查询的正确索引答案

【问题标题】：The right indexes for query查询的正确索引
【发布时间】：2017-11-14 11:01:13
【问题描述】：

我有这个查询需要 1.7 秒才能执行：

SELECT
    i.id_items AS id, 
    i.names, 
    GROUP_CONCAT(DISTINCT iafv.face_value ORDER BY iafv.face_value ASC SEPARATOR ', ') AS facesValues, 
    GROUP_CONCAT(DISTINCT iacut.translation ORDER BY iacut.translation ASC SEPARATOR ', ') AS currencies, 
    GROUP_CONCAT(DISTINCT cout.translation ORDER BY cout.translation ASC SEPARATOR ', ') AS countries, 
    GROUP_CONCAT(DISTINCT iacot.translation ORDER BY iacot.translation ASC SEPARATOR ', ') AS compositions, 
    GROUP_CONCAT(DISTINCT iacc.catalog_code ORDER BY iacc.catalog_code ASC SEPARATOR ', ') AS catalogCodes, 
    GROUP_CONCAT(DISTINCT iaio.issues_on ORDER BY iaio.issues_on ASC SEPARATOR ', ') AS issuesOn, 
    GROUP_CONCAT(DISTINCT iali.last_issues ORDER BY iali.last_issues ASC SEPARATOR ', ') AS latestIssues, 
    MIN(ucfs.price) AS minPrice, 
    SUM(ucfs.units) AS totalUnits           
FROM
    (SELECT
        i.id_items, 
        GROUP_CONCAT(DISTINCT iant.translation ORDER BY iant.translation asc SEPARATOR ', ') AS names   
    FROM
       items i 
    INNER JOIN languages AS l ON l.language_code = "en"
    LEFT JOIN items_atr_names_match AS ianm ON ianm.id_items = i.id_items
    LEFT JOIN items_atr_names_translations AS iant ON iant.id_items_atr_names = ianm.id_items_atr_names AND iant.id_language_code = l.id_languages 
    LEFT JOIN items_atr_names AS ian ON ian.id_items_atr_names = ianm.id_items_atr_names 

    GROUP BY id_items
    ORDER BY names asc

    LIMIT 0, 20) AS i

INNER JOIN languages AS l ON l.language_code = "en"

LEFT JOIN items_atr_faces_values_match AS iafvm ON iafvm.id_items = i.id_items 
LEFT JOIN items_atr_faces_values AS iafv ON iafv.id_items_atr_faces_values = iafvm.id_items_atr_faces_values

LEFT JOIN items_atr_currencies_match AS iacum ON iacum.id_items = i.id_items 
LEFT JOIN items_atr_currencies_translations AS iacut ON iacut.id_items_atr_currencies = iacum.id_items_atr_currencies AND iacut.id_language_code = l.id_languages 
LEFT JOIN items_atr_currencies AS iacu ON iacu.id_items_atr_currencies = iacum.id_items_atr_currencies

LEFT JOIN items_atr_countries_match AS iacoum ON iacoum.id_items = i.id_items 
LEFT JOIN items_atr_countries_translations AS cout ON cout.id_items_atr_countries = iacoum.id_items_atr_countries AND cout.id_language_code = l.id_languages 
LEFT JOIN items_atr_countries AS cou ON cou.id_items_atr_countries = iacoum.id_items_atr_countries

LEFT JOIN items_atr_compositions_match AS iacom ON iacom.id_items = i.id_items 
LEFT JOIN items_atr_compositions_translations AS iacot ON iacot.id_items_atr_compositions = iacom.id_items_atr_compositions AND iacot.id_language_code = l.id_languages 
LEFT JOIN items_atr_compositions AS iaco ON iaco.id_items_atr_compositions = iacom.id_items_atr_compositions

LEFT JOIN items_atr_catalog_codes_match AS iaccm ON iaccm.id_items = i.id_items
LEFT JOIN items_atr_catalog_codes AS iacc ON iacc.id_items_atr_catalog_codes = iaccm.id_items_atr_catalog_codes  

LEFT JOIN items_atr_issues_on_match AS iaiom ON iaiom.id_items = i.id_items
LEFT JOIN items_atr_issues_on AS iaio ON iaio.id_items_atr_issues_on = iaiom.id_items_atr_issues_on

