### 8.11.3 Concurrent Inserts

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 The `MyISAM` storage engine supports concurrent inserts to reduce contention between readers and writers for a given table: If a `MyISAM` table has no holes in the data file (deleted rows in the middle), an [`INSERT`](insert.html) statement can be executed to add rows to the end of the table at the same time that [`SELECT`](select.html) statements are reading rows from the table. If there are multiple [`INSERT`](insert.html) statements, they are queued and performed in sequence, concurrently with the [`SELECT`](select.html) statements. The results of a concurrent [`INSERT`](insert.html) may not be visible immediately.

 The [`concurrent_insert`](server-system-variables.html#sysvar_concurrent_insert) system variable can be set to modify the concurrent-insert processing. By default, the variable is set to `AUTO` (or 1) and concurrent inserts are handled as just described. If [`concurrent_insert`](server-system-variables.html#sysvar_concurrent_insert) is set to `NEVER` (or 0), concurrent inserts are disabled. If the variable is set to `ALWAYS` (or 2), concurrent inserts at the end of the table are permitted even for tables that have deleted rows. See also the description of the [`concurrent_insert`](server-system-variables.html#sysvar_concurrent_insert) system variable.

 If you are using the binary log, concurrent inserts are converted to normal inserts for `CREATE ... SELECT` or [`INSERT ... SELECT`](insert-select.html) statements. This is done to ensure that you can re-create an exact copy of your tables by applying the log during a backup operation. See [Section 5.4.4, “The Binary Log”](binary-log.html). In addition, for those statements a read lock is placed on the selected-from table such that inserts into that table are blocked. The effect is that concurrent inserts for that table must wait as well.

 With [`LOAD DATA`](load-data.html), if you specify `CONCURRENT` with a `MyISAM` table that satisfies the condition for concurrent inserts (that is, it contains no free blocks in the middle), other sessions can retrieve data from the table while [`LOAD DATA`](load-data.html) is executing. Use of the `CONCURRENT` option affects the performance of [`LOAD DATA`](load-data.html) a bit, even if no other session is using the table at the same time.

 If you specify `HIGH_PRIORITY`, it overrides the effect of the [`--low-priority-updates`](server-system-variables.html#sysvar_low_priority_updates) option if the server was started with that option. It also causes concurrent inserts not to be used.

 For [`LOCK TABLE`](lock-tables.html), the difference between `READ LOCAL` and `READ` is that `READ LOCAL` permits nonconflicting [`INSERT`](insert.html) statements (concurrent inserts) to execute while the lock is held. However, this cannot be used if you are going to manipulate the database using processes external to the server while you hold the lock.

