为什么这个 SQL 查询非常慢？
Why is this SQL query extremely slow?
更新：似乎查询缓存未激活，否则第二次执行同一查询时性能会好得多，因为 MySQL 应该已经缓存了它。我有 MariaDB 版本：10.2.41-
协议版本：10，确实从 MariaDB 10.1.7 开始默认禁用查询缓存。
在我的 VPS 配置上启用它是个好主意吗？或者我最好先升级它？
我有一个 MySQL 查询在具有 1GB RAM 和 1 CPU 核心的 CentOS VPS 上执行得非常慢。
查询需要 400 - 900 毫秒 来执行 (!!)，我想弄清楚如何优化。
它超过了几个 table，其中一些相当大（例如 Wordpress 的 wp_postmeta）。
但显然所有涉及的列都已正确编入索引，所以我想就如何操作寻求建议，或者唯一可能的优化是升级 VPS.
提前致谢！
这是查询和 EXPLAIN 输出
EXPLAIN SELECT c.post_id AS wc_variation_id,v.original_price_rand,v.price,c.meta_value AS color,s.meta_value AS size,bp.tier,t.discount_percent,MAX(IFNULL(tag.extra_price_tag,0)) AS extra_price_tag,MAX(IFNULL(lab.extra_price_label,0)) AS extra_price_label,
i.uri,i.branded,i.design_cost,ib.uri AS uri_back,ib.branded AS branded_back,ib.design_cost AS design_cost_back
FROM wp_postmeta c
JOIN brandly_tiers_bp_balances bp
JOIN brandly_tiers t ON t.role_name=bp.tier
JOIN wix_images i
JOIN wix_images_back ib
JOIN wix_imported_product_variations v ON v.wc_id=c.post_id
LEFT JOIN brandly_reseller_tags rt ON rt.user_id=bp.user_id
LEFT JOIN brandly_reseller_branded_packaging_sizes tag ON tag.size_label=rt.size_label
LEFT JOIN brandly_reseller_labels rl ON rl.user_id=bp.user_id
LEFT JOIN brandly_reseller_branded_packaging_sizes lab ON lab.size_label=rl.size_label
JOIN wp_postmeta s ON s.post_id=c.post_id
WHERE c.post_id IN (SELECT DISTINCT(wc_id) FROM wix_imported_product_variations WHERE wc_product_id = 181189)
AND s.meta_key = 'attribute_pa_size'
AND c.meta_key = 'attribute_pa_colour'
AND i.wix_variation_id=v.wix_id
AND ib.wix_variation_id=v.wix_id
AND v.store_id=36
AND bp.user_id=11
GROUP BY c.post_id
ORDER BY color,size
输出：
id
select_type
table
type
possible_keys
key
key_len
ref
rows
Extra
1
PRIMARY
bp
const
PRIMARY,tier
PRIMARY
8
const
1
Using temporary; Using filesort
1
PRIMARY
t
const
PRIMARY
PRIMARY
52
const
1
1
PRIMARY
wix_imported_product_variations
ref
wc_product_id,wc_id
wc_product_id
8
const
3
Start temporary
1
PRIMARY
rt
ref
PRIMARY
PRIMARY
8
const
5
Using index
1
PRIMARY
tag
eq_ref
PRIMARY
PRIMARY
32
brandly_xvfkg.rt.size_label
1
Using where
1
PRIMARY
c
ref
post_id,meta_key
post_id
8
brandly_xvfkg.wix_imported_product_variations.wc_id
14
Using index condition; Using where; End temporary
1
PRIMARY
v
ref
PRIMARY,store_id,wc_id
wc_id
8
brandly_xvfkg.c.post_id
1
Using index condition; Using where
1
PRIMARY
ib
ref
wix_variation_id
wix_variation_id
103
brandly_xvfkg.v.wix_id
3
1
PRIMARY
i
ref
wix_variation_id
wix_variation_id
103
brandly_xvfkg.v.wix_id
3
1
PRIMARY
s
ref
post_id,meta_key
post_id
8
brandly_xvfkg.c.post_id
14
Using where
1
PRIMARY
rl
ref
PRIMARY
PRIMARY
8
const
5
Using index
1
PRIMARY
lab
eq_ref
PRIMARY
PRIMARY
32
brandly_xvfkg.rl.size_label
1
Using where
编辑：子查询需要 0.0005 秒，所以它不是罪魁祸首
EDIT2：wp_postmeta table 有 1709642 条记录，它是 MyISAM：
SHOW CREATE TABLE `wp_postmeta`;
CREATE TABLE `wp_postmeta` (
`meta_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`post_id` bigint(20) unsigned NOT NULL DEFAULT 0,
`meta_key` varchar(255) DEFAULT NULL,
`meta_value` longtext DEFAULT NULL,
PRIMARY KEY (`meta_id`),
KEY `post_id` (`post_id`),
KEY `meta_key` (`meta_key`(191))
) ENGINE=MyISAM AUTO_INCREMENT=3945402 DEFAULT CHARSET=utf8
EDIT3：实际上所有 table 都是 MyISAM，除了 wix_images、wix_images_back、wix_imported_product_variations 是 innoDB
EDIT4：刚刚将所有 table 转换为 innoDB。现在性能更好，需要 200-400 毫秒。不过还是慢。
EDIT5：在将 table 切换到 innoDB 之后，EXPLAIN 输出现在发生了一些变化。参见 https://codebeautify.org/htmlviewer/y225302bb
EDIT6：这里是table涉及的定义
CREATE TABLE `wp_postmeta` (
`meta_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`post_id` bigint(20) unsigned NOT NULL DEFAULT 0,
