CODEWITHSUNDEEP
mysqlstored-procedures
Zaar Hai
Zaar Hai

Reputation: 9889

How to avoid mysql from locking ALL tables from SELECT ... INNER JOIN ... FOR UPDATE

I have two mysql tables:

tbl_jobs (
    id INTEGER PRIMARY KEY,
    status ENUM("runnable", "running", "finished"),
    server_id INTEGER
)

tbl_servers (
    is INTEGER PRIMARY KEY,
    name VARCHAR(50)
)

I have a lot of clients that connect, take one job for a particular server that is runnable and set it to running.

Here is how I currently do it (in stored procedure):

DELIMITER $$
CREATE PROCEDURE GET_JOB(serverName VARCHAR(50))
BEGIN
    DECLARE jobId INTEGER DEFAULT NULL;
    SELECT id FROM tbl_jobs j INNER JOIN tbl_servers s on j.server_id=s.id
              WHERE j.state='runnable' AND s.server_name=serverName
              ORDER BY job_id ASC LIMIT 1
              INTO jobId FOR UPDATE;
    IF IFNULL(jobId, 0) = 0 THEN
        SELECT 0;
    END IF;
    UPDATE tbl_jobs SET state='running' WHERE job_id=jobId;
    SELECT jobId;
END $$

That works just fine, but when number of concurrent clients become large (hundreds), I see that there is a big lock congestion going on tbl_servers table. I understand that FOR UPDATE statement locks all of the tables, but I'm actually using tbl_servers as read-only table.

Question: How to avoid lock congestion (i.e. locking at all) on tbl_servers?

The one thing I can think of is to separate my query into two - first convert server name to id and then query just tbl_jobs, but in my real application one server name can have many ids (I know it sounds odd, but I've just simplified my application here for illustration). So the second query on tbl_jobs would require prepared statement.

I'm sure there is a more elegant solution.

The system:

Upvotes: 0

Views: 1264

Answers (1)

Sir Rufo
Sir Rufo

Reputation: 19106

This should avoid the locking on table tbl_servers

DELIMITER $$
CREATE PROCEDURE GET_JOB(serverName VARCHAR(50))
BEGIN
    DECLARE jobId INTEGER DEFAULT NULL;

    -- get the server id's
    CREATE TEMPORARY TABLE tmp_srvID 
    SELECT id
    INTO srvID 
    FROM tbl_servers
    WHERE name = serverName;

    SELECT id 
    INTO jobId 
    FROM tbl_jobs 
    WHERE state='runnable'

      AND server_id IN ( SELECT id FROM tmp_srvID ) 

    ORDER BY job_id ASC 
    LIMIT 1
    FOR UPDATE;

    DROP TABLE tmp_srvID;

    IF IFNULL(jobId, 0) = 0 THEN
        SELECT 0;
    ELSE
        UPDATE tbl_jobs SET state='running' WHERE job_id=jobId;
        SELECT jobId;
    END IF;
END $$

Upvotes: 1

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