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
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, pgsql.ru/db/pgsearch). 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@xxxxxxxxxxx> 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
http://www.sai.msu.su/~megera/oddmuse/index.cgi/Tsearch_V2_internals
Unfortunately, we're rather busy right now and couldn't be very useful.
Regards,
Oleg
_____________________________________________________________
Oleg Bartunov, Research Scientist, Head of AstroNet (www.astronet.ru),
Sternberg Astronomical Institute, Moscow University, Russia
Internet: oleg@xxxxxxxxxx, http://www.sai.msu.su/~megera/
phone: +007(495)939-16-83, +007(495)939-23-83
