Re: queries with lots of UNIONed relations

On 1/13/2011 4:49 PM, Robert Haas wrote:
> On Thu, Jan 13, 2011 at 5:47 PM, Andy Colson<andy(at)squeakycode(dot)net> wrote:
>>>>> 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?
>
> Doesn't make sense. The re-de-duplication at the end should use the
> same amount of memory regardless of whether the individual relations
> have already been de-duplicated.
>

Unless I missed something in the thread:

distinctList + distinctList + ... -> [fit in mem] -> last distinct ->
[fit in mem]

vs.

fullList + fullList + ... -> [swapped to disk] -> last distinct -> [fit
in mem]

-Andy