From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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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 15:24:22 |
Message-ID: | 32715.1480951462@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> writes:
> 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.
> Same issue is a expression a = a .. can be replaced by true
Wrong; those expressions yield NULL for NULL input. You could perhaps
optimize them slightly into some form of is-null test, but it hardly
seems worth the planner cycles to check for.
If you write something like "1 <> 1", it will be folded.
regards, tom lane
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