MySQL - Table 'my_table' was not locked with Lock Tables

If in one session, you locked one table but want to select from another table, you must either lock that table too or unlock all tables.

mysql> LOCK TABLES t1 READ;
mysql> SELECT COUNT(*) FROM t1;
+----------+
| COUNT(*) |
+----------+
|        3 |
+----------+
mysql> SELECT COUNT(*) FROM t2;
ERROR 1100 (HY000): Table 't2' was not locked with LOCK TABLES

The solution for me was to unlock the tables. They had been locked by a previous query which failed before reaching the unlock tables statement.

UNLOCK TABLES
SELECT ...

http://dev.mysql.com/doc/refman/5.7/en/lock-tables.html

MySQL enables client sessions to acquire table locks explicitly for the purpose of cooperating with other sessions for access to tables, or to prevent other sessions from modifying tables during periods when a session requires exclusive access to them. A session can acquire or release locks only for itself. One session cannot acquire locks for another session or release locks held by another session.

Locks may be used to emulate transactions or to get more speed when updating tables. This is explained in more detail later in this section.

LOCK TABLES explicitly acquires table locks for the current client session. Table locks can be acquired for base tables or views. You must have the LOCK TABLES privilege, and the SELECT privilege for each object to be locked.

For view locking, LOCK TABLES adds all base tables used in the view to the set of tables to be locked and locks them automatically. If you lock a table explicitly with LOCK TABLES, any tables used in triggers are also locked implicitly, as described in Section 13.3.5.2, “LOCK TABLES and Triggers”.

UNLOCK TABLES explicitly releases any table locks held by the current session. LOCK TABLES implicitly releases any table locks held by the current session before acquiring new locks.

Another use for UNLOCK TABLES is to release the global read lock acquired with the FLUSH TABLES WITH READ LOCK statement, which enables you to lock all tables in all databases. See Section 13.7.6.3, “FLUSH Syntax”. (This is a very convenient way to get backups if you have a file system such as Veritas that can take snapshots in time.)

Syntax for LOCK and UNLOCK

 LOCK TABLES
    tbl_name [[AS] alias] lock_type
    [, tbl_name [[AS] alias] lock_type] ...

lock_type:
    READ [LOCAL]
  | [LOW_PRIORITY] WRITE

Eg:-

LOCK TABLE t WRITE, t AS t1 READ;

Unlock tables

 UNLOCK TABLES

One of the most important lines in the MySQL docs relating to the "Table 'my_table' was not locked with LOCK TABLES" message is as follows:

"While the locks thus obtained are held, the session can access only the locked tables" https://dev.mysql.com/doc/refman/8.0/en/lock-tables.html

This means that if you are trying to access any other table in the database while the LOCK is in place you will get the error message "Table 'my_table' was not locked with LOCK TABLES"

The fix is to apply the lock to all of the tables you want to have access to during the lock like this. "LOCK TABLES table_1 WRITE, table_2 WRITE"

Where table_1 is the one you really want to lock but you also want to access table_2 during the same process.

This was confusing because I was locking only table_1 but the error message was telling me Table 'table_2' was not locked with LOCK TABLES

Took me a while to figure out why table_2 was even involved. I hope that this helps someone else with the same issue.

In my case the problem was the aliases.

From the docs:

If your statements refer to a table by means of an alias, you must lock the table using that same alias. It does not work to lock the table without specifying the alias.

Conversely, if you lock a table using an alias, you must refer to it in your statements using that alias.

Wrong:

LOCK TABLE my_table READ;
SELECT * FROM my_table t;
#ERROR 1100: Table 't' was not locked with LOCK TABLES

Correct:

LOCK TABLE my_table t READ;
SELECT * FROM my_table t;