我每个月都有一个分区的表（时间戳列）。
查询数据时，解释说明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，只会扫描目标分区。
所以当这样查询时：
SELECT * FROM vw_comments
WHERE created >= '2019-4-1'
AND created <= '2019-4-30'
limit 100;
它仅扫描1个分区（1个月，很好！） 但是为了使其更具动态性，我传入了类似的内容（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;
上述日期方法中的日期与第一个查询的日期完全相同，但EXPLAIN显示所有分区均被扫描。
如何使其工作？
编辑：添加表定义并说明
应@a_horse_with_no_name的要求，我添加了实际表并进行了说明。这样做之后，我发现了更多的东西：进行联接时，动态日期不起作用。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;
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
答案 0 :(得分：1)
找到了一个不错的答案here
因为计划者无法知道now（）在运行时将产生什么时间，所以它会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功能，所以我去了一个创建日期的不可变函数：
new Date(1993,8,1,0,0,0,0)所以现在而不是使用now（），我只将这个函数用于事务中日期不更改的函数：
CREATE FUNCTION now_immutable()
RETURNS timestamp AS
$func$
SELECT now() AT TIME ZONE current_setting('TimeZone')
$func$ LANGUAGE sql IMMUTABLE;
我还创建了另一个方便的函数，可从代码months_back调用：
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;
当每月进行分区时，这很方便，因为如果您知道第一个注释是3个月前，而您只能调用months_back（3），而postgres将仅搜索3个分区，则传递0将为您提供本月的开始。 / p>
希望这对某人有帮助。