Mysql – Does limit impact affected rows or not

explainMySQLoptimizationperformancequery-performance

I have this table:

CREATE TABLE IF NOT EXISTS `usergroups` (
      `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
      `user_id` int(11) unsigned NOT NULL,
      `group_id` smallint(5) unsigned NOT NULL,
      PRIMARY KEY (`group_id`,`user_id`),
      KEY `id` (`id`),
      KEY `user_id` (`user_id`)
    ) ENGINE=InnoDB  DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci AUTO_INCREMENT=5496 ;

when run this query:

EXPLAIN SELECT `UserGroup`.`user_id` FROM   `usergroups` AS `UserGroup` WHERE  `UserGroup`.`group_id` = 1 limit 30

its out put is:

1       SIMPLE        UserGroup      ref  PRIMARY  PRIMARY  2        const  543      Using index

I think something is wrong! because it affected 543 rows but I think it should affect 30 rows at maximum. Is it true?

Best Answer

Limit does have an impact on how many rows are selected/affected. 543 is just the number of rows which match the joins and the where clause.

Related Solutions

Mysql – Are two indexes needed

An index can seek by a subset of characters, as long as you're searching from the left. E.g., "Inter%" can seek, "%net" will not.

However, the first character is not necessarily the character under which the article would be sorted. "The Internet" should go under "I", not "T". You probably need two fields, DisplayTitle and SortTitle; a single-character index on the latter may be worthwhile, but most likely a full-length index will be just fine.

Indexes are typically B-trees, and a seek will jump to the right location about equally quickly whether you have 10 or 100 entries per page. Scans are another matter, but I'd start with the simplest solution and add an extra index only if performance proves inadequate in practice.

Mysql – Finding rows for a specified date range

Another way to get the result is this. It finds first all groups that the teacher has surely taught (or is going to) by checking that she has started within the month and then in another subquery it finds - for every group - the last teacher that started at the first day of the month or earlier.

With the unique index you have on the table, the second subquery should be quite efficient. The first subquery would benefit from an index on (teacherid, startdate, groupid):

SELECT groupid
FROM pupilgroupteacher
WHERE teacherid = @teacher 
  AND startdate >= @month + INTERVAL 1 DAY
  AND startdate < @month + INTERVAL 1 MONTH

UNION DISTINCT

SELECT gg.groupid
FROM 
    ( SELECT DISTINCT groupid
      FROM pupilgroupteacher
    ) AS gd
  JOIN pupilgroupteacher AS gg
    ON  gg.groupid = gd.groupid
    AND gg.startdate =
        ( SELECT MAX(gi.startdate)
          FROM pupilgroupteacher AS gi 
          WHERE gi.groupid = gd.groupid
            AND gi.startdate < @month + INTERVAL 1 DAY
        )
WHERE gg.teacherid = @teacher ;

Best Answer

Related Solutions

Mysql – Are two indexes needed

Mysql – Finding rows for a specified date range

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