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POMySQL Slow Query Analysis and Indexing
 

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    <p>Recently, we've noticed a particular query popping up in our slow query logs taking quite some time. I've analyzed it to the best of my ability, but can't figure out for the life of me why it's taking so long, and why the indexes we've set up aren't being used. </p> <p>Here's a simplified (i.e., readable) version of the query for the purpose of example:</p> <pre><code>SELECT processstage.id AS processstage_id, processstage.job_id AS processstage_job_id, processstage.event_id AS processstage_event_id, ... FROM processstage INNER JOIN jobevent ON jobevent.id = processstage.event_id WHERE processstage.due_date &lt;= '2009-10-28 16:07:59' AND (EXISTS ( SELECT 1 FROM job WHERE jobevent.job_id = job.id AND job.brand_id = 1 )) ORDER BY processstage.due_date; </code></pre> <p>Also, for good measure, it's important to note the size of the table, <code>processstage</code>:</p> <pre><code>mysql&gt; SELECT COUNT(id) FROM processstage; +-----------+ | COUNT(id) | +-----------+ | 596183 | +-----------+ </code></pre> <p>When I run EXPLAIN on the query, I find that the processstage table is reading over a <em>huge</em> number of rows (see "Using where; Using filesort"), as no index is being used (that I can tell):</p> <pre><code>mysql&gt; EXPLAIN SELECT processstage.id AS processstage_id, processstage.job_id AS processstage_job_id, processstage.event_id AS processstage_event_id FROM processstage INNER JOIN jobevent ON jobevent.id = processstage.event_id WHERE processstage.due_date &lt;= '2009-10-28 16:07:59' AND (EXISTS (SELECT 1 FROM job WHERE jobevent.job_id = job.id AND job.brand_id = 1)) ORDER BY processstage.due_date; +----+--------------------+--------------+--------+---------------------------------------------------+---------+---------+------------------------------+--------+-----------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+--------------------+--------------+--------+---------------------------------------------------+---------+---------+------------------------------+--------+-----------------------------+ | 1 | PRIMARY | processstage | ALL | ix_processstage_due_date,processstage_event_id_fk | NULL | NULL | NULL | 606045 | Using where; Using filesort | | 1 | PRIMARY | jobevent | eq_ref | PRIMARY | PRIMARY | 4 | processstage.event_id | 1 | Using where | | 2 | DEPENDENT SUBQUERY | job | eq_ref | PRIMARY,ix_job_brand_id | PRIMARY | 4 | jobevent.job_id | 1 | Using where | +----+--------------------+--------------+--------+---------------------------------------------------+---------+---------+------------------------------+--------+-----------------------------+ 3 rows in set (0.00 sec) </code></pre> <p>Oddly enough, we've got indexes on the columns utilized in the query's WHERE clause:</p> <pre><code>mysql&gt; SHOW INDEXES FROM processstage; +--------------+------------+----------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | +--------------+------------+----------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ | processstage | 0 | PRIMARY | 1 | id | A | 614150 | NULL | NULL | | BTREE | | | processstage | 1 | ix_processstage_job_id | 1 | job_id | A | 47242 | NULL | NULL | YES | BTREE | | | processstage | 1 | ix_processstage_stop_date | 1 | stop_date | A | 614150 | NULL | NULL | YES | BTREE | | | processstage | 1 | ix_processstage_order | 1 | order | A | 16 | NULL | NULL | YES | BTREE | | | processstage | 1 | ix_processstage_start_date | 1 | start_date | A | 122830 | NULL | NULL | YES | BTREE | | | processstage | 1 | ix_processstage_milestone | 1 | milestone | A | 12794 | NULL | NULL | YES | BTREE | | | processstage | 1 | ix_processstage_due_date | 1 | due_date | A | 51179 | NULL | NULL | YES | BTREE | | | processstage | 1 | ix_processstage_process_id | 1 | process_id | A | 76768 | NULL | NULL | YES | BTREE | | | processstage | 1 | processstage_event_id_fk | 1 | event_id | A | 3722 | NULL | NULL | YES | BTREE | | +--------------+------------+----------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ </code></pre> <p>Any ideas as to why our indexes are going seemingly unused? Perhaps I'm missing something very obvious or am approaching this entirely the wrong way. Any input or suggestion is very much appreciated!</p>
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