按日期范围分区 PostgreSQL 扫描所有分区

Posted 2023-03-31

【中文标题】按日期范围分区 PostgreSQL 扫描所有分区

【英文标题】：Partition by date range PostgreSQL scans all partitions

【发布时间】：2019-04-11 14:10:54

【问题描述】：

我有一个每月分区的表（时间戳列）。

查询数据时，explain 显示当我使用日期函数构造日期时正在查询所有分区，而当我使用硬编码日期时，仅扫描目标分区。

所以当这样查询时：

SELECT * FROM vw_comments 
      WHERE created >= '2019-4-1'
        AND created <= '2019-4-30'
limit 100;

它只扫描 1 个分区（1 个月，很好！） 但是为了让它更有活力，我传递了这样的东西（简化）

SELECT * FROM vw_comments 
      WHERE created >= (date_trunc('month', now()))::timestamp
        AND created <= (date_trunc('month', now() + interval '1 month') - interval '1 day') ::timestamp
limit 100;

上述日期方法的日期与第一个查询完全相同，但 EXPLAIN 显示所有分区都已扫描。

如何让它发挥作用？

编辑：添加表定义并解释

应@a_horse_with_no_name 的要求，我添加了实际表格并进行了解释。这样做后，我想出了更多的东西：动态日期在加入时不起作用。因此，在下面的查询中省略“用户”表可以使动态日期起作用。

CREATE TABLE public.comments
(
    comment_id integer NOT NULL GENERATED BY DEFAULT AS IDENTITY ( INCREMENT 1 START 1 MINVALUE 1 MAXVALUE 2147483647 CACHE 1 ),
    comment_id integer NOT NULL GENERATED BY DEFAULT AS IDENTITY,
    from_user_id integer NOT NULL,
    fk_topic_id integer NOT NULL,
    comment_text text COLLATE pg_catalog."default",
    parent_comment_id integer,
    created timestamp without time zone NOT NULL,
    comment_type integer NOT NULL DEFAULT 0,
    CONSTRAINT comments_pkey PRIMARY KEY (comment_id, created)
) PARTITION BY RANGE (created) 
WITH (
    OIDS = FALSE
)
TABLESPACE pg_default;

ALTER TABLE public.comments
    OWNER to soichat;

CREATE INDEX ix_comments_comment_id
    ON public.comments USING btree
    (comment_id DESC)
    TABLESPACE pg_default;

CREATE INDEX ix_comments_created
    ON public.comments USING btree
    (created DESC)
    TABLESPACE pg_default;

CREATE INDEX ix_comments_fk_topic_id
    ON public.comments USING btree
    (fk_topic_id)
    TABLESPACE pg_default;

CREATE INDEX ix_comments_from_user_id
    ON public.comments USING btree
    (from_user_id)
    TABLESPACE pg_default;

CREATE INDEX ix_comments_parent_comment_id
    ON public.comments USING btree
    (parent_comment_id)
    TABLESPACE pg_default;

-- Partitions SQL

CREATE TABLE public.comments_2019_2 PARTITION OF public.ix_comments_parent_comment_id
    FOR VALUES FROM ('2019-02-01 00:00:00') TO ('2019-03-01 00:00:00');

CREATE TABLE public.comments_2019_3 PARTITION OF public.ix_comments_parent_comment_id
    FOR VALUES FROM ('2019-03-01 00:00:00') TO ('2019-04-01 00:00:00');

CREATE TABLE public.comments_2019_4 PARTITION OF public.ix_comments_parent_comment_id
    FOR VALUES FROM ('2019-04-01 00:00:00') TO ('2019-05-01 00:00:00');

CREATE TABLE public.comments_2019_5 PARTITION OF public.ix_comments_parent_comment_id
    FOR VALUES FROM ('2019-05-01 00:00:00') TO ('2019-06-01 00:00:00');

查询：

explain (analyse, buffers)
 SELECT comments.comment_id,
    comments.from_user_id,
    comments.fk_topic_id,
    comments.comment_text,
    comments.parent_comment_id,
    comments.created,
    users.user_name,
    users.picture_path
   FROM comments
     LEFT JOIN users ON comments.from_user_id = users.user_id
    WHERE comments.created >= (date_trunc('month', now()))::timestamp
        AND comments.created <= (date_trunc('month', now() + interval '1 month') - interval '1 day') ::timestamp
limit 100;

explain (analyze, buffers)

