基于检查约束的分区修剪未按预期工作

Question

为什么table "events_201504"包含在下面的查询计划中？根据我的查询和 table 的检查约束，我希望查询规划器能够完全修剪它：

database=# \d events_201504
                                   Table "public.events_201504"
    Column     |            Type             |                           Modifiers
---------------+-----------------------------+---------------------------------------------------------------
 id            | bigint                      | not null default nextval('events_id_seq'::regclass)
 created_at    | timestamp without time zone |
Indexes:
    "events_201504_pkey" PRIMARY KEY, btree (id)
    "events_201504_created_at" btree (created_at)
Check constraints:
    "events_201504_created_at_check" CHECK (created_at >= '2015-04-01 00:00:00'::timestamp without time zone AND created_at <= '2015-04-30 23:59:59.999999'::timestamp without time zone)
Inherits: events

时间和配置：

database=# select now();
              now
-------------------------------
 2015-05-25 16:49:20.037815-05

database=# show constraint_exclusion;
 constraint_exclusion
----------------------
 on

查询计划：

database=# explain select count(1) from events where created_at > now() - '1 hour'::interval;
                                                                QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------
 Aggregate  (cost=3479.86..3479.87 rows=1 width=0)
   ->  Append  (cost=0.00..3327.90 rows=60784 width=0)
         ->  Seq Scan on events  (cost=0.00..0.00 rows=1 width=0)
               Filter: (created_at > (now() - '01:00:00'::interval))
         ->  Index Only Scan using events_201504_created_at on events_201504  (cost=0.57..4.59 rows=1 width=0)
               Index Cond: (created_at > (now() - '01:00:00'::interval))
         ->  Index Only Scan using events_201505_created_at on events_201505  (cost=0.57..3245.29 rows=60765 width=0)
               Index Cond: (created_at > (now() - '01:00:00'::interval))

Answer 1

您的列 created_at 是 timestamp without time zone。

但是now()returnstimestamp with time zone。表达式 now() - '1 hour'::interval 被强制转换为 timestamp [without time zone]，其中包含 两个问题：

1.)这个不是你求的，只是表达不靠谱。其结果取决于正在执行查询的会话的当前时区设置。详情在这里：

Ignoring timezones altogether in Rails and PostgreSQL

为了表达清楚，您可以使用：

now() AT TIME ZONE 'Europe/London' -- your time zone here

或者只是 (read the manual here):

LOCALTIMESTAMP  -- explicitly take the local time

我会考虑使用 timestamptz。
都没有解决你的第二个问题：

2.) 回答你的问题。约束排除不起作用。 Per documentation:

The following caveats apply to constraint exclusion:

Constraint exclusion only works when the query's WHERE clause contains constants (or externally supplied parameters). For example, a comparison against a non-immutable function such as CURRENT_TIMESTAMP cannot be optimized, since the planner cannot know which partition the function value might fall into at run time.

大胆强调我的。

now() 是 CURRENT_TIMESTAMP 的 Postgres 实现。在系统目录中可以看到，只有STABLE，没有IMMUTABLE:

SELECT proname, provolatile FROM pg_proc WHERE proname = 'now';

proname | provolatile
--------+------------
now     | s              -- meaning: STABLE

解决方案

1.) 您可以通过在 WHERE 条件（始终为 "immutable"）中提供常量来克服限制：

select count(*) from events
where created_at > '2015-05-25 15:49:20.037815'::timestamp;  -- derived from your example

2.)或者通过"faking"一个不可变函数：

CREATE FUNCTION f_now_immutable()
  RETURNS timestamp AS
$func$
SELECT now() AT TIME ZONE 'UTC'  -- your time zone here
$func$  LANGUAGE sql IMMUTABLE;

然后：

select count(*) from events
where created_at > f_now_immutable() - interval '1 hour'

请注意如何使用它：虽然 now() 是 STABLE（在交易期间不会改变），但它会改变在事务之间，所以注意不要在准备好的语句（作为参数值除外）或索引或任何可能会咬你的东西中使用它。

3.) 或者您可以将看似冗余的常量 WHERE 子句添加到与您的分区约束相匹配的当前查询：

SELECT count(*)
FROM   events
WHERE  created_at > now() - '1 hour'::interval
<b>AND    created_at >= '2015-04-01 00:00:00'::timestamp
AND    created_at <= '2015-04-30 23:59:59.999999'::timestamp</b>;

只需确保 now() - '1 hour'::interval 属于正确的分区，否则您显然得不到任何结果。

旁白：我宁愿在 CHECK 约束和查询中使用这个表达式。更容易处理和做同样的事情：

       created_at >= '2015-04-01 0:0'::timestamp
AND    created_at <  '2015-05-01 0:0'::timestamp

基于检查约束的分区修剪未按预期工作

Partition pruning based on check constraint not working as expected

sql

postgresql

postgresql-9.3

database-partitioning

postgresql-performance

解决方案