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(mysql.info) concurrent-inserts

Info Catalog (mysql.info) table-locking (mysql.info) locking-issues
 
 7.3.3 Concurrent Inserts
 ------------------------
 
 For a `MyISAM' table, you can use concurrent inserts to add rows at the
 same time that `SELECT' statements are running if there are no deleted
 rows in middle of the table.
 
 Under circumstances where concurrent inserts can be used, there is
 seldom any need to use the `DELAYED' modifier for `INSERT' statements.
 See  insert-delayed.
 
 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.
 
 With `LOAD DATA INFILE', 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 threads can retrieve data
 from the table while `LOAD DATA' is executing. Using this option
 affects the performance of `LOAD DATA' a bit, even if no other thread is
 using the table at the same time.
 
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