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Re: [GENERAL] Creation of tsearch2 index is very

From: Oleg Bartunov <oleg(at)sai(dot)msu(dot)su>
To: Ron <rjpeace(at)earthlink(dot)net>
Cc: pgsql-performance(at)postgresql(dot)org, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Subject: Re: [GENERAL] Creation of tsearch2 index is very
Date: 2006-01-21 16:33:27
Message-ID: (view raw or whole thread)
Lists: pgsql-generalpgsql-performance
On Sat, 21 Jan 2006, Ron wrote:

> Perhaps a different approach to this problem is called for:
> _Managing Gigabytes: Compressing and Indexing Documents and Images_  2ed
> Witten, Moffat, Bell
> ISBN 1-55860-570-3
> This is a VERY good book on the subject.
> I'd also suggest looking at the publicly available work on indexing and 
> searching for search engines like Inktomi (sp?) and Google.
> Ron


you completely miss the problem ! We do know MG and other SE.  Actually,
we've implemented several search engines based on inverted index technology 
(see, for example, tsearch2 was designed for
online indexing, while keeping inverted index online is rather difficult
problem. We do have plan to implement inverted index as an option for
large read-only archives, but now we discuss how to organize online
index and if possible to optimize current storage for signatures 
without breaking search performance.

> At 08:34 AM 1/21/2006, Oleg Bartunov wrote:
>> On Sat, 21 Jan 2006, Ron wrote:
>>> At 07:23 PM 1/20/2006, Tom Lane wrote:
>>>> "Steinar H. Gunderson" <sgunderson(at)bigfoot(dot)com> writes:
>>>> > On Fri, Jan 20, 2006 at 06:52:37PM -0500, Tom Lane wrote:
>>>> >> It's also worth considering that the entire approach is a heuristic,
>>>> >> really --- getting the furthest-apart pair of seeds doesn't guarantee
>>>> >> an optimal split as far as I can see.  Maybe there's some totally
>>>> >> different way to do it.
>>>> > For those of us who don't know what tsearch2/gist is trying to 
>>>> accomplish
>>>> > here, could you provide some pointers? :-)
>>>> Well, we're trying to split an index page that's gotten full into two 
>>>> index pages, preferably with approximately equal numbers of items in
>>>> each new page (this isn't a hard requirement though).
>>> Maybe we are over thinking this.  What happens if we do the obvious and 
>>> just make a new page and move the "last" n/2 items on the full page to the 
>>> new page?
>>> Various forms of "move the last n/2 items" can be tested here:
>>> 0= just split the table in half.  Sometimes KISS  works. O(1).
>>> 1= the one's with the highest (or lowest) "x" value.
>>> 2= the one's with the highest sum of coordinates (x+y+...= values in the 
>>> top/bottom n/2 of entries).
>>> 3= split the table so that each table has entries whose size_waste values 
>>> add up to approximately the same value.
>>> 4= I'm sure there are others.
>>> 1-5 can be done in O(n) time w/o auxiliary data.  They can be done in O(1) 
>>> if we've kept track of the appropriate metric as we've built the current 
>>> page.
>>>> I think the true figure of merit for a split is how often will subsequent 
>>>> searches have to descend into *both* of the resulting pages as opposed to 
>>>> just one
>>>> --- the less often that is true, the better.  I'm not very clear on what 
>>>> tsearch2 is doing with these bitmaps, but it looks like an upper page's 
>>>> downlink has the union (bitwise OR) of the one-bits in the values on the 
>>>> lower page, and you have to visit the lower page if this union has a 
>>>> nonempty intersection with the set you are looking for.  If that's 
>>>> correct, what you really want is to divide the values so that the unions 
>>>> of the two sets have minimal overlap ... which seems to me to have little 
>>>> to do with what the code does at present.
>>> I'm not sure what "upper page" and "lower page" mean here?
>>>> Teodor, Oleg, can you clarify what's needed here?
>>> Ditto.  Guys what is the real motivation and purpose for this code?
>> we want not just split the page by two very distinct parts, but to keep
>> resulted signatures which is ORed signature of all signatures in the page
>> as much 'sparse' as can. some information available here
>> Unfortunately, we're rather busy right now and couldn't be very useful.

Oleg Bartunov, Research Scientist, Head of AstroNet (,
Sternberg Astronomical Institute, Moscow University, Russia
Internet: oleg(at)sai(dot)msu(dot)su,
phone: +007(495)939-16-83, +007(495)939-23-83

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