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The DELAYED modifier for INSERT acts as follows. When an INSERT DELAYED request arrives for a table, the server puts the rows in a queue and returns a status to the client immediately so that the client can proceed even before the rows have been inserted. If readers are reading from the table, the rows in the queue are held. When there are no readers, the server begins inserting the rows in the delayed-row queue. Every now and then, the server checks whether any new read requests have arrived and are waiting. If so, the delayed-row queue is suspended and the readers are allowed to proceed. When there are no readers left, the server begins inserting delayed rows again. This process continues until the queue is empty.
LOW_PRIORITY and DELAYED are similar in the sense that both allow row insertion to be deferred, but they are quite different in how they affect client operation. LOW_PRIORITY forces the client to wait until the rows can be inserted. DELAYED allows the client to continue and the server buffers the rows until it has time to process them.
INSERT DELAYED is useful if other clients may be running lengthy SELECT statements and you don't want to block waiting for completion of the insertion. The client issuing the INSERT DELAYED can proceed more quickly because the server simply queues the row to be inserted.
However, you should be aware of certain other differences between normal INSERT and INSERT DELAYED behavior. The client gets back an error if the INSERT DELAYED statement contains a syntax error, but other information that would normally be available is not. For example, you can't rely on getting the AUTO_INCREMENT value when the statement returns. You also won't get a count for the number of duplicates on unique indexes. This happens because the insert operation returns a status before the operation actually has been completed. Another implication is that if rows from INSERT DELAYED statements are queued while waiting to be inserted, and the server crashes or is killed with kill -9, the rows are lost. This is not true for a normal kill -TERM kill; in that case, the server inserts the rows before exiting.
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