LEFT JOIN items_atr_last_issues_match AS ialim ON ialim.id_items = i.id_items
LEFT JOIN items_atr_last_issues AS iali ON iali.id_items_atr_last_issues = ialim.id_items_atr_last_issues

LEFT JOIN users_coins_for_sale AS ucfs ON ucfs.id_items = i.id_items
LEFT JOIN users AS u ON u.id_users = ucfs.id_users AND u.status = 'active'

GROUP BY id

ORDER BY names asc

这是带有索引信息的表的创建sql：

CREATE TABLE IF NOT EXISTS `items` (
    `id_items` int(11) unsigned NOT NULL AUTO_INCREMENT,
    `id_users` int(11) unsigned NOT NULL DEFAULT '0',
    `date` date NOT NULL,
    `views` bigint(20) NOT NULL DEFAULT '0',
    `status` enum('draft','published') NOT NULL DEFAULT 'draft',
    PRIMARY KEY (`id_items`),
    UNIQUE KEY `id_items` (`id_items`),
    KEY `id_users` (`id_users`),
    CONSTRAINT `FK_items_users` FOREIGN KEY (`id_users`) REFERENCES `users` (`id_users`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `items_atr_catalog_codes` (
    `id_items_atr_catalog_codes` int(11) unsigned NOT NULL AUTO_INCREMENT,
    `catalog_code` varchar(100) NOT NULL DEFAULT '0',
    PRIMARY KEY (`id_items_atr_catalog_codes`),
    UNIQUE KEY `id_items_atr_catalog_codes` (`id_items_atr_catalog_codes`),
    FULLTEXT KEY `catalog_code` (`catalog_code`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `items_atr_catalog_codes_match` (
    `id_items_atr_catalog_codes_match` int(11) unsigned NOT NULL AUTO_INCREMENT,
    `id_items_atr_catalog_codes` int(11) unsigned NOT NULL,
    `id_items` int(11) unsigned NOT NULL,
    PRIMARY KEY (`id_items_atr_catalog_codes_match`),
    UNIQUE KEY `id_items_atr_catalog_codes_match` (`id_items_atr_catalog_codes_match`),
    KEY `id_items` (`id_items`),
    KEY `id_items_atr_catalog_codes` (`id_items_atr_catalog_codes`),
    CONSTRAINT `FK_catalog_codes_items_match_catalog_codes` FOREIGN KEY (`id_items_atr_catalog_codes`) REFERENCES `items_atr_catalog_codes` (`id_items_atr_catalog_codes`),
    CONSTRAINT `FK_catalog_codes_items_match_items` FOREIGN KEY (`id_items`) REFERENCES `items` (`id_items`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `items_atr_compositions` (
    `id_items_atr_compositions` int(11) unsigned NOT NULL AUTO_INCREMENT,
    PRIMARY KEY (`id_items_atr_compositions`),
    UNIQUE KEY `id_items_atr_compositions` (`id_items_atr_compositions`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `items_atr_compositions_match` (
    `id_items_atr_compositions_match` int(11) unsigned NOT NULL AUTO_INCREMENT,
    `id_items_atr_compositions` int(11) unsigned NOT NULL DEFAULT '0',
    `id_items` int(11) unsigned NOT NULL DEFAULT '0',
    PRIMARY KEY (`id_items_atr_compositions_match`),
    UNIQUE KEY `id_items_atr_compositions_match` (`id_items_atr_compositions_match`),
    KEY `id_items` (`id_items`),
    KEY `id_items_atr_compositions` (`id_items_atr_compositions`),
    CONSTRAINT `FK_compositions_items_match_compositions` FOREIGN KEY (`id_items_atr_compositions`) REFERENCES `items_atr_compositions` (`id_items_atr_compositions`),
    CONSTRAINT `FK_compositions_items_match_items` FOREIGN KEY (`id_items`) REFERENCES `items` (`id_items`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `items_atr_compositions_translations` (
    `id_items_atr_compositions_translations` int(11) unsigned NOT NULL AUTO_INCREMENT,
    `id_items_atr_compositions` int(11) unsigned NOT NULL DEFAULT '0',
    `id_language_code` int(11) unsigned NOT NULL DEFAULT '0',
    `translation` varchar(250) NOT NULL DEFAULT '0',
    PRIMARY KEY (`id_items_atr_compositions_translations`),
    UNIQUE KEY `id_items_atr_compositions_translations` (`id_items_atr_compositions_translations`),
    KEY `id_language_code` (`id_language_code`),
    KEY `id_items_atr_compositions` (`id_items_atr_compositions`),
    FULLTEXT KEY `translation` (`translation`),
    CONSTRAINT `FK_compositions_translations_compositions` FOREIGN KEY (`id_items_atr_compositions`) REFERENCES `items_atr_compositions` (`id_items_atr_compositions`),
    CONSTRAINT `FK_compositions_translations_languages` FOREIGN KEY (`id_language_code`) REFERENCES `languages` (`id_languages`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `items_atr_countries` (
    `id_items_atr_countries` int(11) unsigned NOT NULL AUTO_INCREMENT,
    PRIMARY KEY (`id_items_atr_countries`),
    UNIQUE KEY `id_items_atr_countries` (`id_items_atr_countries`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE IF NOT EXISTS `items_atr_countries_match` (
    `id_items_atr_countries_match` int(11) unsigned NOT NULL AUTO_INCREMENT,
    `id_items_atr_countries` int(11) unsigned NOT NULL,
    `id_items` int(11) unsigned NOT NULL,
    PRIMARY KEY (`id_items_atr_countries_match`),
    UNIQUE KEY `items_atr_countries_match` (`id_items_atr_countries_match`),
    KEY `id_items` (`id_items`),
    KEY `id_items_atr_countries` (`id_items_atr_countries`),
