This section discusses internal locking; that is, locking
performed within the MySQL server itself to manage contention
for table contents by multiple sessions. This type of locking is
internal because it is performed entirely by the server and
involves no other programs. External locking occurs when the
server and other programs lock
MyISAM table files to coordinate
among themselves which program can access the tables at which
time. See Section 8.8.4, “External Locking”.
MySQL uses row-level
InnoDB tables to support
simultaneous write access by multiple sessions, making them
suitable for multi-user, highly concurrent, and OLTP
applications. MySQL uses
table-level locking for
MERGE tables, allowing only one session to
update those tables at a time, making them more suitable for
read-only, read-mostly, or single-user applications.
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.
MySQL grants table write locks 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.
MySQL grants table read locks 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.
Table updates are given higher priority than table retrievals.
Therefore, when a lock is released, the lock is made available
to the requests in the write lock queue and then to the requests
in the read lock queue. This ensures that updates to a table are
not “starved” even if there is heavy
SELECT activity for the table.
However, if you have many updates for a table,
SELECT statements wait until
there are no more updates.
For information on altering the priority of reads and writes, see Section 8.8.2, “Table Locking Issues”.
You can analyze the table lock contention on your system by
variables, which indicate the number of times that requests for
table locks could be granted immediately and the number that had
to wait, respectively:
SHOW STATUS LIKE 'Table%';+-----------------------+---------+ | Variable_name | Value | +-----------------------+---------+ | Table_locks_immediate | 1151552 | | Table_locks_waited | 15324 | +-----------------------+---------+
MyISAM storage engine supports concurrent
inserts to reduce contention between readers and writers for a
given table: If a
MyISAM table has no free
blocks in the middle of the data file, rows are always inserted
at the end of the data file. In this case, you can freely mix
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 enabled again automatically when all holes have been filled
with new data. This behavior is altered by the
variable. See Section 8.8.3, “Concurrent Inserts”.
If you acquire a table lock explicitly with
LOCK TABLES, you can request a
READ LOCAL lock rather than a
READ lock to enable other sessions to perform
concurrent inserts while you have the table locked.
To perform many
SELECT operations on a table
real_table when concurrent inserts are not
possible, you can insert rows into a temporary table
temp_table and update the real table with the
rows from the temporary table periodically. This can be done
with the following code:
LOCK TABLES real_table WRITE, temp_table WRITE;mysql>
INSERT INTO real_table SELECT * FROM temp_table;mysql>
DELETE FROM temp_table;mysql>
InnoDB uses row locks. Deadlocks are possible
InnoDB because it automatically acquires
locks during the processing of SQL statements, not at the start
of the transaction.
Advantages of row-level locking:
Fewer lock conflicts when different sessions access different rows
Fewer changes for rollbacks
Possible to lock a single row for a long time
Disadvantages of row-level locking:
Requires more memory than table-level locks
Slower than table-level locks when used on a large part of the table because you must acquire many more locks
Slower than other locks if you often do
BY operations on a large part of the data or if
you must scan the entire table frequently
Generally, table locks are superior to row-level locks in the following cases:
Most statements for the table are reads
Statements for the table are a mix of reads and writes, where writes are updates or deletes for a single row that can be fetched with one key read:
key_value; DELETE FROM
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 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 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 those provided by
RELEASE_LOCK() in MySQL.
These are advisory locks, so they work only with
applications that cooperate with each other. See
Section 12.16, “Miscellaneous Functions”.