MySQL using different index depending on limit value with ORDER BY query

问题

This is weird to me: One table 'ACTIVITIES' with one index on ACTIVITY_DATE. The exact same query with different LIMIT value results in different execution plan.

Here it is:

mysql> explain select * from ACTIVITIES order by ACTIVITY_DATE desc limit 20
    -> ;
+----+-------------+------------+-------+---------------+-------------+---------+------+------+-------+
| id | select_type | table      | type  | possible_keys | key         | key_len | ref  | rows | Extra |
+----+-------------+------------+-------+---------------+-------------+---------+------+------+-------+
|  1 | SIMPLE      | ACTIVITIES | index | NULL          | ACTI_DATE_I | 4       | NULL |   20 |       |
+----+-------------+------------+-------+---------------+-------------+---------+------+------+-------+
1 row in set (0.00 sec)

mysql> explain select * from ACTIVITIES order by ACTIVITY_DATE desc limit 150
    -> ;
+----+-------------+------------+------+---------------+------+---------+------+-------+----------------+
| id | select_type | table      | type | possible_keys | key  | key_len | ref  | rows  | Extra          |
+----+-------------+------------+------+---------------+------+---------+------+-------+----------------+
|  1 | SIMPLE      | ACTIVITIES | ALL  | NULL          | NULL | NULL    | NULL | 10629 | Using filesort |
+----+-------------+------------+------+---------------+------+---------+------+-------+----------------+
1 row in set (0.00 sec)

How come when I limit 150 it is not using the index? I mean, scanning 150 lines seems faster than scanning 10629 rows, right?

EDIT

The query uses the index till "limit 96" and starts filesort at "limit 97". The table has nothing specific, even not a foreign key, here is the complete create table:

mysql> show create table ACTIVITIES\G
*************************** 1. row ***************************
       Table: ACTIVITIES
Create Table: CREATE TABLE `ACTIVITIES` (
  `ACTIVITY_ID` int(11) NOT NULL AUTO_INCREMENT,
  `ACTIVITY_DATE` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP,
  `USER_KEY` varchar(50) NOT NULL,
  `ITEM_KEY` varchar(50) NOT NULL,
  `ACTIVITY_TYPE` varchar(1) NOT NULL,
  `EXTRA` varchar(500) DEFAULT NULL,
  `IS_VISIBLE` varchar(1) NOT NULL DEFAULT 'Y',
  PRIMARY KEY (`ACTIVITY_ID`),
  KEY `ACTI_USER_I` (`USER_KEY`,`ACTIVITY_DATE`),
  KEY `ACTIVITY_ITEM_I` (`ITEM_KEY`,`ACTIVITY_DATE`),
  KEY `ACTI_ITEM_TYPE_I` (`ITEM_KEY`,`ACTIVITY_TYPE`,`ACTIVITY_DATE`),
  KEY `ACTI_DATE_I` (`ACTIVITY_DATE`)
) ENGINE=InnoDB AUTO_INCREMENT=10091 DEFAULT CHARSET=utf8 COMMENT='Logs    activity'
1 row in set (0.00 sec)

mysql>

I also tried to run "ANALYSE TABLE ACTIVITIES" but that did not change a thing.

回答1:

That's the way things go. Bear with me a minute...

The Optimizer would like to use an INDEX, in this case ACTI_DATE_I. But it does not want to use it if that would be slower.

Plan A: Use the index.

Reach into the BTree-structured index at the end (because of DESC)
Scan backward
For each row in the index, look up the corresponding row in the data. Note: The index has (ACTIVITY_DATE, ACTIVITY_ID) because the PRIMARY KEY is implicitly appended to any secondary key. To reach into the "data" using the PK (ACTIVITY_ID) is another BTree lookup, potentially random. Hence, it is potentially slow. (But not very slow in your case.)
This stops after LIMIT rows.

Plan B: Ignore the table

Scan the table, building a tmp table. (Likely to be in-memory.)
Sort the tmp table
Peel off LIMIT rows.

In your case (96 -- 1% of 10K) it is surprising that it picked the table scan. Normally, the cutoff is somewhere around 10%-30% of the number of rows in the table.

ANALYZE TABLE should have caused a recalculation of the statistics, which could have convinced it to go with the other Plan.

What version of MySQL are you using? (No, I don't know of any changes in this area.)

One thing you could try: OPTIMIZE TABLE ACTIVITIES; That will rebuild the table, thereby repacking the blocks and leading to potentially different statistics. If that helps, I would like to know it -- since I normally say "Optimize table is useless".

来源：https://stackoverflow.com/questions/29125634/mysql-using-different-index-depending-on-limit-value-with-order-by-query

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