    CONSTRAINT `FK__countries` FOREIGN KEY (`id_items_atr_countries`) REFERENCES `items_atr_countries` (`id_items_atr_countries`),
    CONSTRAINT `FK__items` FOREIGN KEY (`id_items`) REFERENCES `items` (`id_items`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `items_atr_countries_translations` (
    `id_items_atr_countries_translations` int(11) unsigned NOT NULL AUTO_INCREMENT,
    `id_items_atr_countries` int(11) unsigned NOT NULL DEFAULT '0',
    `id_language_code` int(11) unsigned NOT NULL DEFAULT '0',
    `translation` varchar(250) NOT NULL DEFAULT '0',
    PRIMARY KEY (`id_items_atr_countries_translations`),
    UNIQUE KEY `id_items_atr_countries_translations` (`id_items_atr_countries_translations`),
    KEY `id_items_atr_countries` (`id_items_atr_countries`),
    KEY `id_language_code` (`id_language_code`),
    FULLTEXT KEY `translation` (`translation`),
    CONSTRAINT `FK__items_atr_countries` FOREIGN KEY (`id_items_atr_countries`) REFERENCES `items_atr_countries` (`id_items_atr_countries`),
    CONSTRAINT `FK_items_atr_countries_translations_languages` FOREIGN KEY (`id_language_code`) REFERENCES `languages` (`id_languages`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `items_atr_currencies` (
    `id_items_atr_currencies` int(11) unsigned NOT NULL AUTO_INCREMENT,
    PRIMARY KEY (`id_items_atr_currencies`),
    UNIQUE KEY `id_items_atr_currencies` (`id_items_atr_currencies`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `items_atr_currencies_match` (
    `id_items_atr_currencies_match` int(11) unsigned NOT NULL AUTO_INCREMENT,
    `id_items_atr_currencies` int(11) unsigned NOT NULL DEFAULT '0',
    `id_items` int(11) unsigned NOT NULL DEFAULT '0',
    PRIMARY KEY (`id_items_atr_currencies_match`),
    UNIQUE KEY `id_items_atr_currencies_match` (`id_items_atr_currencies_match`),
    KEY `id_items_atr_currencies` (`id_items_atr_currencies`),
    KEY `id_items` (`id_items`),
    CONSTRAINT `FK_items_atr_currencies_match_items` FOREIGN KEY (`id_items`) REFERENCES `items` (`id_items`),
    CONSTRAINT `FK_items_atr_currencies_match_items_atr_currencies` FOREIGN KEY (`id_items_atr_currencies`) REFERENCES `items_atr_currencies` (`id_items_atr_currencies`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `items_atr_currencies_translations` (
    `id_items_atr_currencies_translations` int(11) unsigned NOT NULL AUTO_INCREMENT,
    `id_items_atr_currencies` int(11) unsigned NOT NULL DEFAULT '0',
    `id_language_code` int(11) unsigned NOT NULL DEFAULT '0',
    `translation` varchar(255) NOT NULL DEFAULT '0',
    PRIMARY KEY (`id_items_atr_currencies_translations`),
    UNIQUE KEY `id_items_atr_currencies_translations` (`id_items_atr_currencies_translations`),
    KEY `id_items_atr_currencies` (`id_items_atr_currencies`),
    KEY `id_language_code` (`id_language_code`),
    FULLTEXT KEY `translation` (`translation`),
    CONSTRAINT `FK_items_atr_currencies_translations_items_atr_currencies` FOREIGN KEY (`id_items_atr_currencies`) REFERENCES `items_atr_currencies` (`id_items_atr_currencies`),
    CONSTRAINT `FK_items_atr_currencies_translations_languages` FOREIGN KEY (`id_language_code`) REFERENCES `languages` (`id_languages`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `items_atr_faces_values` (
    `id_items_atr_faces_values` int(11) unsigned NOT NULL AUTO_INCREMENT,
    `face_value` decimal(11,2) unsigned NOT NULL DEFAULT '0.00',
    PRIMARY KEY (`id_items_atr_faces_values`),
    KEY `face_value` (`face_value`),
    KEY `id-fv` (`id_items_atr_faces_values`,`face_value`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `items_atr_faces_values_match` (
    `id_items_atr_faces_values_match` int(11) unsigned NOT NULL AUTO_INCREMENT,
    `id_items_atr_faces_values` int(11) unsigned NOT NULL DEFAULT '0',
    `id_items` int(11) unsigned NOT NULL DEFAULT '0',
    PRIMARY KEY (`id_items_atr_faces_values_match`),
    UNIQUE KEY `id_items_atr_faces_values_match` (`id_items_atr_faces_values_match`),
    KEY `id_items_atr_faces_values` (`id_items_atr_faces_values`),
    KEY `id_items` (`id_items`),
    KEY `idItems-IdFaceValue` (`id_items`,`id_items_atr_faces_values`),
    CONSTRAINT `FK_items_atr_faces_values_match_items` FOREIGN KEY (`id_items`) REFERENCES `items` (`id_items`),
    CONSTRAINT `FK_items_atr_faces_values_match_items_atr_faces_values` FOREIGN KEY (`id_items_atr_faces_values`) REFERENCES `items_atr_faces_values` (`id_items_atr_faces_values`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `items_atr_issues_on` (
    `id_items_atr_issues_on` int(11) unsigned NOT NULL AUTO_INCREMENT,
    `issues_on` varchar(6) NOT NULL DEFAULT '0',
    PRIMARY KEY (`id_items_atr_issues_on`),
    UNIQUE KEY `id_items_atr_issues_on` (`id_items_atr_issues_on`),
    FULLTEXT KEY `issues_on` (`issues_on`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `items_atr_issues_on_match` (
    `id_items_atr_issues_on_match` int(11) unsigned NOT NULL AUTO_INCREMENT,
