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7.3.1. Locking Methods

MySQL uses table-level locking for MyISAM and MEMORY tables, page-level locking for BDB tables, and row-level locking for InnoDB tables.

In many cases, you can make an educated guess about which locking type is best for an application, but generally it is difficult to say that a given lock type is better than another. Everything depends on the application and different parts of an application may require different lock types.

To decide whether you want to use a storage engine with row-level locking, you should look at what your application does and what mix of select and update statements it uses. For example, most Web applications perform many selects, relatively few deletes, updates based mainly on key values, and inserts into a few specific tables. The base MySQL MyISAM setup is very well tuned for this.

Table locking in MySQL is deadlock-free for storage engines that use table-level locking. Deadlock avoidance is managed by always requesting all needed locks at once at the beginning of a query and always locking the tables in the same order.

The table-locking method MySQL uses for WRITE locks works as follows:

  • If there are no locks on the table, put a write lock on it.

  • Otherwise, put the lock request in the write lock queue.

The table-locking method MySQL uses for READ locks works as follows:

  • If there are no write locks on the table, put a read lock on it.

  • Otherwise, put the lock request in the read lock queue.

When a lock is released, the lock is made available to the threads in the write lock queue and then to the threads in the read lock queue. This means that if you have many updates for a table, SELECT statements wait until there are no more updates.

You can analyze the table lock contention on your system by checking the Table_locks_waited and Table_locks_immediate status variables:

mysql> SHOW STATUS LIKE 'Table%';
+-----------------------+---------+
| Variable_name         | Value   |
+-----------------------+---------+
| Table_locks_immediate | 1151552 |
| Table_locks_waited    | 15324   |
+-----------------------+---------+

If a MyISAM table contains no free blocks in the middle, rows always are inserted at the end of the data file. In this case, you can freely mix concurrent INSERT and SELECT statements for a MyISAM table without locks. That is, you can insert rows into a MyISAM table at the same time other clients are reading from it. (Holes can result from rows having been deleted from or updated in the middle of the table. If there are holes, concurrent inserts are disabled but are re-enabled automatically when all holes have been filled with new data.)

If you want to perform many INSERT and SELECT operations on a table when concurrent inserts are not possible, you can insert rows in a temporary table and update the real table with the rows from the temporary table once in a while. This can be done with the following code:

mysql> LOCK TABLES real_table WRITE, insert_table WRITE;
mysql> INSERT INTO real_table SELECT * FROM insert_table;
mysql> TRUNCATE TABLE insert_table;
mysql> UNLOCK TABLES;

InnoDB uses row locks and BDB uses page locks. For these two storage engines, deadlocks are possible because they automatically acquire locks during the processing of SQL statements, not at the start of the transaction.

Advantages of row-level locking:

  • Fewer lock conflicts when accessing different rows in many threads.

  • Fewer changes for rollbacks.

  • Possible to lock a single row for a long time.

Disadvantages of row-level locking:

  • Requires more memory than page-level or table-level locks.

  • Slower than page-level or table-level locks when used on a large part of the table because you must acquire many more locks.

  • Definitely much slower than other locks if you often do GROUP BY operations on a large part of the data or if you must scan the entire table frequently.

Table locks are superior to page-level or row-level locks in the following cases:

  • Most statements for the table are reads.

  • A mix of reads and writes, where writes are updates or deletes for a single row that can be fetched with one key read:

    UPDATE tbl_name SET column=value WHERE unique_key_col=key_value;
    DELETE FROM tbl_name WHERE unique_key_col=key_value;
    
  • SELECT combined with concurrent INSERT statements, and very few UPDATE or DELETE statements.

  • Many scans or GROUP BY operations on the entire table without any writers.

With higher-level locks, you can more easily tune applications by supporting locks of different types, because the lock overhead is less than for row-level locks.

Options other than row-level or page-level locking:

  • Versioning (such as that used in MySQL for concurrent inserts) where it is possible to have one writer at the same time as many readers. This means that the database or table supports different views for the data depending on when access begins. Other common terms for this are “time travel,” “copy on write,” or “copy on demand.

  • Copy on demand is in many cases superior to page-level or row-level locking. However, in the worst case, it can use much more memory than using normal locks.

  • Instead of using row-level locks, you can employ application-level locks, such as GET_LOCK() and RELEASE_LOCK() in MySQL. These are advisory locks, so they work only in well-behaved applications.


 
 
  Published under the terms of the GNU General Public License Design by Interspire