`meta_key` varchar(255) DEFAULT NULL,
`meta_value` longtext DEFAULT NULL,
PRIMARY KEY (`meta_id`),
KEY `post_id` (`post_id`),
KEY `meta_key` (`meta_key`(191))
) ENGINE=InnoDB AUTO_INCREMENT=3945402 DEFAULT CHARSET=utf8
CREATE TABLE `brandly_tiers_bp_balances` (
`user_id` bigint(20) NOT NULL,
`bp_balance` bigint(20) NOT NULL DEFAULT 0,
`bp_pending_balance` bigint(20) NOT NULL DEFAULT 0,
`tier` varchar(50) COLLATE latin1_general_ci NOT NULL DEFAULT 'Earth',
`tier_change_ts` bigint(20) DEFAULT NULL,
PRIMARY KEY (`user_id`),
KEY `tier` (`tier`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci
CREATE TABLE `brandly_tiers` (
`role_name` varchar(50) COLLATE latin1_general_ci NOT NULL,
`bp_threshold` int(11) NOT NULL DEFAULT 0,
`discount_percent` smallint(6) NOT NULL,
`bp_percent_branded_orders` smallint(6) NOT NULL,
`bp_percent_bulk_orders` smallint(6) NOT NULL,
`bp_percent_courier` smallint(6) NOT NULL,
`bp_percent_bulk_branded_packaging` smallint(6) NOT NULL,
`bp_percent_monthly_subscription_fee` smallint(6) NOT NULL,
`bulk_order_minimum_quantity` int(11) NOT NULL DEFAULT 10,
`bp_bulk_order_cap` int(11) NOT NULL DEFAULT 7500,
`tier_monthly_subscription_price` int(11) NOT NULL DEFAULT 0,
PRIMARY KEY (`role_name`),
UNIQUE KEY `bp_threshold` (`bp_threshold`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci
CREATE TABLE `wix_images` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`filename` text COLLATE latin1_general_ci NOT NULL,
`uri` longtext COLLATE latin1_general_ci NOT NULL,
`background_image_uri` longtext COLLATE latin1_general_ci NOT NULL,
`branded` tinyint(1) DEFAULT 0,
`wix_id` varchar(100) COLLATE latin1_general_ci DEFAULT NULL,
`wix_product_id` varchar(100) COLLATE latin1_general_ci DEFAULT NULL,
`wix_variation_id` varchar(100) COLLATE latin1_general_ci DEFAULT NULL,
`design_cost` decimal(18,2) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `wix_id` (`wix_id`),
KEY `wix_product_id` (`wix_product_id`),
KEY `wix_variation_id` (`wix_variation_id`),
KEY `branded` (`branded`),
KEY `background_image_uri` (`background_image_uri`(8))
) ENGINE=InnoDB AUTO_INCREMENT=10680 DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci
CREATE TABLE `wix_images_back` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`filename` text COLLATE latin1_general_ci NOT NULL,
`uri` longtext COLLATE latin1_general_ci NOT NULL,
`background_image_uri` longtext COLLATE latin1_general_ci NOT NULL,
`branded` tinyint(1) DEFAULT 0,
`wix_id` varchar(100) COLLATE latin1_general_ci DEFAULT NULL,
`wix_product_id` varchar(100) COLLATE latin1_general_ci DEFAULT NULL,
`wix_variation_id` varchar(100) COLLATE latin1_general_ci DEFAULT NULL,
`design_cost` decimal(18,2) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `wix_id` (`wix_id`),
KEY `wix_product_id` (`wix_product_id`),
KEY `wix_variation_id` (`wix_variation_id`),
KEY `branded` (`branded`),
KEY `background_image_uri` (`background_image_uri`(8))
) ENGINE=InnoDB AUTO_INCREMENT=7733 DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci
CREATE TABLE `wix_imported_product_variations` (
`wix_id` varchar(100) COLLATE latin1_general_ci NOT NULL,
`store_id` int(11) NOT NULL,
`wc_id` bigint(20) NOT NULL,
`wc_product_id` bigint(20) NOT NULL,
`sku` varchar(100) COLLATE latin1_general_ci NOT NULL,
`price` decimal(18,2) NOT NULL,
`original_price_rand` decimal(18,2) NOT NULL,
`has_markup` tinyint(1) DEFAULT 1,
`wix_image_id` varchar(1000) COLLATE latin1_general_ci NOT NULL,
`imported_at` datetime DEFAULT NULL,
