I didn’t know you could actually do this before addressing this problem in a benchmark using 5.0.36.

MySQL allows you to update two tables with a single UPDATE statement. I knew you could reference two tables in an UPDATE statement but not update both. However when working with a client after benchmarking I observed a large number of Created_tmp_disk_tables via SHOW GLOBAL STATUS and found that this query was the offending query only for certain circumstances. Understanding took a little longer.

schema.sql

DROP TABLE IF EXISTS a;
CREATE TABLE a(
id  INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
i   INT UNSIGNED NOT NULL,
c   CHAR(1) NOT NULL)
ENGINE=INNODB;
DROP TABLE IF EXISTS b;
CREATE TABLE b(
id  INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
i   INT UNSIGNED NOT NULL,
c   CHAR(1) NOT NULL,
t   TEXT NOT NULL)
ENGINE=INNODB;

data.sql

INSERT INTO a VALUES(1,1,'a');
INSERT INTO a VALUES(2,2,'b');
INSERT INTO a VALUES(3,3,'c');
INSERT INTO a VALUES(4,4,'d');
INSERT INTO b VALUES(1,1,'a',REPEAT('The quick brown fox jumped over the lazy dog',100));
INSERT INTO b VALUES(2,2,'b',REPEAT('The quick brown fox jumped over the lazy dog',200));
INSERT INTO b VALUES(3,3,'c',REPEAT('The quick brown fox jumped over the lazy dog',300));
INSERT INTO b VALUES(4,4,'d',REPEAT('The quick brown fox jumped over the lazy dog',400));

test.sql

SHOW SESSION STATUS LIKE '%tmp%tables';
SHOW SESSION STATUS LIKE '%tmp%tables';
UPDATE a,b SET a.i = 10, a.c = 'x', b.i = 20 WHERE a.id = b.id and a.id=1;
SHOW SESSION STATUS LIKE '%tmp%tables';
UPDATE a,b SET a.i = 10, a.c = 'x', b.i = 20,b.t=REPEAT('b',5000) WHERE a.id = b.id and a.id=3;
SHOW SESSION STATUS LIKE '%tmp%tables';
UPDATE b SET t = REPEAT('a',5000) WHERE b.id=2;
SHOW SESSION STATUS LIKE '%tmp%tables';

Output

The following output shows the following.

1. Impact of a SHOW SESSION STATUS increments Created_tmp_tables
2. A two table update increments Created_tmp_tables
3. A two table update including a text field increments Created_tmp_tables and Created_tmp_disk_tables
4. A single table update with the text field increments Created_tmp_tables

So the culprit was the combination of a two table update and a TEXT field.

mysql> source test.sql
--------------
SHOW SESSION STATUS LIKE '%tmp%tables'
--------------

+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| Created_tmp_disk_tables | 0     |
| Created_tmp_tables      | 3     |
+-------------------------+-------+
2 rows in set (0.00 sec)

--------------
SHOW SESSION STATUS LIKE '%tmp%tables'
--------------

+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| Created_tmp_disk_tables | 0     |
| Created_tmp_tables      | 4     |
+-------------------------+-------+
2 rows in set (0.00 sec)

--------------
UPDATE a,b SET a.i = 10, a.c = 'x', b.i = 20 WHERE a.id = b.id and a.id=1
--------------

Query OK, 2 rows affected (0.02 sec)
Rows matched: 2  Changed: 2  Warnings: 0

--------------
SHOW SESSION STATUS LIKE '%tmp%tables'
--------------

+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| Created_tmp_disk_tables | 0     |
| Created_tmp_tables      | 6     |
+-------------------------+-------+
2 rows in set (0.00 sec)

--------------
UPDATE a,b SET a.i = 10, a.c = 'x', b.i = 20,b.t=REPEAT('b',5000) WHERE a.id = b.id and a.id=3
--------------

Query OK, 2 rows affected (0.00 sec)
Rows matched: 2  Changed: 2  Warnings: 0

--------------
SHOW SESSION STATUS LIKE '%tmp%tables'
--------------

+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| Created_tmp_disk_tables | 1     |
| Created_tmp_tables      | 8     |
+-------------------------+-------+
2 rows in set (0.00 sec)

--------------
UPDATE b SET t = REPEAT('a',5000) WHERE b.id=2
--------------

Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

--------------
SHOW SESSION STATUS LIKE '%tmp%tables'
--------------

+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| Created_tmp_disk_tables | 1     |
| Created_tmp_tables      | 9     |
+-------------------------+-------+
2 rows in set (0.00 sec)

This entry was posted on Thursday, June 21st, 2007 at 2:55 pm and is filed under Databases, MySQL, Professional. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.