使用多个选择作为列优化查询答案

【问题标题】：Optimising query with multiple selects as columns使用多个选择作为列优化查询
【发布时间】：2020-10-30 05:40:32
【问题描述】：

我有一个查询：

 SELECT DISTINCT player_1 AS player, 
                (SELECT COUNT(*) FROM results WHERE player_1=player OR player_2=player) AS since_start_matches, 
                (SELECT COUNT(*) FROM results WHERE (player_1=player OR player_2=player) AND ht_total_goals=0) AS since_start_ht_0,
                (SELECT COUNT(*) FROM results WHERE (player_1=player OR player_2=player) AND ht_total_goals=1) AS since_start_ht_1,
                (SELECT COUNT(*) FROM results WHERE (player_1=player OR player_2=player) AND ht_total_goals=2) AS since_start_ht_2,
                (SELECT COUNT(*) FROM results WHERE (player_1=player OR player_2=player) AND ht_total_goals=3) AS since_start_ht_3,
                (SELECT COUNT(*) FROM results WHERE (player_1=player OR player_2=player) AND ht_total_goals=4) AS since_start_ht_4,
                (SELECT COUNT(*) FROM results WHERE (player_1=player OR player_2=player) AND ht_total_goals>=5) AS since_start_ht_5_plus
FROM results ORDER BY player

results 表有 25000 个条目，执行此查询大约需要 7 秒，这太长了。该查询效率极低，因为我创建的每一列都在同一个表上再次搜索，但条件不同。

我尝试在 where 子句中索引感兴趣的列。这会关闭几秒钟。但是还是太慢了。

处理此类查询的最佳方法是什么？

我正在使用 MariaDB 10.2

【问题讨论】：

也就是说，最好的方法可能是处理应用程序代码中数据显示的所有问题

标签： mysql sql performance indexing mariadb

【解决方案1】：

取消透视数据然后聚合：

 SELECT player, 
        COUNT(*) AS since_start_matches, 
        SUM(ht_total_goals=0) AS since_start_ht_0,
        SUM(ht_total_goals=1) AS since_start_ht_1,
        SUM(ht_total_goals=2) AS since_start_ht_2,
        SUM(ht_total_goals=3) AS since_start_ht_3,
        SUM(ht_total_goals=4) AS since_start_ht_4,
        SUM( ht_total_goals>=5) AS since_start_ht_5_plus
FROM ((SELECT player_1 as player, ht_total_goals
       FROM results
      ) UNION ALL
      (SELECT player_2 as player, ht_total_goals
       FROM results
      )
     ) p
GROUP BY player

【讨论】：

【解决方案2】：

您可以使用LEFT JOIN 和conditional aggregation，如下所示：

SELECT  player_1 AS player, 
                COUNT(T2.player_1) AS since_start_matches, 
                SUM(CASE WHEN T2.ht_total_goals=0 THEN 1 END) AS since_start_ht_0,
                SUM(CASE WHEN T2.ht_total_goals=1 THEN 1 END) AS since_start_ht_1,
                SUM(CASE WHEN T2.ht_total_goals=2 THEN 1 END) AS since_start_ht_2,
                SUM(CASE WHEN T2.ht_total_goals=3 THEN 1 END) AS since_start_ht_3,
                SUM(CASE WHEN T2.ht_total_goals=4 THEN 1 END) AS since_start_ht_4,
                SUM(CASE WHEN T2.ht_total_goals>=5 THEN 1 END) AS since_start_ht_5_PLUS
FROM results T1 LEFT JOIN results T2 
ON (T2.player_1=T.player OR T2.player_2=T.player)
GROUP BY T1.PLA

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【讨论】：