`tmp_design_image` text COLLATE latin1_general_ci DEFAULT NULL,
`tmp_branded` tinyint(1) DEFAULT NULL,
`tmp_design_cost` decimal(18,2) DEFAULT NULL,
`tmp_design_image_back` text COLLATE latin1_general_ci DEFAULT NULL,
`tmp_branded_back` tinyint(1) DEFAULT NULL,
`tmp_design_cost_back` decimal(18,2) DEFAULT NULL,
PRIMARY KEY (`wix_id`),
KEY `sku` (`sku`),
KEY `store_id` (`store_id`),
KEY `has_markup` (`has_markup`),
KEY `imported_at` (`imported_at`),
KEY `wc_product_id` (`wc_product_id`),
KEY `wc_id` (`wc_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci
CREATE TABLE `brandly_reseller_tags` (
`user_id` bigint(20) NOT NULL,
`size_label` varchar(30) COLLATE latin1_general_ci NOT NULL,
`image_front1` mediumtext COLLATE latin1_general_ci NOT NULL,
`image_back1` mediumtext COLLATE latin1_general_ci NOT NULL,
`image_front2` mediumtext COLLATE latin1_general_ci NOT NULL,
`image_back2` mediumtext COLLATE latin1_general_ci NOT NULL,
`image_front3` mediumtext COLLATE latin1_general_ci NOT NULL,
`image_back3` mediumtext COLLATE latin1_general_ci NOT NULL,
`default_index` int(11) DEFAULT NULL,
`design_cost` decimal(18,2) DEFAULT NULL,
PRIMARY KEY (`user_id`,`size_label`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci
CREATE TABLE `brandly_reseller_labels` (
`user_id` bigint(20) NOT NULL,
`size_label` varchar(30) COLLATE latin1_general_ci NOT NULL,
`image_front1` mediumtext COLLATE latin1_general_ci NOT NULL,
`image_back1` mediumtext COLLATE latin1_general_ci NOT NULL,
`image_front2` mediumtext COLLATE latin1_general_ci NOT NULL,
`image_back2` mediumtext COLLATE latin1_general_ci NOT NULL,
`image_front3` mediumtext COLLATE latin1_general_ci NOT NULL,
`image_back3` mediumtext COLLATE latin1_general_ci NOT NULL,
`default_index` int(11) DEFAULT NULL,
`design_cost` decimal(18,2) DEFAULT NULL,
PRIMARY KEY (`user_id`,`size_label`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci
CREATE TABLE `brandly_reseller_branded_packaging_sizes` (
`size_label` varchar(30) COLLATE latin1_general_ci NOT NULL,
`size1_cm` int(11) NOT NULL,
`size2_cm` int(11) NOT NULL,
`size3_cm` int(11) NOT NULL,
`is_bag` tinyint(1) DEFAULT NULL,
`max_weight_kg` decimal(18,2) NOT NULL,
`extra_price` decimal(18,2) NOT NULL,
`extra_price_gift` decimal(18,2) NOT NULL,
`extra_price_tag` decimal(18,2) NOT NULL,
`extra_price_label` decimal(18,2) NOT NULL,
PRIMARY KEY (`size_label`),
KEY `size1_cm` (`size1_cm`),
KEY `size2_cm` (`size2_cm`),
KEY `size3_cm` (`size3_cm`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci
编辑7：
将 ON 条件从 JOIN 移动到 WHERE。执行时间 200-400ms 和以前一样。
EXPLAIN SELECT c.post_id AS wc_variation_id,v.original_price_rand,v.price,c.meta_value AS color,s.meta_value AS size,bp.tier,t.discount_percent,MAX(IFNULL(tag.extra_price_tag,0)) AS extra_price_tag,MAX(IFNULL(lab.extra_price_label,0)) AS extra_price_label,
i.uri,i.branded,i.design_cost,ib.uri AS uri_back,ib.branded AS branded_back,ib.design_cost AS design_cost_back
FROM wp_postmeta c
JOIN brandly_tiers_bp_balances bp
JOIN brandly_tiers t
JOIN wix_images i
JOIN wix_images_back ib
JOIN wix_imported_product_variations v
LEFT JOIN brandly_reseller_tags rt ON rt.user_id=bp.user_id
LEFT JOIN brandly_reseller_branded_packaging_sizes tag ON tag.size_label=rt.size_label
LEFT JOIN brandly_reseller_labels rl ON rl.user_id=bp.user_id
LEFT JOIN brandly_reseller_branded_packaging_sizes lab ON lab.size_label=rl.size_label
JOIN wp_postmeta s
WHERE c.post_id IN (SELECT DISTINCT(wc_id) FROM wix_imported_product_variations WHERE wc_product_id = 181189)
AND s.meta_key = 'attribute_pa_size'
