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From: Andy Colson <andy@squeakycode.net>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Jon Nelson <jnelson+pgsql@jamponi.net>
Cc: pgsql-performance@postgresql.org
Subject: Re: queries with lots of UNIONed relations
Date: Thu, 13 Jan 2011 16:47:31 -0600
Message-ID: <4D2F8103.7010805@squeakycode.net> (raw)
In-Reply-To: <AANLkTinXs1v8zPJiL-di7TZok6CNgOfv-Dan8H+Ot2DN@mail.gmail.com>
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On 1/13/2011 4:42 PM, Robert Haas wrote:
> On Thu, Jan 13, 2011 at 5:41 PM, Robert Haas<robertmhaas@gmail.com>  wrote:
>> On Thu, Jan 13, 2011 at 5:26 PM, Tom Lane<tgl@sss.pgh.pa.us>  wrote:
>>> Robert Haas<robertmhaas@gmail.com>  writes:
>>>> On Thu, Jan 13, 2011 at 3:12 PM, Jon Nelson<jnelson+pgsql@jamponi.net>  wrote:
>>>>> I still think that having UNION do de-duplication of each contributory
>>>>> relation is a beneficial thing to consider -- especially if postgresql
>>>>> thinks the uniqueness is not very high.
>>>
>>>> This might be worth a TODO.
>>>
>>> I don't believe there is any case where hashing each individual relation
>>> is a win compared to hashing them all together.  If the optimizer were
>>> smart enough to be considering the situation as a whole, it would always
>>> do the latter.
>>
>> You might be right, but I'm not sure.  Suppose that there are 100
>> inheritance children, and each has 10,000 distinct values, but none of
>> them are common between the tables.  In that situation, de-duplicating
>> each individual table requires a hash table that can hold 10,000
>> entries.  But deduplicating everything at once requires a hash table
>> that can hold 1,000,000 entries.
>>
>> Or am I all wet?
>
> Yeah, I'm all wet, because you'd still have to re-de-duplicate at the
> end.  But then why did the OP get a speedup?  *scratches head*
>

Because it all fix it memory and didnt swap to disk?

-Andy



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