如何在MySQL中创建或删除索引时使用“if exists”？

Question

我想知道是否有办法在创建索引或在MySQL上销毁它之前检查索引是否存在。几年前似乎有一个功能请求，但我找不到任何解决方案的文档。这需要在使用MDB2的PHP应用程序中完成。

Answer 1

这是我的4班轮：

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;

Answer 2

IF EXISTS修饰符尚未针对DROP INDEX或CREATE INDEX构建。但您可以在创建/删除索引之前手动检查是否存在。

使用此句来检查索引是否已存在。

SHOW INDEX FROM table_name WHERE KEY_NAME = 'index_name'

• 如果查询返回零（0），则索引不存在，则可以创建它。
• 如果查询返回正数，则索引存在，则可以删除它。

Answer 3

这是DROP INDEX IF EXISTS程序：

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 ;

此代码是根据此处的过程创建的：Determining if MySQL table index exists before creating

Answer 4

我调整了在这里找到的答案，以及在哪里提出以下的sprocs for drop＆amp;创建索引。请注意，如果需要，AddTableIndex sproc可以删除索引。他们还接受一个对我的用途至关重要的模式名称。

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 ;

Answer 5

我在MySQL中使用SELECT IF（）语句有类似的东西。

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;

使用if（）语句的优点是不需要编写任何过程。

Answer 6

MySQL Workbench版本6.3（MySql fork MariaDb）

DROP INDEX IF EXISTS FK_customer__client_school__school_id ON dbname.tablename;

Answer 7

我遇到了这里提出的一些解决方案的问题。这就是我想出的：

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 ;

按如下方式使用：

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

这是在使用MySQL 5.5.24的MAC OS X 10.8.2和使用MySQL 5.5.21的Windows 7上测试的

Answer 8

我认为这对您删除现有索引很有帮助。

        DELIMITER //
        CREATE PROCEDURE dropIndexing
        ()
        BEGIN

        IF EXISTS(
                    SELECT * FROM information_schema.statistics 
                    WHERE TABLE_SCHEMA = DATABASE() 
                    AND `table_name`='mytable' 
                    AND `index_name` = 'myindex'
                )
        THEN
        ALTER TABLE `mytable` DROP INDEX `myindex`;
        END IF;

        END //
        DELIMITER ;

        CALL dropIndexing();
        DROP PROCEDURE dropIndexing;

Answer 9

这是DROP INDEX IF EXISTS的一种解决方法，在v10.1.4之前的MySQL和MariaDB版本中缺少。您还可以将其用于所需的其他所有语句，具体取决于INDEX的存在（例如，对于SELECT "info: index exists."，如下例所示）。

-- DROP INDEX IF EXISTS
SELECT
    COUNT(*)
INTO
    @INDEX_my_index_ON_TABLE_my_table_EXISTS
FROM
    `information_schema`.`statistics`
WHERE
    `table_schema` = 'my_database'
    AND `index_name` = 'my_index'
    AND `table_name` = 'my_table'
;
SET @statement := IF(
    @INDEX_my_index_ON_TABLE_my_table_EXISTS > 0,
    -- 'SELECT "info: index exists."',
    'DROP INDEX `my_index` ON `my_table`',
    'SELECT "info: index does not exist."'
);
PREPARE statement FROM @statement;
EXECUTE statement;