AND c.meta_key = 'attribute_pa_colour'
AND i.wix_variation_id=v.wix_id
AND ib.wix_variation_id=v.wix_id
AND v.store_id=36
AND bp.user_id=11
AND s.post_id=c.post_id
AND t.role_name=bp.tier
AND v.wc_id=c.post_id
GROUP BY c.post_id
ORDER BY color,size
解释输出
id
select_type
table
type
possible_keys
key
key_len
ref
rows
Extra
1
PRIMARY
bp
const
PRIMARY,tier
PRIMARY
8
const
1
Using temporary; Using filesort
1
PRIMARY
t
const
PRIMARY
PRIMARY
52
const
1
1
PRIMARY
wix_imported_product_variations
ref
wc_product_id,wc_id
wc_product_id
8
const
3
Start temporary
1
PRIMARY
rt
ref
PRIMARY
PRIMARY
8
const
5
Using index
1
PRIMARY
tag
eq_ref
PRIMARY
PRIMARY
32
brandly_xvfkg.rt.size_label
1
Using where
1
PRIMARY
c
ref
post_id,meta_key
post_id
8
brandly_xvfkg.wix_imported_product_variations.wc_id
8
Using index condition; Using where; End temporary
1
PRIMARY
v
ref
PRIMARY,store_id,wc_id
wc_id
8
brandly_xvfkg.c.post_id
1
Using index condition; Using where
1
PRIMARY
ib
ref
wix_variation_id
wix_variation_id
103
brandly_xvfkg.v.wix_id
3
1
PRIMARY
i
ref
wix_variation_id
wix_variation_id
103
brandly_xvfkg.v.wix_id
3
1
PRIMARY
s
ref
post_id,meta_key
post_id
8
brandly_xvfkg.v.wc_id
8
Using index condition; Using where
1
PRIMARY
rl
ref
PRIMARY
PRIMARY
8
const
5
Using index
1
PRIMARY
lab
eq_ref
PRIMARY
PRIMARY
32
brandly_xvfkg.rl.size_label
1
Using where
在更大的服务器上花更多的钱不太可能对你有多大好处。
200-400 毫秒这样的查询还不错。可能没有 SQL 魔法可以让它低于 100 毫秒；它做了很多。 极慢 一词通常指的是需要几分钟或更长时间的查询。
column 的所有值都是正数，MAX(IFNULL(column, 0)) 就会产生与 MAX(column) 完全相同的结果。聚合函数会跳过空值。您也许可以简化使用该表达式的两个地方。
您的查询很难推理。如果这是我的查询，我会重写它以摆脱短 table 别名，而是将它们拼写出来。而且，我会重写它，以便每个 JOIN（包括 LEFT 和普通）都有一个 ON-clause 。注意 ON-clauses 可以是复合的：
FROM a
JOIN wp_postmeta ON a.id = wp_postmeta.postid
AND wp_postmeta.meta_key = 'attribute_pa_size'
这是我们 WordPress 黑客立即识别的查询模式。
对于普通 JOIN，将 ON-clauses 移动到 WHERE-clauses 对执行计划没有影响。
您的计划中的连接类型都是“const”、“eq-ref”或“ref”。太好了。
WordPress 的 postmeta table 索引有 field-proven 返工。您的 postmeta table 足够大——几乎有四兆行——您可能可以使用它。它将主键更改为满足查询模式要求的复合键。你的是 WooCommerce 安装中的常见模式。对于 MySQL 5.7 及更高版本，它是 this。
ALTER TABLE wp_postmeta
ADD UNIQUE KEY meta_id (meta_id),
DROP PRIMARY KEY,
ADD PRIMARY KEY (post_id, meta_key, meta_id),
DROP KEY meta_key,
ADD KEY meta_key (meta_key, meta_value(32), post_id, meta_id),
ADD KEY meta_value (meta_value(32), meta_id),
DROP KEY post_id;
Index WP MySQL For Speed 插件旨在为您安装这套索引，并在 MySQL / MariaDB 5.5 及更高版本上发挥最大作用。
我没有发现在您的 non-WordPress table 上建立索引有任何大问题。 可能有一些机会，但同样，您的查询很难阅读。
95% 的系统在关闭查询缓存后效果会更好。请记住，对 table 的 任何 修改会导致 QC 清除 table 的 所有 条目。如果 table 有很多写入，那么可能需要很多 CPU 周期来清除。
如果你只有1GB的RAM，QC的空间不大；改用 InnoDB 的 buffer_pool 空间。当您从 MyISAM 更改时，您是否降低了 key_buffer_size 并提高了 innodb_buffer_pool_size？
其中一些复合索引可能会有所帮助：
c: INDEX(post_id, meta_key, meta_value)
v: INDEX(wix_id, store_id, wc_id, original_price_rand, price)
s: INDEX(meta_key, post_id, meta_value)
bp: INDEX(user_id, tier)
t: INDEX(role_name, discount_percent)
tag: INDEX(size_label, extra_price_tag)
lab: INDEX(size_label, extra_price_label)
i: INDEX(wix_variation_id, uri, branded, design_cost)
ib: INDEX(wix_variation_id, uri, branded, design_cost)
rt: INDEX(user_id, size_label)
rl: INDEX(user_id, size_label)
添加复合索引时，删除具有相同前导列的索引。
也就是说，当你同时拥有INDEX(a)和INDEX(a,b)时，把前者扔掉。
更新：似乎查询缓存未激活，否则第二次执行同一查询时性能会好得多，因为 MySQL 应该已经缓存了它。我有 MariaDB 版本：10.2.41- 协议版本：10，确实从 MariaDB 10.1.7 开始默认禁用查询缓存。 在我的 VPS 配置上启用它是个好主意吗？或者我最好先升级它？
我有一个 MySQL 查询在具有 1GB RAM 和 1 CPU 核心的 CentOS VPS 上执行得非常慢。
查询需要 400 - 900 毫秒 来执行 (!!)，我想弄清楚如何优化。
它超过了几个 table，其中一些相当大（例如 Wordpress 的 wp_postmeta）。 但显然所有涉及的列都已正确编入索引，所以我想就如何操作寻求建议，或者唯一可能的优化是升级 VPS.
