Recently I had an interesting surprise with concurrent inserts into a MyISAM table. The inserts were not happening concurrently with SELECT statements; they were blocking and the process list was filling up with queries in Locked status.
My first thought was that the customer had deleted from the table, which leaves “holes” in the middle of it and prevents concurrent inserts. (You can configure the server to permit concurrent inserts even when there are holes, but it’s disabled by default.) However, that turned out not to be the cause; the table was only inserted into (and selected from). Instead, the blocked statements were because of INSERT… SELECT statements that were running against the table, selecting data from it and inserting into another table.
Let’s look at what happens here: suppose you have two tables tbl1 and tbl2 and concurrent inserts into tbl2 are running fine. If you now run the following query,
INSERT INTO tbl1 SELECT * FROM tbl2
The concurrent inserts into tbl2 can block. This happens if you have the binary log enabled. If you think about it, this makes sense and is correct behavior. The statements have to be serialized for the binary log; otherwise replaying the binary log can result in a different order of execution.
The MySQL manual actually says this, but not in the clearest way. It just says
If you are using the binary log, concurrent inserts are converted to normal inserts for CREATE … SELECT or INSERT … SELECT statements.
If you use mysqladmin debug, you’ll see an ordinary SELECT gets a lock on the table like this:
Locked - read Low priority read lock
But on INSERT…SELECT, you’ll see this:
Read lock without concurrent inserts
That read lock is what’s blocking the concurrent inserts from happening.
There’s no solution to this, if you need the binary log enabled. (It needs to be enabled for replication.) There are workarounds, though. You can use the old trick of SELECT INTO OUTFILE followed by LOAD DATA INFILE. You can use InnoDB instead. Or you can do something more elaborate and application-specific, but that’s a topic for another post.