7.23 LOCK TABLES/UNLOCK TABLES syntax

LOCK TABLES tbl_name [AS alias] {READ | [LOW_PRIORITY] WRITE}
            [, tbl_name {READ | [LOW_PRIORITY] WRITE} ...]
...
UNLOCK TABLES

LOCK TABLES locks tables for the current thread. UNLOCK TABLES releases any locks held by the current thread. All tables that are locked by the current thread are automatically unlocked when the thread issues another LOCK TABLES, or when the connection to the server is closed.

If a thread obtains a READ lock on a table, that thread (and all other threads) can only read from the table. If a thread obtains a WRITE lock on a table, then only the thread holding the lock can READ from or WRITE to the table. Other threads are blocked.

Each thread waits (without timing out) until it obtains all the locks it has requested.

WRITE locks normally have higher priority than READ locks, to ensure that updates are processed as soon as possible. This means that if one thread obtains a READ lock and then another thread requests a WRITE lock, subsequent READ lock requests will wait until the WRITE thread has gotten the lock and released it. You can use LOW_PRIORITY WRITE locks to allow other threads to obtain READ locks while the thread is waiting for the WRITE lock. You should only use LOW_PRIORITY WRITE locks if you are sure that there will eventually be a time when no threads will have a READ lock.

When you use LOCK TABLES, you must lock all tables that you are going to use! If you are using a table multiple times in a query (with aliases), you must get a lock for each alias! This policy ensures that table locking is deadlock free.

Note that you should NOT lock any tables that you are using with INSERT DELAYED. This is because that in this case the INSERT is done by a separate thread.

Normally, you don't have to lock tables, as all single UPDATE statements are atomic; no other thread can interfere with any other currently executing SQL statement. There are a few cases when you would like to lock tables anyway:

• If you are going to run many operations on a bunch of tables, it's much faster to lock the tables you are going to use. The downside is, of course, that no other thread can update a READ-locked table and no other thread can read a WRITE-locked table.
• MySQL doesn't support a transaction environment, so you must use LOCK TABLES if you want to ensure that no other thread comes between a SELECT and an UPDATE. The example shown below requires LOCK TABLES in order to execute safely:

  mysql> LOCK TABLES trans READ, customer WRITE;
  mysql> select sum(value) from trans where customer_id= some_id;
  mysql> update customer set total_value=sum_from_previous_statement
             where customer_id=some_id;
  mysql> UNLOCK TABLES;
  

  Without LOCK TABLES, there is a chance that another thread might insert a new row in the trans table between execution of the SELECT and UPDATE statements.

By using incremental updates (UPDATE customer SET value=value+new_value) or the LAST_INSERT_ID() function, you can avoid using LOCK TABLES in many cases.

You can also solve some cases by using the user-level lock functions GET_LOCK() and RELEASE_LOCK(). These locks are saved in a hash table in the server and implemented with pthread_mutex_lock() and pthread_mutex_unlock() for high speed. 7.3.12 Miscellaneous functions.

See 10.11 How MySQL locks tables, for more information on locking policy.