提前致谢！
这是查询和 EXPLAIN 输出
EXPLAIN SELECT c.post_id AS wc_variation_id,v.original_price_rand,v.price,c.meta_value AS color,s.meta_value AS size,bp.tier,t.discount_percent,MAX(IFNULL(tag.extra_price_tag,0)) AS extra_price_tag,MAX(IFNULL(lab.extra_price_label,0)) AS extra_price_label,
i.uri,i.branded,i.design_cost,ib.uri AS uri_back,ib.branded AS branded_back,ib.design_cost AS design_cost_back
FROM wp_postmeta c
JOIN brandly_tiers_bp_balances bp
JOIN brandly_tiers t ON t.role_name=bp.tier
JOIN wix_images i
JOIN wix_images_back ib
JOIN wix_imported_product_variations v ON v.wc_id=c.post_id
LEFT JOIN brandly_reseller_tags rt ON rt.user_id=bp.user_id
LEFT JOIN brandly_reseller_branded_packaging_sizes tag ON tag.size_label=rt.size_label
LEFT JOIN brandly_reseller_labels rl ON rl.user_id=bp.user_id
LEFT JOIN brandly_reseller_branded_packaging_sizes lab ON lab.size_label=rl.size_label
JOIN wp_postmeta s ON s.post_id=c.post_id
WHERE c.post_id IN (SELECT DISTINCT(wc_id) FROM wix_imported_product_variations WHERE wc_product_id = 181189)
AND s.meta_key = 'attribute_pa_size'
AND c.meta_key = 'attribute_pa_colour'
AND i.wix_variation_id=v.wix_id
AND ib.wix_variation_id=v.wix_id
AND v.store_id=36
AND bp.user_id=11
GROUP BY c.post_id
ORDER BY color,size
输出：
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
|---|---|---|---|---|---|---|---|---|---|
| 1 | PRIMARY | bp | const | PRIMARY,tier | PRIMARY | 8 | const | 1 | Using temporary; Using filesort |
| 1 | PRIMARY | t | const | PRIMARY | PRIMARY | 52 | const | 1 | |
| 1 | PRIMARY | wix_imported_product_variations | ref | wc_product_id,wc_id | wc_product_id | 8 | const | 3 | Start temporary |
| 1 | PRIMARY | rt | ref | PRIMARY | PRIMARY | 8 | const | 5 | Using index |
| 1 | PRIMARY | tag | eq_ref | PRIMARY | PRIMARY | 32 | brandly_xvfkg.rt.size_label | 1 | Using where |
| 1 | PRIMARY | c | ref | post_id,meta_key | post_id | 8 | brandly_xvfkg.wix_imported_product_variations.wc_id | 14 | Using index condition; Using where; End temporary |
| 1 | PRIMARY | v | ref | PRIMARY,store_id,wc_id | wc_id | 8 | brandly_xvfkg.c.post_id | 1 | Using index condition; Using where |
| 1 | PRIMARY | ib | ref | wix_variation_id | wix_variation_id | 103 | brandly_xvfkg.v.wix_id | 3 | |
| 1 | PRIMARY | i | ref | wix_variation_id | wix_variation_id | 103 | brandly_xvfkg.v.wix_id | 3 | |
| 1 | PRIMARY | s | ref | post_id,meta_key | post_id | 8 | brandly_xvfkg.c.post_id | 14 | Using where |
| 1 | PRIMARY | rl | ref | PRIMARY | PRIMARY | 8 | const | 5 | Using index |
| 1 | PRIMARY | lab | eq_ref | PRIMARY | PRIMARY | 32 | brandly_xvfkg.rl.size_label | 1 | Using where |
编辑：子查询需要 0.0005 秒，所以它不是罪魁祸首
EDIT2：wp_postmeta table 有 1709642 条记录，它是 MyISAM：
SHOW CREATE TABLE `wp_postmeta`;
CREATE TABLE `wp_postmeta` (
`meta_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`post_id` bigint(20) unsigned NOT NULL DEFAULT 0,
`meta_key` varchar(255) DEFAULT NULL,
`meta_value` longtext DEFAULT NULL,
PRIMARY KEY (`meta_id`),
KEY `post_id` (`post_id`),
KEY `meta_key` (`meta_key`(191))
) ENGINE=MyISAM AUTO_INCREMENT=3945402 DEFAULT CHARSET=utf8
EDIT3：实际上所有 table 都是 MyISAM，除了 wix_images、wix_images_back、wix_imported_product_variations 是 innoDB
EDIT4：刚刚将所有 table 转换为 innoDB。现在性能更好，需要 200-400 毫秒。不过还是慢。
EDIT5：在将 table 切换到 innoDB 之后，EXPLAIN 输出现在发生了一些变化。参见 https://codebeautify.org/htmlviewer/y225302bb
EDIT6：这里是table涉及的定义
CREATE TABLE `wp_postmeta` (
`meta_id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`post_id` bigint(20) unsigned NOT NULL DEFAULT 0,
