Re: Why won't it index scan?

Sorry for the naive question, but: is there a problem with analyze doing
full table scans? Analyze will not lock anything, will it?

Peter

Greg Stark wrote:
> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
>
>
>> "Ed L." <pgsql(at)bluepolka(dot)net> writes:
>>
>>> So, does this sound like we just happened to get repeatedly
>>> horribly unrepresentative random samples with stats target at
>>> 10? Are we at the mercy of randomness here? Or is there a
>>> better preventive procedure we can follow to systematically
>>> identify this kind of situation?
>>>
>> I think the real issue is that stats target 10 is too small for large
>> tables: the samples are just not large enough to support a decent
>> numdistinct estimate, which is the critical stat for cases such as this
>> (ie, estimating the number of hits on a value that's not in the
>> most-common-values list).
>>
>
> There's been some discussion on -hackers about this area. Sadly the idea of
> using samples to calculate numdistinct estimates is fundamentally on pretty
> shaky ground.
>
> Whereas a fixed sample size works fine for calculating distribution of values,
> in order to generate consistent precision for numdistinct estimates the
> samples will have to be a constant fraction of the table -- and unfortunately
> a pretty large fraction at that.
>
> So sadly I think "at the mercy of randomness" is pretty accurate. You'll have
> to raise the statistics target as the table grows and I expect you'll
> eventually run into some downsides of large stats targets.
>
> Some better algorithms were posted, but they would require full table scans
> during analyze, not just samples.
>
>