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POMySQL: 'EXPLAIN' showing different explanations for similar queries
 

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  1. POMySQL: 'EXPLAIN' showing different explanations for similar queries
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    <p>I have a table 'posts' in database which has non-unique index on user_id (Key: MUL).</p> <pre><code>mysql&gt; show columns from posts; +---------+--------------+------+-----+-------------------+----------------+ | Field | Type | Null | Key | Default | Extra | +---------+--------------+------+-----+-------------------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | user_id | int(11) | YES | MUL | NULL | | | post | varchar(140) | NO | | NULL | | +---------+--------------+------+-----+-------------------+----------------+ </code></pre> <p>For this table, explain gives expected explanation where type is '<strong>REF</strong>'</p> <pre><code>mysql&gt; explain select * from posts where posts.user_id=1; +----+-------------+-------+------+---------------+---------+---------+-------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+---------+---------+-------+------+-------------+ | 1 | SIMPLE | posts | ref | user_id | user_id | 5 | const | 74 | Using where | +----+-------------+-------+------+---------------+---------+---------+-------+------+-------------+ </code></pre> <p>I have a second table 'followers' where 'user_id' and 'follower' are part of non-unique index</p> <pre><code>mysql&gt; show columns from followers; +---------------+-----------+------+-----+---------------------+----------------+ | Field | Type | Null | Key | Default | Extra | +---------------+-----------+------+-----+---------------------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | user_id | int(11) | YES | MUL | NULL | | | follower | int(11) | YES | MUL | NULL | | +---------------+-----------+------+-----+---------------------+----------------+ </code></pre> <p>But in this table, type is '<strong>ALL</strong>'. I expected it to be '<strong>REF</strong>' as similar to 'user_id' in previous table, this 'user_id' also has non-unique index. Is there any explanation for this?</p> <pre><code>mysql&gt; explain select * from followers where followers.user_id=1; +----+-------------+-----------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-----------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | followers | ALL | user_id | NULL | NULL | NULL | 6 | Using where | +----+-------------+-----------+------+---------------+------+---------+------+------+-------------+ </code></pre>
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    1. PO
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      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. This table or related slice is empty.
    1. USdb42
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    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
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    1. VO
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      1. POMySQL: 'EXPLAIN' showing different explanations for similar queries
      1. This table or related slice is empty.
      1. VTUpMod
    1. COMost likely because second table has a composite key from `(user_id, follower)` combined. Please post `show create table followers`
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      1. POMySQL: 'EXPLAIN' showing different explanations for similar queries
      1. USfavoretti
    2. COI hope @favoretti is correct.
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      1. POMySQL: 'EXPLAIN' showing different explanations for similar queries
      1. USJava Developers Guide
 

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