https://bugzilla.wikimedia.org/show_bug.cgi?id=24479
--- Comment #8 from Reedy <s...@reedyboy.net> 2010-12-31 06:00:11 UTC ---
mysql> describe SELECT cr_id,cr_repo_id,cr_status,COUNT(DISTINCT cc_id) AS
comments,cr_path,cr_message,cr_author,cr_timestamp FROM
`mw_code_tags`,`mw_code_rev` LEFT JOIN `mw_code_comment` ON ((cc_repo_id =
cr_repo_id AND cc_rev_id = cr_id)) WHERE cr_repo_id = '3' AND (ct_repo_id =
'3') AND (cr_id=ct_rev_id) AND ct_tag = 'api' LIMIT 51;
+----+-------------+-----------------+--------+-----------------------------------+------------+---------+--------------------------------------------------------+------+--------------------------+
| id | select_type | table | type | possible_keys
| key | key_len | ref
| rows | Extra |
+----+-------------+-----------------+--------+-----------------------------------+------------+---------+--------------------------------------------------------+------+--------------------------+
| 1 | SIMPLE | mw_code_tags | ref | PRIMARY,ct_repo_id
| PRIMARY | 4 | const
| 1 | Using where; Using index |
| 1 | SIMPLE | mw_code_rev | eq_ref |
PRIMARY,cr_repo_id,cr_repo_author | PRIMARY | 8 |
const,wikidb.mw_code_tags.ct_rev_id | 1 |
|
| 1 | SIMPLE | mw_code_comment | ref | cc_repo_id,cc_repo_time
| cc_repo_id | 8 |
wikidb.mw_code_rev.cr_repo_id,wikidb.mw_code_rev.cr_id | 1 | Using index
|
+----+-------------+-----------------+--------+-----------------------------------+------------+---------+--------------------------------------------------------+------+--------------------------+
3 rows in set (0.00 sec)
mysql> describe SELECT cr_id,cr_repo_id,cr_status,COUNT(DISTINCT cc_id) AS
comments,cr_path,cr_message,cr_author,cr_timestamp FROM
`mw_code_tags`,`mw_code_rev` LEFT JOIN `mw_code_comment` ON ((cc_repo_id =
cr_repo_id AND cc_rev_id = cr_id)) WHERE cr_repo_id = '3' AND (ct_repo_id =
'3') AND (cr_id=ct_rev_id) AND ct_tag = 'api' GROUP BY cr_id ORDER BY cr_id
DESC LIMIT 51;
+----+-------------+-----------------+--------+-----------------------------------+------------+---------+--------------------------------------------------------+------+-----------------------------------------------------------+
| id | select_type | table | type | possible_keys
| key | key_len | ref
| rows | Extra |
+----+-------------+-----------------+--------+-----------------------------------+------------+---------+--------------------------------------------------------+------+-----------------------------------------------------------+
| 1 | SIMPLE | mw_code_tags | ref | PRIMARY,ct_repo_id
| PRIMARY | 4 | const
| 1 | Using where; Using index; Using temporary; Using filesort |
| 1 | SIMPLE | mw_code_rev | eq_ref |
PRIMARY,cr_repo_id,cr_repo_author | PRIMARY | 8 |
const,wikidb.mw_code_tags.ct_rev_id | 1 |
|
| 1 | SIMPLE | mw_code_comment | ref | cc_repo_id,cc_repo_time
| cc_repo_id | 8 |
wikidb.mw_code_rev.cr_repo_id,wikidb.mw_code_rev.cr_id | 1 | Using index
|
+----+-------------+-----------------+--------+-----------------------------------+------------+---------+--------------------------------------------------------+------+-----------------------------------------------------------+
3 rows in set (0.00 sec)
mysql> describe SELECT cr_id,cr_repo_id,cr_status,COUNT(DISTINCT cc_id) AS
comments,cr_path,cr_message,cr_author,cr_timestamp FROM
`mw_code_tags`,`mw_code_rev` LEFT JOIN `mw_code_comment` ON ((cc_repo_id =
cr_repo_id AND cc_rev_id = cr_id)) WHERE cr_repo_id = '3' AND (ct_repo_id =
'3') AND (cr_id=ct_rev_id) AND ct_tag = 'api' ORDER BY cr_id DESC LIMIT 51;
+----+-------------+-----------------+--------+-----------------------------------+------------+---------+--------------------------------------------------------+------+-------------------------------------------+
| id | select_type | table | type | possible_keys
| key | key_len | ref
| rows | Extra |
+----+-------------+-----------------+--------+-----------------------------------+------------+---------+--------------------------------------------------------+------+-------------------------------------------+
| 1 | SIMPLE | mw_code_tags | ref | PRIMARY,ct_repo_id
| PRIMARY | 4 | const
| 1 | Using where; Using index; Using temporary |
| 1 | SIMPLE | mw_code_rev | eq_ref |
PRIMARY,cr_repo_id,cr_repo_author | PRIMARY | 8 |
const,wikidb.mw_code_tags.ct_rev_id | 1 |
|
| 1 | SIMPLE | mw_code_comment | ref | cc_repo_id,cc_repo_time
| cc_repo_id | 8 |
wikidb.mw_code_rev.cr_repo_id,wikidb.mw_code_rev.cr_id | 1 | Using index
|
+----+-------------+-----------------+--------+-----------------------------------+------------+---------+--------------------------------------------------------+------+-------------------------------------------+
Order BY only uses temporary
Group By (With/without order by) uses temporary and file sorts
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