`meta_key` varchar(255) DEFAULT NULL,
`meta_value` longtext DEFAULT NULL,
PRIMARY KEY (`meta_id`),
KEY `post_id` (`post_id`),
KEY `meta_key` (`meta_key`(191))
) ENGINE=InnoDB AUTO_INCREMENT=3945402 DEFAULT CHARSET=utf8
CREATE TABLE `brandly_tiers_bp_balances` (
`user_id` bigint(20) NOT NULL,
`bp_balance` bigint(20) NOT NULL DEFAULT 0,
`bp_pending_balance` bigint(20) NOT NULL DEFAULT 0,
`tier` varchar(50) COLLATE latin1_general_ci NOT NULL DEFAULT 'Earth',
`tier_change_ts` bigint(20) DEFAULT NULL,
PRIMARY KEY (`user_id`),
KEY `tier` (`tier`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci
CREATE TABLE `brandly_tiers` (
`role_name` varchar(50) COLLATE latin1_general_ci NOT NULL,
`bp_threshold` int(11) NOT NULL DEFAULT 0,
`discount_percent` smallint(6) NOT NULL,
`bp_percent_branded_orders` smallint(6) NOT NULL,
`bp_percent_bulk_orders` smallint(6) NOT NULL,
`bp_percent_courier` smallint(6) NOT NULL,
`bp_percent_bulk_branded_packaging` smallint(6) NOT NULL,
`bp_percent_monthly_subscription_fee` smallint(6) NOT NULL,
`bulk_order_minimum_quantity` int(11) NOT NULL DEFAULT 10,
`bp_bulk_order_cap` int(11) NOT NULL DEFAULT 7500,
`tier_monthly_subscription_price` int(11) NOT NULL DEFAULT 0,
PRIMARY KEY (`role_name`),
UNIQUE KEY `bp_threshold` (`bp_threshold`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci
CREATE TABLE `wix_images` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`filename` text COLLATE latin1_general_ci NOT NULL,
`uri` longtext COLLATE latin1_general_ci NOT NULL,
`background_image_uri` longtext COLLATE latin1_general_ci NOT NULL,
`branded` tinyint(1) DEFAULT 0,
`wix_id` varchar(100) COLLATE latin1_general_ci DEFAULT NULL,
`wix_product_id` varchar(100) COLLATE latin1_general_ci DEFAULT NULL,
`wix_variation_id` varchar(100) COLLATE latin1_general_ci DEFAULT NULL,
`design_cost` decimal(18,2) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `wix_id` (`wix_id`),
KEY `wix_product_id` (`wix_product_id`),
KEY `wix_variation_id` (`wix_variation_id`),
KEY `branded` (`branded`),
KEY `background_image_uri` (`background_image_uri`(8))
) ENGINE=InnoDB AUTO_INCREMENT=10680 DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci
CREATE TABLE `wix_images_back` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`filename` text COLLATE latin1_general_ci NOT NULL,
`uri` longtext COLLATE latin1_general_ci NOT NULL,
`background_image_uri` longtext COLLATE latin1_general_ci NOT NULL,
`branded` tinyint(1) DEFAULT 0,
`wix_id` varchar(100) COLLATE latin1_general_ci DEFAULT NULL,
`wix_product_id` varchar(100) COLLATE latin1_general_ci DEFAULT NULL,
`wix_variation_id` varchar(100) COLLATE latin1_general_ci DEFAULT NULL,
`design_cost` decimal(18,2) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `wix_id` (`wix_id`),
KEY `wix_product_id` (`wix_product_id`),
KEY `wix_variation_id` (`wix_variation_id`),
KEY `branded` (`branded`),
KEY `background_image_uri` (`background_image_uri`(8))
) ENGINE=InnoDB AUTO_INCREMENT=7733 DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci
CREATE TABLE `wix_imported_product_variations` (
`wix_id` varchar(100) COLLATE latin1_general_ci NOT NULL,
`store_id` int(11) NOT NULL,
`wc_id` bigint(20) NOT NULL,
`wc_product_id` bigint(20) NOT NULL,
`sku` varchar(100) COLLATE latin1_general_ci NOT NULL,
`price` decimal(18,2) NOT NULL,
`original_price_rand` decimal(18,2) NOT NULL,
`has_markup` tinyint(1) DEFAULT 1,
`wix_image_id` varchar(1000) COLLATE latin1_general_ci NOT NULL,
`imported_at` datetime DEFAULT NULL,