    `id_items_atr_issues_on` int(11) unsigned NOT NULL,
    `id_items` int(11) unsigned NOT NULL,
    PRIMARY KEY (`id_items_atr_issues_on_match`),
    UNIQUE KEY `id_items_atr_issues_on_match` (`id_items_atr_issues_on_match`),
    KEY `id_items_atr_issues_on` (`id_items_atr_issues_on`),
    KEY `id_items` (`id_items`),
    CONSTRAINT `FK_items_atr_issues_on_match_items` FOREIGN KEY (`id_items`) REFERENCES `items` (`id_items`),
    CONSTRAINT `FK_items_atr_issues_on_match_items_atr_issues_on` FOREIGN KEY (`id_items_atr_issues_on`) REFERENCES `items_atr_issues_on` (`id_items_atr_issues_on`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `items_atr_last_issues` (
    `id_items_atr_last_issues` int(11) unsigned NOT NULL AUTO_INCREMENT,
    `last_issues` varchar(6) NOT NULL DEFAULT '0',
    PRIMARY KEY (`id_items_atr_last_issues`),
    UNIQUE KEY `id_items_atr_last_issues` (`id_items_atr_last_issues`),
    FULLTEXT KEY `last_issues` (`last_issues`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `items_atr_last_issues_match` (
    `id_items_atr_last_issues_match` int(11) unsigned NOT NULL AUTO_INCREMENT,
    `id_items_atr_last_issues` int(11) unsigned NOT NULL DEFAULT '0',
    `id_items` int(11) unsigned NOT NULL DEFAULT '0',
    PRIMARY KEY (`id_items_atr_last_issues_match`),
    UNIQUE KEY `id_items_atr_last_issues_match` (`id_items_atr_last_issues_match`),
    KEY `id_items_atr_last_issues` (`id_items_atr_last_issues`),
    KEY `id_items` (`id_items`),
    CONSTRAINT `FK_items_atr_last_issues_match_items` FOREIGN KEY (`id_items`) REFERENCES `items` (`id_items`),
    CONSTRAINT `FK_items_atr_last_issues_match_items_atr_last_issues` FOREIGN KEY (`id_items_atr_last_issues`) REFERENCES `items_atr_last_issues` (`id_items_atr_last_issues`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `items_atr_names` (
    `id_items_atr_names` int(11) unsigned NOT NULL AUTO_INCREMENT,
    PRIMARY KEY (`id_items_atr_names`),
    UNIQUE KEY `id_items_atr_names` (`id_items_atr_names`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `items_atr_names_match` (
    `id_items_atr_names_match` int(11) unsigned NOT NULL AUTO_INCREMENT,
    `id_items_atr_names` int(11) unsigned NOT NULL DEFAULT '0',
    `id_items` int(11) unsigned NOT NULL DEFAULT '0',
    PRIMARY KEY (`id_items_atr_names_match`),
    UNIQUE KEY `id_items_atr_names_match` (`id_items_atr_names_match`),
    KEY `id_items_atr_names` (`id_items_atr_names`),
    KEY `id_items` (`id_items`),
    CONSTRAINT `FK_items_atr_names_match_items` FOREIGN KEY (`id_items`) REFERENCES `items` (`id_items`),
    CONSTRAINT `FK_items_atr_names_match_items_atr_names` FOREIGN KEY (`id_items_atr_names`) REFERENCES `items_atr_names` (`id_items_atr_names`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `items_atr_names_translations` (
    `id_items_atr_names_translations` int(11) unsigned NOT NULL AUTO_INCREMENT,
    `id_items_atr_names` int(11) unsigned NOT NULL DEFAULT '0',
    `id_language_code` int(11) unsigned NOT NULL DEFAULT '0',
    `translation` varchar(250) NOT NULL DEFAULT '0',
    PRIMARY KEY (`id_items_atr_names_translations`),
    UNIQUE KEY `id_items_atr_names_translations` (`id_items_atr_names_translations`),
    KEY `id_language_code` (`id_language_code`),
    KEY `id_items_atr_names` (`id_items_atr_names`),
    KEY `translation_indx` (`translation`),
    FULLTEXT KEY `translation` (`translation`),
    CONSTRAINT `FK_items_atr_names_translations_items_atr_names` FOREIGN KEY (`id_items_atr_names`) REFERENCES `items_atr_names` (`id_items_atr_names`),
    CONSTRAINT `FK_items_atr_names_translations_languages` FOREIGN KEY (`id_language_code`) REFERENCES `languages` (`id_languages`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `languages` (
    `id_languages` int(11) unsigned NOT NULL,
    `language_code` char(2) NOT NULL,
    PRIMARY KEY (`id_languages`),
    UNIQUE KEY `language_code` (`language_code`),
    KEY `id-language_code` (`id_languages`,`language_code`),
    KEY `language-code` (`language_code`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `languages_translations` (
    `id_languages_translations` int(11) unsigned NOT NULL,
    `id_languages` int(11) unsigned NOT NULL,
    `id_language_code` int(11) unsigned NOT NULL,
    `translation` varchar(255) NOT NULL,
    PRIMARY KEY (`id_languages_translations`),
    UNIQUE KEY `id_languages_translations` (`id_languages_translations`),
    KEY `id_languages` (`id_languages`),
    KEY `language_code` (`id_language_code`),
    FULLTEXT KEY `translation` (`translation`),
    CONSTRAINT `FK_languages_translations_languages` FOREIGN KEY (`id_languages`) REFERENCES `languages` (`id_languages`),
    CONSTRAINT `FK_languages_translations_languages_2` FOREIGN KEY (`id_language_code`) REFERENCES `languages` (`id_languages`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


