Re: queries with lots of UNIONed relations

On 1/13/2011 4:42 PM, Robert Haas wrote:
> 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*
>

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

-Andy