`tmp_design_image` text COLLATE latin1_general_ci DEFAULT NULL,
`tmp_branded` tinyint(1) DEFAULT NULL,
`tmp_design_cost` decimal(18,2) DEFAULT NULL,
`tmp_design_image_back` text COLLATE latin1_general_ci DEFAULT NULL,
`tmp_branded_back` tinyint(1) DEFAULT NULL,
`tmp_design_cost_back` decimal(18,2) DEFAULT NULL,
PRIMARY KEY (`wix_id`),
KEY `sku` (`sku`),
KEY `store_id` (`store_id`),
KEY `has_markup` (`has_markup`),
KEY `imported_at` (`imported_at`),
KEY `wc_product_id` (`wc_product_id`),
KEY `wc_id` (`wc_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci
CREATE TABLE `brandly_reseller_tags` (
`user_id` bigint(20) NOT NULL,
`size_label` varchar(30) COLLATE latin1_general_ci NOT NULL,
`image_front1` mediumtext COLLATE latin1_general_ci NOT NULL,
`image_back1` mediumtext COLLATE latin1_general_ci NOT NULL,
`image_front2` mediumtext COLLATE latin1_general_ci NOT NULL,
`image_back2` mediumtext COLLATE latin1_general_ci NOT NULL,
`image_front3` mediumtext COLLATE latin1_general_ci NOT NULL,
`image_back3` mediumtext COLLATE latin1_general_ci NOT NULL,
`default_index` int(11) DEFAULT NULL,
`design_cost` decimal(18,2) DEFAULT NULL,
PRIMARY KEY (`user_id`,`size_label`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci
CREATE TABLE `brandly_reseller_labels` (
`user_id` bigint(20) NOT NULL,
`size_label` varchar(30) COLLATE latin1_general_ci NOT NULL,
`image_front1` mediumtext COLLATE latin1_general_ci NOT NULL,
`image_back1` mediumtext COLLATE latin1_general_ci NOT NULL,
`image_front2` mediumtext COLLATE latin1_general_ci NOT NULL,
`image_back2` mediumtext COLLATE latin1_general_ci NOT NULL,
`image_front3` mediumtext COLLATE latin1_general_ci NOT NULL,
`image_back3` mediumtext COLLATE latin1_general_ci NOT NULL,
`default_index` int(11) DEFAULT NULL,
`design_cost` decimal(18,2) DEFAULT NULL,
PRIMARY KEY (`user_id`,`size_label`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci
CREATE TABLE `brandly_reseller_branded_packaging_sizes` (
`size_label` varchar(30) COLLATE latin1_general_ci NOT NULL,
`size1_cm` int(11) NOT NULL,
`size2_cm` int(11) NOT NULL,
`size3_cm` int(11) NOT NULL,
`is_bag` tinyint(1) DEFAULT NULL,
`max_weight_kg` decimal(18,2) NOT NULL,
`extra_price` decimal(18,2) NOT NULL,
`extra_price_gift` decimal(18,2) NOT NULL,
`extra_price_tag` decimal(18,2) NOT NULL,
`extra_price_label` decimal(18,2) NOT NULL,
PRIMARY KEY (`size_label`),
KEY `size1_cm` (`size1_cm`),
KEY `size2_cm` (`size2_cm`),
KEY `size3_cm` (`size3_cm`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci
编辑7： 将 ON 条件从 JOIN 移动到 WHERE。执行时间 200-400ms 和以前一样。
EXPLAIN SELECT c.post_id AS wc_variation_id,v.original_price_rand,v.price,c.meta_value AS color,s.meta_value AS size,bp.tier,t.discount_percent,MAX(IFNULL(tag.extra_price_tag,0)) AS extra_price_tag,MAX(IFNULL(lab.extra_price_label,0)) AS extra_price_label,
i.uri,i.branded,i.design_cost,ib.uri AS uri_back,ib.branded AS branded_back,ib.design_cost AS design_cost_back
FROM wp_postmeta c
JOIN brandly_tiers_bp_balances bp
JOIN brandly_tiers t
JOIN wix_images i
JOIN wix_images_back ib
JOIN wix_imported_product_variations v
LEFT JOIN brandly_reseller_tags rt ON rt.user_id=bp.user_id
LEFT JOIN brandly_reseller_branded_packaging_sizes tag ON tag.size_label=rt.size_label
LEFT JOIN brandly_reseller_labels rl ON rl.user_id=bp.user_id
LEFT JOIN brandly_reseller_branded_packaging_sizes lab ON lab.size_label=rl.size_label
JOIN wp_postmeta s
WHERE c.post_id IN (SELECT DISTINCT(wc_id) FROM wix_imported_product_variations WHERE wc_product_id = 181189)