CREATE TABLE IF NOT EXISTS `users_coins_for_sale` (
    `id_users_coins_for_sale` int(11) unsigned NOT NULL AUTO_INCREMENT,
    `id_items` int(11) unsigned NOT NULL,
    `id_users` int(11) unsigned NOT NULL,
    `units` int(5) unsigned NOT NULL,
    `year` varchar(5) NOT NULL,
    `grade` enum('good','very_good','fine','very_fine','extra_fine','about_uncirculated','uncirculated','brilliant_uncirculated','proof') NOT NULL,
    `price` decimal(8,2) unsigned NOT NULL,
    `picture` varchar(500) DEFAULT NULL,
    `comment` varchar(500) DEFAULT NULL,
    `date` datetime NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
    PRIMARY KEY (`id_users_coins_for_sale`),
    UNIQUE KEY `id_users_coins_for_sale_unique` (`id_users_coins_for_sale`),
    KEY `id_items` (`id_items`),
    KEY `id_users` (`id_users`),
    KEY `year` (`year`),
    KEY `grade` (`grade`),
    KEY `id_users_coins_for_sale` (`id_users_coins_for_sale`),
    KEY `idItems-idUsers` (`id_items`,`id_users`),
    CONSTRAINT `FK_users_coins_for_sale_items` FOREIGN KEY (`id_items`) REFERENCES `items` (`id_items`),
    CONSTRAINT `FK_users_coins_for_sale_users` FOREIGN KEY (`id_users`) REFERENCES `users` (`id_users`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

