Question

I have the following stored procedure (Taken from a comment on http://dev.mysql.com/doc/refman/5.0/en/create-index.html, used to check if an index exists on a table):

DELIMITER $$
DROP PROCEDURE IF EXISTS `create_index_if_not_exists`$$

CREATE DEFINER=`user`@`%` PROCEDURE `create_index_if_not_exists`(table_name_vc varchar(50), index_name_vc varchar(50), field_list_vc varchar(200))
SQL SECURITY INVOKER
BEGIN

set @Index_cnt = (
select  count(1) cnt
FROM    INFORMATION_SCHEMA.STATISTICS
WHERE   table_name = table_name_vc
and index_name = index_name_vc
);

IF ifnull(@Index_cnt,0) = 0 THEN set @index_sql = concat('Alter table ',table_name_vc,' ADD INDEX ',index_name_vc,'(',field_list_vc,');');

PREPARE stmt FROM @index_sql;
EXECUTE stmt;

DEALLOCATE PREPARE stmt;

END IF;

END$$
DELIMITER ;

The strange thing is that it works perfectly on CentOS 5.4 with MySQL 5.5.25, but does not work on Mac OS X 10.8.1 with MySQL 5.5.24. On Mac, the @Index_cnt is always 0 if I add

select @Index_cnt

in the stored procedure. If I do the SELECT COUNT(1) cnt ... statement on it’s own, then 1 or 0 is returned as it should be.

Any ideas?

Answer 1

I have re-written the stored procedure and this now works on all platforms:

DELIMITER $$

DROP PROCEDURE IF EXISTS myschema.create_index_if_not_exists $$
CREATE PROCEDURE myschema.create_index_if_not_exists(in p_tableName VARCHAR(128), in p_indexName VARCHAR(128), in p_columnName VARCHAR(128) )
BEGIN

PREPARE stmt FROM 'SELECT @indexCount := COUNT(1) from information_schema.statistics WHERE `table_name` = ? AND `index_name` = ?';
SET @table_name = p_tableName;
SET @index_name = p_indexName;
EXECUTE stmt USING @table_name, @index_name;
DEALLOCATE PREPARE stmt;

-- select @indexCount;

IF( @indexCount = 0 ) THEN
  SELECT 'Creating index';
  SET @createIndexStmt = CONCAT('CREATE INDEX ', p_indexName, ' ON ', p_tableName, ' ( ', p_columnName ,')');
  PREPARE stmt FROM @createIndexStmt;
  EXECUTE stmt;
  DEALLOCATE PREPARE stmt;
END IF;

END $$

DELIMITER ;

Use it as follows:

call myschema.create_index_if_not_exists('MyTable','end_time_index','end_time');

This was tested on MAC OS X 10.8.2 with MySQL 5.5.24 and on Windows 7 with MySQL 5.5.21