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5f911fa874
Bug#54678: InnoDB, TRUNCATE, ALTER, I_S SELECT, crash or deadlock - Incompatible change: truncate no longer resorts to a row by row delete if the storage engine does not support the truncate method. Consequently, the count of affected rows does not, in any case, reflect the actual number of rows. - Incompatible change: it is no longer possible to truncate a table that participates as a parent in a foreign key constraint, unless it is a self-referencing constraint (both parent and child are in the same table). To work around this incompatible change and still be able to truncate such tables, disable foreign checks with SET foreign_key_checks=0 before truncate. Alternatively, if foreign key checks are necessary, please use a DELETE statement without a WHERE condition. Problem description: The problem was that for storage engines that do not support truncate table via a external drop and recreate, such as InnoDB which implements truncate via a internal drop and recreate, the delete_all_rows method could be invoked with a shared metadata lock, causing problems if the engine needed exclusive access to some internal metadata. This problem originated with the fact that there is no truncate specific handler method, which ended up leading to a abuse of the delete_all_rows method that is primarily used for delete operations without a condition. Solution: The solution is to introduce a truncate handler method that is invoked when the engine does not support truncation via a table drop and recreate. This method is invoked under a exclusive metadata lock, so that there is only a single instance of the table when the method is invoked. Also, the method is not invoked and a error is thrown if the table is a parent in a non-self-referencing foreign key relationship. This was necessary to avoid inconsistency as some integrity checks are bypassed. This is inline with the fact that truncate is primarily a DDL operation that was designed to quickly remove all data from a table.
68 lines
2.3 KiB
Text
68 lines
2.3 KiB
Text
#
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# TRUNCATE TABLE
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#
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# Truncating is disallowed for parent tables unless such table
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# participates in self-referencing foreign keys only.
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#
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CREATE TABLE t1 (pk INT PRIMARY KEY) ENGINE=INNODB;
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CREATE TABLE t2 (fk INT NOT NULL, FOREIGN KEY (fk) REFERENCES t1 (pk)) ENGINE=INNODB;
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TRUNCATE TABLE t1;
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ERROR 42000: Cannot truncate a table referenced in a foreign key constraint (`test`.`t2`, CONSTRAINT `t2_ibfk_1` FOREIGN KEY (`fk`) REFERENCES `test`.`t1` (`pk`))
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# Truncation of child should succeed.
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TRUNCATE TABLE t2;
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DROP TABLE t2;
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DROP TABLE t1;
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CREATE TABLE t1 (pk INT PRIMARY KEY, fk INT,
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FOREIGN KEY (fk) REFERENCES t1 (pk)) ENGINE=INNODB;
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# Truncation of self-referencing table should succeed.
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TRUNCATE TABLE t1;
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DROP TABLE t1;
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#
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# Also, truncating such tables is allowed if foreign key
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# checks are disabled.
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#
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SET @old_foreign_key_checks = @@SESSION.foreign_key_checks;
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CREATE TABLE t1 (pk INT PRIMARY KEY) ENGINE=INNODB;
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CREATE TABLE t2 (fk INT NOT NULL, FOREIGN KEY (fk) REFERENCES t1 (pk)) ENGINE=INNODB;
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CREATE TABLE t3 (pk INT PRIMARY KEY, fk INT,
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FOREIGN KEY (fk) REFERENCES t1 (pk)) ENGINE=INNODB;
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SET @@SESSION.foreign_key_checks = 0;
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TRUNCATE TABLE t1;
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TRUNCATE TABLE t2;
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TRUNCATE TABLE t3;
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SET @@SESSION.foreign_key_checks = 1;
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TRUNCATE TABLE t1;
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ERROR 42000: Cannot truncate a table referenced in a foreign key constraint (`test`.`t2`, CONSTRAINT `t2_ibfk_1` FOREIGN KEY (`fk`) REFERENCES `test`.`t1` (`pk`))
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TRUNCATE TABLE t2;
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TRUNCATE TABLE t3;
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LOCK TABLES t1 WRITE;
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SET @@SESSION.foreign_key_checks = 0;
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TRUNCATE TABLE t1;
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SET @@SESSION.foreign_key_checks = 1;
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TRUNCATE TABLE t1;
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ERROR 42000: Cannot truncate a table referenced in a foreign key constraint (`test`.`t2`, CONSTRAINT `t2_ibfk_1` FOREIGN KEY (`fk`) REFERENCES `test`.`t1` (`pk`))
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UNLOCK TABLES;
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DROP TABLE t3,t2,t1;
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SET @@SESSION.foreign_key_checks = @old_foreign_key_checks;
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#
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# Test that TRUNCATE resets auto-increment.
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#
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CREATE TABLE t1 (a INT PRIMARY KEY NOT NULL AUTO_INCREMENT);
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INSERT INTO t1 VALUES (NULL), (NULL);
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SELECT AUTO_INCREMENT FROM INFORMATION_SCHEMA.TABLES WHERE table_name = 't1';
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AUTO_INCREMENT
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3
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SELECT * FROM t1 ORDER BY a;
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a
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1
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2
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TRUNCATE TABLE t1;
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SELECT AUTO_INCREMENT FROM INFORMATION_SCHEMA.TABLES WHERE table_name = 't1';
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AUTO_INCREMENT
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1
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INSERT INTO t1 VALUES (NULL), (NULL);
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SELECT * FROM t1 ORDER BY a;
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a
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1
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2
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DROP TABLE t1;
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