Sql将多个类似的子查询压缩为单个子查询

Question

美好的一天,我的问题是关于优化SQL查询.以下查询很慢:

SELECT id, name,
, (SELECT rank_time FROM stage_rank WHERE stage = stage.id ORDER BY rank_time DESC LIMIT 1)::date AS rank_time
, (SELECT host_c    FROM stage_rank WHERE stage = stage.id ORDER BY rank_time DESC LIMIT 1) AS host_c
, (SELECT index_pa  FROM stage_rank WHERE stage = stage.id ORDER BY rank_time DESC LIMIT 1) AS index_pa
, (SELECT links_pa  FROM stage_rank WHERE stage = stage.id ORDER BY rank_time DESC LIMIT 1) AS links_pa
, (SELECT index_pb  FROM stage_rank WHERE stage = stage.id ORDER BY rank_time DESC LIMIT 1) AS index_pb
, (SELECT links_pb  FROM stage_rank WHERE stage = stage.id ORDER BY rank_time DESC LIMIT 1) AS links_pb
FROM stage
ORDER BY name;

我认为这主要是因为重复选择stage_rank,是否有可能使这个选择完成一次,并获得单击中的所有字段？

任何postgresql特定的功能可能在这里有所帮助？

Answer 1

在PostgreSQL,您可以选择整个记录作为字段,并在以后展开它:

SELECT  id, name, (sr).*
FROM    (
        SELECT  id, name,
                (
                SELECT  stage_rank
                FROM    stage_rank
                WHERE   stage = stage.id
                ORDER BY
                        rank_time DESC
                LIMIT 1
                ) sr
        FROM    stage
        ) q
ORDER BY
        name

或重写查询:

SELECT  DISTINCT ON (s.name, s.id, sr.rank_time, sr.id)
        s.id, s.name, sr.*
FROM    stage s
JOIN    stage_rank sr
ON      sr.stage = s.id
ORDER BY
        s.name, s.id, sr.rank_time DESC, sr.id DESC

或者用另一种方式重写它:

SELECT  id, name, (sr).*
FROM    (
        SELECT  s.id, s.name, sr, ROW_NUMBER() OVER (PARTITION BY s.id ORDER BY sr.rank_time DESC, sr.id DESC) rn
        FROM    stage
        JOIN    stage_rank sr
        ON      sr.stage = s.id
        ) q
WHERE   rn = 1
ORDER BY
        name