这里是查询的解释：

在解释中我可以看到有时使用了索引，但有时没有使用，我想知道如果我正确创建了索引或者我缺少任何索引，我可以做些什么来提高此查询的性能。

items 表有 90k 个条目，但我认为它不够大，无法花费 1.7 秒...

我一直在关注这个链接来学习最佳实践：https://stackoverflow.com/a/3049329

您对如何提高性能有什么建议吗？

提前致谢

【问题讨论】：

1.7 秒对我来说似乎没问题。最佳实践当然是不使用 GROUP_CONCAT，而是处理应用程序代码中的数据显示问题
谢谢@Strawberry，那么在查询解释的“额外”列中出现空值是否正常？如果我从选择中删除 GROUP_CONCAT，大约需要相同的时间（1.4 秒）。瓶颈一定在我看不到的其他部分
如果从查询中删除users、items_atr_compositions、items_atr_compositions_translations 和items_atr_countries，结果会怎样？
如果我拿出你说的那些，执行需要1.359秒到1.484秒
节省 20%。万岁。

标签： mysql performance indexing database-indexes

【解决方案1】：

如果JOINs 不是 1:1，SUM() 可能会被夸大。检查值！
当GROUP BY 和ORDER BY 不同时，没有索引将有助于避免在LIMIT 之前收集暂定结果集。否则，某些索引可能一直到达LIMIT，从而避免了全扫描。
一个PRIMARY KEY是一个UNIQUE KEY，所以后者是多余的； DROP它。（我在所有（？）表格中都看到了这种模式。）
This 讨论了 many:many 映射表的最佳索引。
这闻起来像“键值”或“EAV”架构，很糟糕。
有时最好改变

这个

GROUP_CONCAT(DISTINCT ... )
...
LEFT JOIN ...

进入

( SELECT GROUP_CONCAT( ... ) FROM ... )

上述建议在派生查询中尤为重要，因为它可能是最慢的部分。建议您拉出该子查询并对其计时并进行优化。请注意，它可能会扫描很多行，但交付的行数不超过 20。假设您至少有最少的索引，那么这 20 行不会对其余查询造成太大负担。
唉，这只是用了一半——它可能会去掉GROUP BY，但它不能去掉完整的扫描。这是因为 ORDER BY 取决于计算值 (names)。尽管如此，它应该可以摆脱由JOINs + GROUP BY 引起的“充气-放气”。
摆脱表languages。它所做的只是将像'en' 这样的2 字节语言映射到4 字节INT。这是对 (1) 空间和 (2) JOINs 的浪费。

抱歉，此建议列表可能不完整。请尽你所能来改进，然后回来寻求更多帮助。

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