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POWhy is the index on s not used for sort here?
 

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    <p>Setup:</p> <pre><code>mysql&gt; create table test(id integer unsigned,s varchar(30)); Query OK, 0 rows affected (0.05 sec) mysql&gt; insert into test(id,s) value(1,'s'); Query OK, 1 row affected (0.00 sec) mysql&gt; insert into test(id,s) value(1,'tsr'); Query OK, 1 row affected (0.00 sec) mysql&gt; insert into test(id,s) value(1,'ts3r'); Query OK, 1 row affected (0.00 sec) mysql&gt; create index i_test_id on test(id); Query OK, 3 rows affected (0.08 sec) Records: 3 Duplicates: 0 Warnings: 0 mysql&gt; create index i_test_s on test(s); Query OK, 3 rows affected (0.05 sec) Records: 3 Duplicates: 0 Warnings: 0 mysql&gt; insert into test(id,s) value(21,'ts3r'); Query OK, 1 row affected (0.00 sec) </code></pre> <p>And then run this:</p> <pre><code>mysql&gt; explain select * from test where id in (1) order by s desc; +----+-------------+-------+------+---------------+-----------+---------+-------+------+-----------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+-----------+---------+-------+------+-----------------------------+ | 1 | SIMPLE | test | ref | i_test_id | i_test_id | 5 | const | 2 | Using where; Using filesort | +----+-------------+-------+------+---------------+-----------+---------+-------+------+-----------------------------+ 1 row in set (0.02 sec) </code></pre> <p>We can see it uses filesort instead of using the index on <code>s</code>,which will be slow when the selected result set is big.How to optimize it?</p>
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