How can I employ "if exists" for creating or dropping an index in MySQL?
Solution 1
Here is my 4 liner:
set @exist := (select count(*) from information_schema.statistics where table_name = 'table' and index_name = 'index' and table_schema = database());
set @sqlstmt := if( @exist > 0, 'select ''INFO: Index already exists.''', 'create index i_index on tablename ( columnname )');
PREPARE stmt FROM @sqlstmt;
EXECUTE stmt;
Solution 2
IF EXISTS
modifier is not built for DROP INDEX
or CREATE INDEX
yet. But you can check manually for the existence before creating/dropping an index.
Use this sentence to check whether the index already exists.
SHOW INDEX FROM table_name WHERE KEY_NAME = 'index_name'
- If the query returns zero (0) then the index does not exists, then you can create it.
- If the query returns a positive number, then the index exists, then you can drop it.
Solution 3
Here is a DROP INDEX IF EXISTS procedure:
DELIMITER $$
DROP PROCEDURE IF EXISTS drop_index_if_exists $$
CREATE PROCEDURE drop_index_if_exists(in theTable varchar(128), in theIndexName varchar(128) )
BEGIN
IF((SELECT COUNT(*) AS index_exists FROM information_schema.statistics WHERE TABLE_SCHEMA = DATABASE() and table_name =
theTable AND index_name = theIndexName) > 0) THEN
SET @s = CONCAT('DROP INDEX ' , theIndexName , ' ON ' , theTable);
PREPARE stmt FROM @s;
EXECUTE stmt;
END IF;
END $$
DELIMITER ;
This code was created based on the procedure from here: Determining if MySQL table index exists before creating
Solution 4
I tweaked answers found here and else where to come up with the following sprocs for dropping & creating indexes. Note that the AddTableIndex sproc can drop the index if need be. They also accept a schema name which was critical for my uses.
DELIMITER //
DROP PROCEDURE IF EXISTS migrate.DropTableIndex //
CREATE PROCEDURE migrate.DropTableIndex
(
in schemaName varchar(128) -- If null use name of current schema;
, in tableName varchar(128) -- If null an exception will be thrown.
, in indexName varchar(128) -- If null an exception will be thrown.
)
BEGIN
SET schemaName = coalesce(schemaName, schema());
IF((SELECT COUNT(*) AS index_exists FROM information_schema.statistics WHERE TABLE_SCHEMA = schemaName and table_name = tableName AND index_name = indexName) > 0) THEN
SET @s = CONCAT('DROP INDEX `' , indexName , '` ON `' , schemaName, '`.`', tableName, '`');
PREPARE stmt FROM @s;
EXECUTE stmt;
END IF;
END //
DROP PROCEDURE IF EXISTS migrate.AddTableIndex//
CREATE PROCEDURE migrate.AddTableIndex
(
IN schemaName varchar(128) -- If null use name of current schema;
, IN tableName varchar(128) -- If null an exception will be thrown.
, IN indexName varchar(128) -- If null an exception will be thrown.
, IN indexDefinition varchar(1024) -- E.g. '(expireTS_ ASC)'
, IN ifPresent ENUM('leaveUnchanged', 'dropAndReplace') -- null=leaveUnchanged.
, OUT outcome tinyint(1) -- 0=unchanged, 1=replaced, 4=added.
)
BEGIN
DECLARE doDrop tinyint(1) DEFAULT NULL;
DECLARE doAdd tinyint(1) DEFAULT NULL;
DECLARE tmpSql varchar(4096) DEFAULT '';
SET schemaName = coalesce(schemaName, schema());
SET ifPresent = coalesce(ifPresent, 'leaveUnchanged');
IF EXISTS (SELECT * FROM INFORMATION_SCHEMA.STATISTICS WHERE table_schema = schemaName AND table_name = tableName AND index_name = indexName) THEN
IF (ifPresent = 'leaveUnchanged') THEN
SET doDrop = 0;
SET doAdd = 0;
SET outcome = 0;
ELSEIF (ifPresent = 'dropAndReplace')
THEN
SET doDrop = 1;
SET doAdd = 1;
SET outcome = 1;
END IF;
ELSE
SET doDrop = 0;
SET doAdd = 1;
SET outcome = 4;
END IF;
IF (doDrop = 1) THEN
SET tmpSql = concat( 'alter table `', schemaName, '`.`', tableName, '` drop index `', indexName, '` ');
SET @sql = tmpSql;
PREPARE tmp_stmt FROM @sql;
EXECUTE tmp_stmt;
DEALLOCATE PREPARE tmp_stmt;
END IF;
IF (doAdd = 1) THEN
SET tmpSql = concat( 'alter table `', schemaName, '`.`', tableName, '` add index `', indexName, '` (', indexDefinition, ')');
SET @sql = tmpSql;
PREPARE tmp_stmt FROM @sql;
EXECUTE tmp_stmt;
DEALLOCATE PREPARE tmp_stmt;
END IF;
END;
//
DELIMITER ;
Solution 5
I have something similar with using SELECT IF() statement in MySQL.
select if (
exists(
select distinct index_name from information_schema.statistics
where table_schema = 'schema_db_name'
and table_name = 'tab_name' and index_name like 'index_1'
)
,'select ''index index_1 exists'' _______;'
,'create index index_1 on tab_name(column_name_names)') into @a;
PREPARE stmt1 FROM @a;
EXECUTE stmt1;
DEALLOCATE PREPARE stmt1;
The advantage of using if() statement is that, it doesn’t need a stored procedures.
Admin
Updated on May 23, 2020Comments
-
Admin almost 4 years
I was wondering if there's a way to check if an index exists before creating it or destroying it on MySQL. It appears that there was a feature request for this a few years back, but I can't find any documentation for a solution. This needs to be done in a PHP app using MDB2.
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still_dreaming_1 over 13 yearsThis query does not work for me. If I delete the WHERE clause it works, but then it is not very helpful. A KEY_name column does get displayed when I delete the WHERE clause though.
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Pablo Venturino over 13 yearsI just checked again to make sure, and worked for me. Maybe it's a version or configuration issue. I'm using version 5.1.45.
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VenerableAgents over 10 yearsAs someone who loads multiple databases with the same schema, I'd add to the WHERE on the first line: "AND TABLE_SCHEMA = DATABASE()"
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Andrew Gee over 10 years@PabloVenturino can this be checked "programatically" within an SQL statement?
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user3518901 over 9 years@VenerableAgents, I just edited the answer to contain that. Saved my life here :)
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ilasno almost 9 yearsNicely done, just tapped this for a migration script! :) You might want to utilize some _ characters in the procedure names, tho ;)
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PodTech.io over 8 yearswhen you select from information_schema, you should also specify the db.... SELECT @indexCount := COUNT(1) from information_schema.statistics WHERE table_schema='DB' ....... Otherwise you could pick out indexes from different DBs that have the same table and index name - caught me out.
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bmlkc about 7 yearsThis answer is great if you don't want to deal with any stored procedures.
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Mark Davich about 7 yearsThis works well on MariaDB. Workbench highlights the 'if' as if it is an error.
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Rob S over 4 yearsI think i should read :table_schema = 'database' rather than table_schema = database()
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famas23 about 2 yearsAny idea why we cant do if( @exist > 0, 'create index i_index on tablename ( columnname )'); ?