| From: | Pantelis Theodosiou <ypercube(at)gmail(dot)com> |
|---|---|
| To: | Corey Huinker <corey(dot)huinker(at)gmail(dot)com> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: missing optimization - column <> column |
| Date: | 2016-12-05 20:59:19 |
| Message-ID: | CAE3TBxyot9f1yEhmEaTpxN4+9rTBgRR_uX3cqcWegLNEFdvKaQ@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Mon, Dec 5, 2016 at 7:02 PM, Corey Huinker <corey(dot)huinker(at)gmail(dot)com>
wrote:
>
>> Would it be worth replacing the condition with the equivalent?
>> I mean would that help optimizing better some queries when it knows that
>> a is (not) nullable or when "a" is more complicated expression?
>>
>> a <> a : (a IS NULL) AND NULL
>> a = a : (a IS NOT NULL) OR NULL
>>
>
>
> I think you're looking for
>
> a IS DISTINCT FROM a
>
>
> And that will work for cases where a might be null.
>
> I have no opinion about whether adding such a test to the planner is worth
> it.
>
No, (a IS DISTINCT FROM a) will evaluate to FALSE when a is NULL. The
other conditions (a <> a) , ((a IS NULL) AND NULL) will evaluate to NULL.
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