Re: queries with lots of UNIONed relations

On Thu, Jan 13, 2011 at 5:41 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Thu, Jan 13, 2011 at 5:26 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>>> On Thu, Jan 13, 2011 at 3:12 PM, Jon Nelson <jnelson+pgsql(at)jamponi(dot)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*

--
Robert Haas
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