (0.00 sec)

Connection 2:

mysql> SELECT * FROM myisam_table;
+------+
| f    |
+------+
|    1 |
+------+
1 row in set (0.00 sec)

Connection 1:

mysql> UNLOCK TABLES;
Query OK, 0 rows affected (0.00 sec)

Let's try the same statements with a write lock.

Connection 1:

mysql> LOCK TABLE myisam_table WRITE;
Query OK, 0 rows affected (0.01 sec)

Connection 2:

mysql> SELECT * FROM myisam_table;

The connection hangs, waiting for the lock to be released.

Connection 1:

mysql> UNLOCK TABLE;
Query OK, 0 rows affected (0.01 sec)

Connection 2:

mysql> SELECT * FROM myisam_table;
+------+
| f    |
+------+
|    1 |
+------+
1 row in set (6.22 sec)

If you have been thinking about this a bit, you may wonder what happens if the connection that creates a READ lock attempts to INSERT a record. Since this is forbidden on a read lock even to the originating connection, and if the connection had to wait, there would be deadlock since it would then be unable to release the lock, MySQL returns an immediate error.

mysql> LOCK TABLE myisam_table READ;
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO myisam_table VALUES(2);
ERROR 1099: Table 'myisam_table' was locked with a READ lock and can't be updated

mysql> UNLOCK TABLE;
Query OK, 0 rows affected (0.00 sec)

Locking priority

Write locks have a higher priority than read locks. A write lock will always be obtained ahead of any prior waiting read locks. Let's demonstrate this:

Connection 1:

mysql> LOCK TABLE myisam_table WRITE;
Query OK, 0 rows affected (0.00 sec)

Connection 2:

mysql> LOCK TABLE myisam_table READ;

The connection hangs, waiting for the earlier lock to be released.

Connection 3:

mysql> LOCK TABLE myisam_table WRITE;

Now there are two locks waiting, a read lock, and a write lock requested after the read lock. When we release the first lock, the lock from the third connection, since it is a higher precedence write lock, is obtained.

Connection 1:

mysql> UNLOCK TABLE;
Query OK, 0 rows affected (0.00 sec)

Connection 2 is still waiting, but connection 3 is ready for action again:

mysql> LOCK TABLE myisam_table WRITE;
Query OK, 0 rows affected (5.52 sec)

mysql> UNLOCK TABLE;
Query OK, 0 rows affected (0.00 sec) 

Only now is the second connection's lock obtained. It does not take much imagination to understand why table locking can lead to performance implications if there are many write locks.

Sometimes you genuinely want a write lock to have a lower priority than a read lock. An example we use at Independent Online is the article tracker. Every article read is tracked, but the INSERT statement is a lower priority than the read, which of course impacts a reader. Using the same MyISAM table and the same set of locks as before except that the third connection is a low priority lock, let's examine what happens.

Connection 1:

mysql> LOCK TABLE myisam_table WRITE;
Query OK, 0 rows affected (0.00 sec)

Connection 2:

mysql> LOCK TABLE myisam_table READ;

The connection hangs, waiting for the earlier lock to be released.

Connection 3:

mysql> LOCK TABLE myisam_table LOW_PRIORITY WRITE;

Connection 1:

mysql> UNLOCK TABLE;
Query OK, 0 rows affected (0.00 sec)

This time, Connection 2 is obtained, while the write lock in connection 3 is still waiting.

Connection 2:

mysql> LOCK TABLE myisam_table READ;
Query OK, 0 rows affected (3 min 8.29 sec)

mysql> UNLOCK TABLE;
Query OK, 0 rows affected (0.00 sec) 

Connection 3:

mysql> LOCK TABLE myisam_table LOW_PRIORITY WRITE;
Query OK, 0 rows affected (2 min 12.23 sec)

mysql> UNLOCK TABLE;
Query OK, 0 rows affected (0.00 sec)

Savepoints

Finally, for this month, we will go back to InnoDB tables, and examine a recent addition to MySQL - savepoints. These allow you to rollback part of a transaction, rather than the all or nothing approach usually found with transactions. Savepoints only work with InnoDB tables - for those who have not been following the series, you can recreate the table as it is with the following statements (though the records are not really necessary):

mysql> CREATE TABLE t (f INT) TYPE = InnoDB;
Query OK, 0 rows affected (0.03 sec)

mysql> INSERT INTO t values (1),(2),(3),(4),(55),(6),(7),(88);
Query OK, 8 rows affected (0.00 sec)
Records: 8  Duplicates: 0  Warnings: 0

Inside the same transaction, we will insert two new records, one before the savepoint, and one after:

mysql> INSERT INTO t VALUES(9);
Query OK, 1 row affected (0.05 sec)

mysql> SAVEPOINT x;
Query OK, 0 rows affected (0.06 sec)

mysql> INSERT INTO t VALUES(10);
Query OK, 1 row affected (0.00 sec)

mysql> ROLLBACK TO SAVEPOINT x;
Query OK, 0 rows affected (0.06 sec)

mysql> SELECT * FROM t;
+------+
| f    |
+------+
|    1 |
|    2 |
|    3 |
|    4 |
|   55 |
|    6 |
|    7 |
|   88 |
|    9 |
+------+
9 rows in set (0.00 sec)

The first insert has been performed - effectively a savepoint could also be termed COMMIT UNTIL, so anything before the savepoint is now committed. The second insert, after the savepoint, is rolled back.

That concludes the series on transactions. I hope that you have gotten a taste for how MySQL handles them, and some of its quirks, but as always, there is no better way to learn than diving in and making all the mistakes yourself. Good luck.

MySQL Transactions, Part III - BDB Tables, Table locking and Savepoints