Re: [GENERAL] 8.2.4 serious slowdown

From: Sim Zacks <sim(at)compulab.co.il>
Date: Sun Jan 13 2008 - 00:59:22 EST

How would you rewrite something like:

   WHERE (COALESCE(b.quantity, 0) - COALESCE(b.deliveredsum, 0)) > 0; I could write:
where case when b.quantity is null then 0 else b.quantity end - case when b.deliveredsum is null then 0 else b.deliveredsum end > 0

It is butt ugly, but is that the most efficient way to write it in 8.2.4?

Sim

Pavel Stehule wrote:
> Hello
>
> On 11/01/2008, Sim Zacks <sim@compulab.co.il> wrote:

>> I changed it to "where f.commited is not true" and the query now takes 1 second as opposed to 60.
>> (much faster then the 3 seconds it took on 8.0.1, which could also be because of the coalesce there)
>> Is it considered better practice (or more efficient) to always use (x is not or x=value)
>> instead of coalesce? Or does it make more sense to turn on the option "transform_null_equals"?
>>

>
> You can use without coalesce() = some operator IS DISTINCT FROM ... .
> Use coalesce only if you need some NON NULL value.
>
> for you sample
>
> where f.commited IS DISTINCT FROM true;
>
> operator IS DISTINCT FROM is NULL insensitive
>
> Regards
> Pavel Stehule
>

>> Thank you much
>> Sim
>>
>>> I assume that the original query is something along the lines of
>>>
>>>       d left join f on (...) where coalesce(f.commited, false) = false
>>>
>>>
>>> In the meantime, Sim would probably have better luck if he restructured
>>> this particular clause in some other way, say
>>>
>>>       where f.commited is not true
>>> or
>>>       where f.commited = false or f.commited is null
>>>
>>> Note also that he really ought to move up to 8.2.6, as 8.2.4 is not
>>> very sane about what IS NULL means for a left join's result.
>>>
>>>                       regards, tom lane
>>>
>>> ---------------------------(end of broadcast)---------------------------
>>> TIP 6: explain analyze is your friend
>>>
>> ---------------------------(end of broadcast)---------------------------
>> TIP 5: don't forget to increase your free space map settings
>>

>
> ---------------------------(end of broadcast)---------------------------
> TIP 3: Have you checked our extensive FAQ?
>
> http://www.postgresql.org/docs/faq
>

---------------------------(end of broadcast)---------------------------

TIP 3: Have you checked our extensive FAQ?

               http://www.postgresql.org/docs/faq Received on Sun Jan 13 01:27:45 2008