Limit  (cost=1.20..11.93 rows=100 width=126) (actual time=1.441..1.865 rows=100 loops=1)
  Buffers: shared hit=499
  ->  Merge Left Join  (cost=1.20..753901.07 rows=7028011 width=126) (actual time=1.440..1.778 rows=100 loops=1)
        Merge Cond: (comments_2019_2.from_user_id = users.user_id)
        Buffers: shared hit=499
        ->  Merge Append  (cost=0.92..665812.08 rows=7028011 width=51) (actual time=0.017..0.259 rows=100 loops=1)
              Sort Key: comments_2019_2.from_user_id
              Buffers: shared hit=15
              ->  Index Scan using comments_2019_2_from_user_id_idx on comments_2019_2  (cost=0.15..58.95 rows=5 width=56) (actual time=0.002..0.003 rows=0 loops=1)
                    Filter: ((created >= (date_trunc('month'::text, now()))::timestamp without time zone) AND (created <= ((date_trunc('month'::text, (now() + '1 mon'::interval)) - '1 day'::interval))::timestamp without time zone))
                    Buffers: shared hit=1
              ->  Index Scan using comments_2019_3_from_user_id_idx on comments_2019_3  (cost=0.15..9790.24 rows=1 width=51) (actual time=0.002..0.003 rows=0 loops=1)
                    Filter: ((created >= (date_trunc('month'::text, now()))::timestamp without time zone) AND (created <= ((date_trunc('month'::text, (now() + '1 mon'::interval)) - '1 day'::interval))::timestamp without time zone))
                    Buffers: shared hit=1
              ->  Index Scan using comments_2019_4_from_user_id_idx on comments_2019_4  (cost=0.43..550483.74 rows=7028000 width=51) (actual time=0.010..0.162 rows=100 loops=1)
                    Filter: ((created >= (date_trunc('month'::text, now()))::timestamp without time zone) AND (created <= ((date_trunc('month'::text, (now() + '1 mon'::interval)) - '1 day'::interval))::timestamp without time zone))
                    Buffers: shared hit=12
              ->  Index Scan using comments_2019_5_from_user_id_idx on comments_2019_5  (cost=0.15..58.95 rows=5 width=56) (actual time=0.001..0.002 rows=0 loops=1)
                    Filter: ((created >= (date_trunc('month'::text, now()))::timestamp without time zone) AND (created <= ((date_trunc('month'::text, (now() + '1 mon'::interval)) - '1 day'::interval))::timestamp without time zone))
                    Buffers: shared hit=1
        ->  Index Scan using pk_users on users  (cost=0.28..234.83 rows=1606 width=79) (actual time=0.005..0.870 rows=1395 loops=1)
              Buffers: shared hit=484
Planning Time: 0.360 ms
Execution Time: 1.942 ms

【问题讨论】：

@a_horse_with_no_name 我添加了实际查询和解释。这样做后，我想出了更多的东西：动态日期在加入时不起作用。因此，在下面的查询中省略“用户”表可以使动态日期起作用。

@a_horse_with_no_name：嗯，我使用 dbeaver 来执行创建脚本。我使用来自 pgadmin 的正确创建脚本更新了问题。对此感到抱歉。

看起来这种情况对于PostgreSQL来说太复杂了，无法进行分区修剪；我不知道确切的原因。但是其他分区上的索引扫描并没有真正受到伤害，不是吗？

@LaurenzAlbe 看起来是这样，但是您看到的是测试数据库的结果（其他月份几乎没有记录），通常每月有数百万个 cmets，我们正在删除它们一年后，所以会有点疼。分区目前尚未在生产中实现。

【参考方案1】：

找到（一个很好的）答案here

因为规划器不知道 now() 将在运行时产生什么时间，所以它选择安全选项并扫描所有分区。因为我不想为每个分区配置新函数，所以我使用了一个创建日期的不可变函数：

CREATE FUNCTION now_immutable()
  RETURNS timestamp AS
$func$
SELECT now() AT TIME ZONE current_setting('TimeZone')
$func$  LANGUAGE sql IMMUTABLE;

所以现在我不使用 now()，而是将此函数用于事务中日期不变的函数：

explain (analyse, buffers)
 SELECT comments.comment_id,
    comments.from_user_id,
    comments.fk_topic_id,
    comments.comment_text,
    comments.parent_comment_id,
    comments.created,
    users.user_name,
    users.picture_path
   FROM comments
     LEFT JOIN users ON comments.from_user_id = users.user_id
    WHERE comments.created >= (date_trunc('month', now_immutable()))
        AND comments.created <= (date_trunc('month', now_immutable() + interval '1 month') - interval '1 day') 
limit 100;

我还创建了另一个方便的函数来从代码months_back调用：

    CREATE OR REPLACE FUNCTION public.months_back(months_back integer)
     RETURNS timestamp without time zone
     LANGUAGE sql
     IMMUTABLE
    AS $function$
        SELECT cast((date_trunc('month', now()) - (months_back || ' month')::interval)::timestamp AT TIME ZONE current_setting('TimeZone') as timestamp)  
    $function$;

这在每月分区时很方便，因为如果您知道第一个评论是 3 个月前，您可以调用months_back(3)，并且 postgres 将仅搜索 3 个分区，传递 0 将为您提供当前月份的开始。

希望这对某人有所帮助。