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Re: tsvector pg_stats seems quite a bit off.

From: Jesper Krogh <jesper(at)krogh(dot)cc>
To: Jan Urbański <wulczer(at)wulczer(dot)org>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: tsvector pg_stats seems quite a bit off.
Date: 2010-05-29 10:34:43
Message-ID: 4C00EDC3.6070008@krogh.cc (view raw or flat)
Thread:
Lists: pgsql-hackers
On 2010-05-28 23:47, Jan Urbański wrote:
> On 28/05/10 22:22, Tom Lane wrote:
>    
>> The idea that I was toying with is to assume a Zipfian distribution of
>> the input (with some reasonable parameter), and use that to estimate
>> what the frequency of the K'th element will be, where K is the target
>> number of MCV entries or perhaps a bit more.  Then use that estimate as
>> the "s" value, and set e = s/10 or so, and then w = 1/e and continue as
>> per the paper.  If the eventual filtering results in a lot less than the
>> target number of MCV entries (because the input wasn't so Zipfian), we
>> lose, but at least we have accurate numbers for the entries we kept.
>>      
> I see what you mean, so the idea would be:
>
>   * assume some value of W as the number of all words in the language
>   * estimate s as 1/(st + 10)*H(W), where H(W) is the W'th harmonic
> number and st is the statistics target, using Zipf's law
>   * set e = s/10 and w = 1/e, that is 10/s
>   * perform LC using that value of w
>   * remove all elements for which f<  (s-e)N, that is f<  0.9*sN, where N
> is the total number of lexemes processed
>   * create the MCELEM entries as (item, f/N)
>
> Now I tried to substitute some numbers there, and so assuming the
> English language has ~1e6 words H(W) is around 6.5. Let's assume the
> statistics target to be 100.
>
> I chose s as 1/(st + 10)*H(W) because the top 10 English words will most
> probably be stopwords, so we will never see them in the input.
>    

I think you should skip the assumption about stop-words, users may
use something where they are needed in the index or have a language
than the typical.  (and they dont seem to influcence the math that much).

Isn't it the same "type" of logic that is used for collecting statistics 
for
"array-types", say integer-arrays and text arrays?
> Using the above estimate s ends up being 6.5/(100 + 10) = 0.06
>
> We then do LC, pruning the D structure every w = 1/0.006 = 167 lexemes
>    

Im not sure I get this one.. does this mean that we prune everytime
we have collected 167 new datapoints .. that would seem too often
for me since that would roughly be once per "row".

> After that, we remove lexemes with f<  0.9 * 0.06 * N = 0.054*N
>
> So assuming that on average a tsvector has 154 elements and that we went
> through 35017 rows (as it would be in Jesper's case, before he raised
> the stats target from 100 to 1000), we will remove lexemes with f<
> 0.054 * 35017 * 154 that is f<  291201.37
>
> I wonder what would happen if Jasper's case if we did that... And I
> wonder how sound that maths is
>    

If it means that I would get an accurate MCE-histogram for all
things that have an occourance of more than 5.4% of the rows
(given the samples chosen), then I think that would be really
reasonable.

I can "fairly easy" try out patches or do other kind of testing.

-- 
Jesper

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Subject: Re: tsvector pg_stats seems quite a bit off.
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