Date: Fri, 25 Aug 2017 10:44:34 -0500
From: Justin Pryzby <pryzby@telsasoft.com>
To: Aniko Belim <anikob@spiceworks.com>
Cc: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: Partitioned table - scans through every partitions
Message-ID: <20170825154434.GC16287@telsasoft.com>
References: <6EB8CBFC-49E5-4745-96D8-532EC912D039@spiceworks.com>
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On Fri, Aug 25, 2017 at 03:36:29PM +0000, Aniko Belim wrote:
> Hi,
> 
> We have an issue with one of our partitioned tables. It has a column with timestamp without time zone type, and we had to partition it daily. To do that, we created the following constraints like this example:
> CHECK (to_char(impression_time, 'YYYYMMDD'::text) = '20170202'::text)
> 
> 
> The problem we’re facing is no matter how we’re trying to select from it, it scans through every partitions.


> It scans through every partitions. Shouldn’t it only scan the dfp_in_network_impressions.dfp_in_network_impressions_20170202 child table? Or we missing something?
> Any advice/help would highly appreciated.

https://www.postgresql.org/docs/9.6/static/ddl-partitioning.html#DDL-PARTITIONING-CAVEATS
|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.

...


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