AND s.meta_key = 'attribute_pa_size'
AND c.meta_key = 'attribute_pa_colour'
AND i.wix_variation_id=v.wix_id
AND ib.wix_variation_id=v.wix_id
AND v.store_id=36
AND bp.user_id=11
AND s.post_id=c.post_id
AND t.role_name=bp.tier
AND v.wc_id=c.post_id
GROUP BY c.post_id
ORDER BY color,size
解释输出
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | |
|---|---|---|---|---|---|---|---|---|---|---|
| 1 | PRIMARY | bp | const | PRIMARY,tier | PRIMARY | 8 | const | 1 | Using temporary; Using filesort | |
| 1 | PRIMARY | t | const | PRIMARY | PRIMARY | 52 | const | 1 | ||
| 1 | PRIMARY | wix_imported_product_variations | ref | wc_product_id,wc_id | wc_product_id | 8 | const | 3 | Start temporary | |
| 1 | PRIMARY | rt | ref | PRIMARY | PRIMARY | 8 | const | 5 | Using index | |
| 1 | PRIMARY | tag | eq_ref | PRIMARY | PRIMARY | 32 | brandly_xvfkg.rt.size_label | 1 | Using where | |
| 1 | PRIMARY | c | ref | post_id,meta_key | post_id | 8 | brandly_xvfkg.wix_imported_product_variations.wc_id | 8 | Using index condition; Using where; End temporary | |
| 1 | PRIMARY | v | ref | PRIMARY,store_id,wc_id | wc_id | 8 | brandly_xvfkg.c.post_id | 1 | Using index condition; Using where | |
| 1 | PRIMARY | ib | ref | wix_variation_id | wix_variation_id | 103 | brandly_xvfkg.v.wix_id | 3 | ||
| 1 | PRIMARY | i | ref | wix_variation_id | wix_variation_id | 103 | brandly_xvfkg.v.wix_id | 3 | ||
| 1 | PRIMARY | s | ref | post_id,meta_key | post_id | 8 | brandly_xvfkg.v.wc_id | 8 | Using index condition; Using where | |
| 1 | PRIMARY | rl | ref | PRIMARY | PRIMARY | 8 | const | 5 | Using index | |
| 1 | PRIMARY | lab | eq_ref | PRIMARY | PRIMARY | 32 | brandly_xvfkg.rl.size_label | 1 | Using where |
在更大的服务器上花更多的钱不太可能对你有多大好处。
200-400 毫秒这样的查询还不错。可能没有 SQL 魔法可以让它低于 100 毫秒；它做了很多。 极慢 一词通常指的是需要几分钟或更长时间的查询。
只要 MAX(IFNULL(column, 0))就会产生与MAX(column)完全相同的结果。聚合函数会跳过空值。您也许可以简化使用该表达式的两个地方。您的查询很难推理。如果这是我的查询，我会重写它以摆脱短 table 别名，而是将它们拼写出来。而且，我会重写它，以便每个 JOIN（包括 LEFT 和普通）都有一个 ON-clause 。注意 ON-clauses 可以是复合的：
FROM a JOIN wp_postmeta ON a.id = wp_postmeta.postid AND wp_postmeta.meta_key = 'attribute_pa_size'这是我们 WordPress 黑客立即识别的查询模式。
对于普通 JOIN，将 ON-clauses 移动到 WHERE-clauses 对执行计划没有影响。
您的计划中的连接类型都是“const”、“eq-ref”或“ref”。太好了。
WordPress 的 postmeta table 索引有 field-proven 返工。您的 postmeta table 足够大——几乎有四兆行——您可能可以使用它。它将主键更改为满足查询模式要求的复合键。你的是 WooCommerce 安装中的常见模式。对于 MySQL 5.7 及更高版本，它是 this。
ALTER TABLE wp_postmeta ADD UNIQUE KEY meta_id (meta_id), DROP PRIMARY KEY, ADD PRIMARY KEY (post_id, meta_key, meta_id), DROP KEY meta_key, ADD KEY meta_key (meta_key, meta_value(32), post_id, meta_id), ADD KEY meta_value (meta_value(32), meta_id), DROP KEY post_id;Index WP MySQL For Speed 插件旨在为您安装这套索引，并在 MySQL / MariaDB 5.5 及更高版本上发挥最大作用。
我没有发现在您的 non-WordPress table 上建立索引有任何大问题。
column 的所有值都是正数，95% 的系统在关闭查询缓存后效果会更好。请记住，对 table 的 任何 修改会导致 QC 清除 table 的 所有 条目。如果 table 有很多写入，那么可能需要很多 CPU 周期来清除。
如果你只有1GB的RAM，QC的空间不大；改用 InnoDB 的 buffer_pool 空间。当您从 MyISAM 更改时，您是否降低了 key_buffer_size 并提高了 innodb_buffer_pool_size？
其中一些复合索引可能会有所帮助：
c: INDEX(post_id, meta_key, meta_value)
v: INDEX(wix_id, store_id, wc_id, original_price_rand, price)
s: INDEX(meta_key, post_id, meta_value)
bp: INDEX(user_id, tier)
t: INDEX(role_name, discount_percent)
tag: INDEX(size_label, extra_price_tag)
lab: INDEX(size_label, extra_price_label)
i: INDEX(wix_variation_id, uri, branded, design_cost)
ib: INDEX(wix_variation_id, uri, branded, design_cost)
rt: INDEX(user_id, size_label)
rl: INDEX(user_id, size_label)
添加复合索引时，删除具有相同前导列的索引。 也就是说，当你同时拥有INDEX(a)和INDEX(a,b)时，把前者扔掉。