From: | Tomas Vondra <tv(at)fuzzy(dot)cz> |
---|---|
To: | pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: 9.5: Memory-bounded HashAgg |
Date: | 2014-09-03 23:41:34 |
Message-ID: | 5407A72E.8030707@fuzzy.cz |
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Lists: | pgsql-hackers |
On 4.9.2014 01:34, Tomas Vondra wrote:
> On 20.8.2014 20:32, Robert Haas wrote:
>>
>> As I see it, the advantage of Jeff's approach is that it doesn't
>> really matter whether our estimates are accurate or not. We don't
>> have to decide at the beginning how many batches to do, and then
>> possibly end up using too much or too little memory per batch if we're
>> wrong; we can let the amount of memory actually used during execution
>> determine the number of batches. That seems good. Of course, a hash
Also, you don't actually have to decide the number of batches at the
very beginning. You can start start with nbatch=1 and decide how many
batches to use when the work_mem is reached. I.e. at exactly the same
moment / using the same amount of info as with Jeff's approach. No?
Tomas
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