Question

我需要优化以下查询。表中有很多记录：a，b，c，d，查询时间很长。请求帮助。

SELECT main.*, items.*
FROM main
LEFT JOIN (
  (SELECT a.*, 1 AS `type` FROM a) UNION
  (SELECT b.*, 2 AS `type` FROM b) UNION
  (SELECT c.*, 3 AS `type` FROM c) UNION
  (SELECT d.*, 4 AS `type` FROM d)
)
items ON main.`to` = items.id AND main.`type` = items.`type`
WHERE main.id > 0
LIMIT 20

Answer 1

呸！像这样：

SELECT main.*, z.*
    FROM (
    ( SELECT  1 AS type, main.id AS mid, a.*, xx
        FROM  main
        JOIN  a  ON a.id = main.to
        WHERE  main.id > 0  AND main.type = 1
        ORDER BY  xx
        LIMIT  20 )
    UNION ALL
    ( SELECT  2 AS type, main.id, b.*, xx
        FROM  main
        JOIN  b  ON a.id = main.to
        WHERE  main.id > 0  AND main.type = 2
        ORDER BY  xx
        LIMIT  20 )
    UNION ALL
    ( SELECT  3 AS type, main.id, c.*, xx
        FROM  main
        JOIN  c  ON a.id = main.to
        WHERE  main.id > 0  AND main.type = 3
        ORDER BY  xx
        LIMIT  20 )
    UNION ALL
    ( SELECT  4 AS type, main.id, d.*, xx
        FROM  main
        JOIN  d  ON a.id = main.to
        WHERE  main.id > 0  AND main.type = 4
        ORDER BY  xx
        LIMIT  20 )
      ORDER BY xx
      LIMIT 20 ) z
    JOIN main  ON main.id = z.mid
ORDER BY xx;

Plus索引：

main: PRIMARY KEY(id), INDEX(type, xx, id, to)
a,b,c,d:  PRIMARY KEY(id)

注意：

我假设你真的想按xx排序main。
我假设所有表格都有PRIMARY KEY(id)
main上的4-col索引在进行过滤和排序方面有所帮助;它是＆＃34;覆盖＆＃34;。
6 ORDER BY + LIMIT将中间表最小化为80行。最后一个是因为子查询不需要保留顺序。
main.id > 0是一种痛苦 - 我们可以摆脱它吗？
ALL而不是DISTINCT。
SELECT *是一种痛苦;可以避免吗？由于它，我搞砸了确切的列列表。
JOIN返回main是为了避免牵引所有main.*;相反，只需要获取80个。

LEFT JOIN有多个表。表现不佳

1 个答案: