MySQL 5.7 Release Notes
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 statement can be executed
to add rows to the end of the table at the same time that
SELECT statements are reading
rows from the table. If there are multiple
INSERT statements, they are
queued and performed in sequence, concurrently with the
SELECT statements. The results of
a concurrent INSERT may not be
visible immediately.
The 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 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 system
variable.
If you are using the binary log, concurrent inserts are
converted to normal inserts for CREATE ...
SELECT or
INSERT ...
SELECT 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”. 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, 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 is executing. Use of the
CONCURRENT option affects the performance of
LOAD DATA 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 option if
the server was started with that option. It also causes
concurrent inserts not to be used.
For LOCK
TABLE, the difference between READ
LOCAL and READ is that
READ LOCAL permits nonconflicting
INSERT 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.