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Employee Index merge annoyances fixed in MySQL 5.6
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While the index merge access types certainly are useful for a number of queries, there has been some frustration expressed both from customers and the community about how it...
  • is not used when it should have been
  • is used when ref access is obviously better
  • merges suboptimal indexes
  • is too restricted in which conditions can be used
  • I could come up with numerous examples of related bugs and feature requests dating back more than six years. To list a few: 17673, 30151, 23322, 65274, 65359.

    The good news is that we have fixed all these issues in the latest released MySQL 5.6 server.

    Consider the problem in BUG#65274 (simplified a bit): 
    CREATE TABLE phpbb_posts (
      ...
      KEY `topic_id` (`topic_id`),
      KEY `forum_id` (`forum_id`)
      KEY `tid_fid` (`topic_id`,`forum_id`),
    );
    SELECT count(*) FROM phpbb_posts WHERE topic_id = 110126 AND forum_id = 19;

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