MyISAM Selects locks inserts inside procedure only

Question

We have a large MyISAM table to which rows get inserted to the bottom of the table only.

While doing some benchmarks, i realized that selects do not (always) lock other inserts to that same table. However, when the inserts are coming from a stored procedure/function they will by locked by the select.

Why is that?

To demonstrate this behavior:

CREATE TABLE Foo (
   ID INT NOT NULL AUTO_INCREMENT,
   Bar VARCHAR(200),
   PRIMARY KEY(ID)) ENGINE=MyISAM;

--INSERT into Foo 10M rows


DELIMITER $$

DROP PROCEDURE IF EXISTS InsertProc$$

CREATE PROCEDURE InsertProc(IN vBar VARCHAR(255))
BEGIN
    INSERT Foo(Bar) VALUES (vBar);
END$$

DELIMITER ;

Run the following query:

SELECT Count(*) FROM Foo WHERE INSTR(Bar, 'abcdefg') > 0;

While that Select is running, open a new connection and run the following insert query:

INSERT Foo(Bar) VALUES ('xyz1234');

That Insert will run and return right away, However if i run the following query:

CALL InsertProc('xyz1234');

Now the query locks and waits for the select to complete.

MySql Version: 5.0.51 running on Window Server 2K3

Thank you.

-- UPDATE Here is the profile output:

Insert Direct:

(initialization)     0.0000432
checking permissions 0.0000074
Opening tables       0.0000077
System lock          0.0000032
Table lock           0.0000025
init                 0.000021
update               0.0002365
end                  0.0000382
query end            0.000002
freeing items        0.0000057
closing tables       0.0000022
logging slow query   0.0000005

Insert via Procedure:

(initialization) 0.0000285
Opening tables   0.0004325
System lock      0.0000022
Table lock       0.0002957
checking permissions 0.0000047
Opening tables   0.000004
System lock      0.0000017
Table lock       3.2365122
init             0.0000422
update           0.000251
end              0.0000025
query end        0.000003
closing tables   0.00004
query end        0.0000074
freeing items    0.0000074
logging slow query 0.000001
cleaning up      0.5790915

Why does the procedure open and "Table lock" twice?

Answer 1

This issue was submitted as a bug: http://bugs.mysql.com/bug.php?id=58689