MySQL Where 子句导致性能大幅下降

Question

我正在尝试解决数据库查询的性能问题。当查询的 WHERE 子句中考虑 ad.entry_exit_ts 时，性能从 ~2.5s 变为 ~330+s。

我只是偶尔在 MySQL 中编写代码，所以我没有足够的知识来正确调试它。如果有人可以帮助甚至向我指出我应该阅读以理解问题的文档，我将不胜感激。

查询的版本如下。

对于上下文，“item_details”是一个连接多个表并检索entry_exit_ts 值的视图。该视图的性能通常为我提到的约 2.5 秒或更快。左连接本身对性能的影响也可以忽略不计，只有在检查entry_exit_ts以过滤数据时才会发生。还有创建和修改的时间字段，其数据类型与entry_exit_ts相同，但它们不会以任何方式导致性能下降。

编辑：尝试将查询格式化为更具可读性

SELECT * FROM  ( 
   SELECT 
       ad.id
  ,    ad.entry_exit_ts
  ,    CASE 
         WHEN sub.sub_id::text = ad.id::character varying::text 
          THEN 'true'::text 
          ELSE 'false'::text 
         END AS subscribed from item_details ad         
       LEFT JOIN (
            SELECT cast(sue.entity_id as varchar) as sub_id 
                 FROM proj_notification.subscription_user su  
               LEFT JOIN proj_notification.subscription_user_entity sue 
                 ON cast(su.user_id as varchar)=cast('35bd1f2a-e523-2a26-cad6-35bd1f2a' as varchar) 
                 AND cast(su.id as varchar)= cast(sue.subscription_user_id as varchar)  
                 AND cast(su.tenant_id as varchar)=cast('5aaaaaa3-5acc-4b6f-aa19-5aaaaaa38' as varchar)
          ) as sub 
            ON sub.sub_id::text = ad.id::character varying::text 
            WHERE (ad.entry_exit_ts >=  1695078000000)
) e 
WHERE e.tenant_id::text='5aaaaaa3-5acc-4b6f-aa19-5aaaaaa38' 

Answer 1

您应该考虑更改缩进和格式化策略，以使您的查询更易于阅读，并且此问题更易于回答。

导致性能下降的 WHERE 子句位于子查询中，执行多次，也许每次都执行全表扫描（如果该字段没有索引）。

因此，除了向entry_exit_ts添加索引之外，您还应该尝试将该WHERE子句移到子查询之外并将其应用到最终结果集，假设这不会使您的结果无效...

SELECT *
FROM   (
    SELECT    ad.id,
            ad.entry_exit_ts,
            CASE
                WHEN sub.sub_id::text = ad.id::character VARYING::text THEN 'true'::text
                ELSE 'false'::text
            END AS subscribed
    FROM      item_details ad
    LEFT JOIN
    (
        SELECT    cast(sue.entity_id AS varchar) AS sub_id
        FROM      proj_notification.subscription_user su
        LEFT JOIN proj_notification.subscription_user_entity sue
        ON        cast(su.user_id AS   varchar)=cast('35bd1f2a-e523-2a26-cad6-35bd1f2a' AS varchar)
        AND       cast(su.id AS        varchar)= cast(sue.subscription_user_id AS varchar)
        AND       cast(su.tenant_id AS varchar)=cast('5aaaaaa3-5acc-4b6f-aa19-5aaaaaa38' AS varchar)) AS sub
    ON        sub.sub_id::text = ad.id::character VARYING::text
    ) e
WHERE  e.tenant_id::text='5aaaaaa3-5acc-4b6f-aa19-5aaaaaa38'
    AND entry_exit_ts >= 1695078000000