我在我的网站上遇到了一些每天都有更多流量的问题，我想知道如何优化这种对 MYSQL DB 的查询。
Query_time: 37
Lock_time: 2
Rows_sent: 10
Rows_examined: 354287
SET timestamp=1323180396;
询问：
Results for explain extended SELECT a.*, UNIX_TIMESTAMP(a.createdon) AS timestamp, ct.cityname, ct.SeoCityName, scat.subcatname, scat.SeoSubcatName, cat.catid, cat.SeoCatName, cat.catname,
COUNT(*) AS piccount, p.picfile, UNIX_TIMESTAMP(feat.featuredtill) AS featuredtill
FROM prefix_ads a
INNER JOIN prefix_es_cities ct ON a.cityid = ct.cityid
INNER JOIN prefix_es_subcats scat ON a.subcatid = scat.subcatid
INNER JOIN prefix_es_cats cat ON scat.catid = cat.catid
LEFT OUTER JOIN prefix_adxfields axf ON a.adid = axf.adid
LEFT OUTER JOIN prefix_adpics p ON a.adid = p.adid AND p.isevent = '0'
LEFT OUTER JOIN prefix_es_featured feat ON a.adid = feat.adid AND feat.adtype = 'A'
WHERE scat.catid = 2
AND a.enabled = '1' AND a.verified = '1' AND a.expireson >= NOW()
GROUP BY a.adid
ORDER BY a.createdon DESC
LIMIT 0, 18:
我们使用 EXPLAIN（我使用 PDO 进行数据库连接）
Array
(
[id] => 1
[0] => 1
[select_type] => SIMPLE
[1] => SIMPLE
[table] => cat
[2] => cat
[type] => const
[3] => const
[possible_keys] => PRIMARY
[4] => PRIMARY
[key] => PRIMARY
[5] => PRIMARY
[key_len] => 2
[6] => 2
[ref] => const
[7] => const
[rows] => 1
[8] => 1
[filtered] => 100.00
[9] => 100.00
[Extra] => Using temporary; Using filesort
[10] => Using temporary; Using filesort
)
Array
(
[id] => 1
[0] => 1
[select_type] => SIMPLE
[1] => SIMPLE
[table] => scat
[2] => scat
[type] => ref
[3] => ref
[possible_keys] => PRIMARY,catid
[4] => PRIMARY,catid
[key] => catid
[5] => catid
[key_len] => 2
[6] => 2
[ref] => const
[7] => const
[rows] => 12
[8] => 12
[filtered] => 100.00
[9] => 100.00
[Extra] =>
[10] =>
)
Array
(
[id] => 1
[0] => 1
[select_type] => SIMPLE
[1] => SIMPLE
[table] => a
[2] => a
[type] => ref
[3] => ref
[possible_keys] => subcatid,cityid,verified,enabled,expireson
[4] => subcatid,cityid,verified,enabled,expireson
[key] => subcatid
[5] => subcatid
[key_len] => 2
[6] => 2
[ref] => some_DB.scat.subcatid
[7] => some_DB.scat.subcatid
[rows] => 354287
[8] => 354287
[filtered] => 100.00
[9] => 100.00
[Extra] => Using where
[10] => Using where
)
Array
(
[id] => 1
[0] => 1
[select_type] => SIMPLE
[1] => SIMPLE
[table] => ct
[2] => ct
[type] => eq_ref
[3] => eq_ref
[possible_keys] => PRIMARY
[4] => PRIMARY
[key] => PRIMARY
[5] => PRIMARY
[key_len] => 2
[6] => 2
[ref] => some_DB.a.cityid
[7] => some_DB.a.cityid
[rows] => 1
[8] => 1
[filtered] => 100.00
[9] => 100.00
[Extra] =>
[10] =>
)
Array
(
[id] => 1
[0] => 1
[select_type] => SIMPLE
[1] => SIMPLE
[table] => axf
[2] => axf
[type] => ref
[3] => ref
[possible_keys] => adid
[4] => adid
[key] => adid
[5] => adid
[key_len] => 4
[6] => 4
[ref] => some_DB.a.adid
[7] => some_DB.a.adid
[rows] => 1
[8] => 1
[filtered] => 100.00
[9] => 100.00
[Extra] => Using index
[10] => Using index
)
Array
(
[id] => 1
[0] => 1
[select_type] => SIMPLE
[1] => SIMPLE
[table] => p
[2] => p
[type] => ref
[3] => ref
[possible_keys] => adid
[4] => adid
[key] => adid
[5] => adid
[key_len] => 5
[6] => 5
[ref] => some_DB.a.adid,const
[7] => some_DB.a.adid,const
[rows] => 3
[8] => 3
[filtered] => 100.00
[9] => 100.00
[Extra] =>
[10] =>
)
Array
(
[id] => 1
[0] => 1
[select_type] => SIMPLE
[1] => SIMPLE
[table] => feat
[2] => feat
[type] => eq_ref
[3] => eq_ref
[possible_keys] => adid
[4] => adid
[key] => adid
[5] => adid
[key_len] => 7
[6] => 7
[ref] => some_DB.a.adid,const
[7] => some_DB.a.adid,const
[rows] => 1
[8] => 1
[filtered] => 100.00
[9] => 100.00
[Extra] =>
[10] =>
)
来自同一个查询的更多解释
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cat const PRIMARY PRIMARY 2 const 1 Using temporary; Using filesort
1 SIMPLE scat ref PRIMARY,catid catid 2 const 12
1 SIMPLE a ref subcatid,cityid,verified,enabled,expireson subcatid 2 some_DB.scat.subcatid 354287 Using where
1 SIMPLE ct eq_ref PRIMARY PRIMARY 2 some_DB.a.cityid 1
1 SIMPLE axf ref adid adid 4 some_DB.a.adid 1 Using index
1 SIMPLE p ref adid adid 5 some_DB.a.adid,const 3
1 SIMPLE feat eq_ref adid adid 7 some_DB.a.adid,const 1
谢谢