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Re: gin_fuzzy_search_limit description

From: Teodor Sigaev <teodor(at)sigaev(dot)ru>
To: Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: gin_fuzzy_search_limit description
Date: 2006-11-30 12:33:52
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
 From docs

     The primary goal of developing GIN indices was support for highly scalable, 
full-text search in PostgreSQL and there are often situations when a full-text 
search returns a very large set of results. Since reading tuples from the disk 
and sorting them could take a lot of time, this is unacceptable for production. 
(Note that the index search itself is very fast.)

     Such queries usually contain very frequent words, so the results are not 
very helpful. To facilitate execution of such queries GIN has a configurable 
soft upper limit of the size of the returned set, determined by the 
gin_fuzzy_search_limit GUC variable. It is set to 0 by default (no limit).

     If a non-zero search limit is set, then the returned set is a subset of the 
whole result set, chosen at random.

     "Soft" means that the actual number of returned results could slightly 
differ from the specified limit, depending on the query and the quality of the 
system's random number generator.
Alvaro Herrera wrote:

> Hi,
> I'm not very clear on what this is supposed to mean.  The description in
> guc.c is this:
> 	Sets the maximum allowed result for exact search by GIN.
> Say again?
> The involved code is this:

So this piece is about "choosen at random", some below dropItem macros use this 
result directly in calculations.
>         if (GinFuzzySearchLimit > 0)
>         {
>             /*
>              * If all of keys more than treshold we will try to reduce result,
>              * we hope (and only hope, for intersection operation of array our
>              * supposition isn't true), that total result will not more than
>              * minimal predictNumberResult.
>              */
>             for (i = 0; i < key->nentries; i++)
>                 if (key->scanEntry[i].predictNumberResult <= key->nentries * GinFuzzySearchLimit)
>                     return;
>             for (i = 0; i < key->nentries; i++)
>                 if (key->scanEntry[i].predictNumberResult > key->nentries * GinFuzzySearchLimit)
This comparing is just an artifact after tuning and debugging... May I remove it 
in RC stage?

Teodor Sigaev                                   E-mail: teodor(at)sigaev(dot)ru

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