Miernik <public(at)public(dot)miernik(dot)name> writes:
> On Wed, Jul 30, 2008 at 11:08:06PM -0400, Tom Lane wrote:
>> Hmm, what have you got work_mem set to? The first one would likely
>> have been a lot faster if it had hashed the subplan; which I'd have
>> thought would happen with only 80K rows in the subplan result,
>> except it didn't.
> work_mem = 1024kB
Try increasing that ... I don't recall the exact per-row overhead
but I'm quite sure it's more than 8 bytes. Ten times that would
likely get you to a hash subquery plan.
> The machine has 48 MB total RAM and is a Xen host.
48MB is really not a sane amount of memory to run a modern database
in. Maybe you could make it go with sqlite or some other tiny-footprint
DBMS, but Postgres isn't focused on that case.
>> The queries are in fact not exactly equivalent, because EXCEPT
>> involves some duplicate-elimination behavior that won't happen
>> in the NOT IN formulation. So I don't apologize for your having
>> gotten different plans.
> But if use EXCEPT ALL?
Fraid not, EXCEPT ALL has yet other rules for how it deals with
>> Another issue is that the NOT IN will probably not do what you
>> expected if the subquery yields any NULLs.
> In this specific query I think it is not possible for the subquery to
> have NULLs,
Okay, just wanted to point out a common gotcha.
regards, tom lane
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