From: | Stephen Frost <sfrost(at)snowman(dot)net> |
---|---|
To: | Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> |
Cc: | PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: missing optimization - column <> column |
Date: | 2016-12-05 15:23:52 |
Message-ID: | 20161205152352.GB23417@tamriel.snowman.net |
Views: | Raw Message | Whole Thread | Download mbox | Resend email |
Thread: | |
Lists: | pgsql-hackers |
Pavel,
* Pavel Stehule (pavel(dot)stehule(at)gmail(dot)com) wrote:
> I found some crazy queries in one customer application. These queries are
> stupid, but it was surprise for me so there are not some simple optimization
>
> create table foo(a int);
> insert into foo select generate_series(1,100000);
> analyze foo;
> explain select * from foo where a <> a;
>
> It does full scan of foo, although it should be replaced by false in
> planner time.
a <> a could go to NULL. Obviously, that'll be false for such a simple
case, but it might not work out that way in a more complicated WHERE
clause.
> Same issue is a expression a = a .. can be replaced by true
a = a can't be replaced unless you know that 'a' can't be NULL.
In short, fix the application.
Thanks!
Stephen
From | Date | Subject | |
---|---|---|---|
Next Message | Tom Lane | 2016-12-05 15:24:22 | Re: missing optimization - column <> column |
Previous Message | Robert Haas | 2016-12-05 15:21:18 | Re: [COMMITTERS] pgsql: Introduce dynamic shared memory areas. |