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Re: Full text searching, anyone interested?

From: mlw <markw(at)mohawksoft(dot)com>
To: Gavin Sherry <swm(at)linuxworld(dot)com(dot)au>
Cc: Hackers List <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Full text searching, anyone interested?
Date: 2001-06-03 17:22:03
Message-ID: 3B1A723B.8734FDAF@mohawksoft.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Gavin Sherry wrote:
> 
> Hi guys,
> 
> On Sat, 2 Jun 2001, mlw wrote:
> 
> > I frequently rant on this about full text searching.
> >
> [snip]
> > I would love to find a way to get a bitmap like index native to Postgres. I
> > would, in fact, love and expect to do an amount of it. The problem is to do it
> > "right" will require a lot of work at very low levels of Postgres.
> >
> > Is anyone interested in pursuing this?
> 
> Yes. I think this would be an important feature of PostgreSQL. I have
> hacked contrib/fulltextindex to bits in order to segment the index so that
> I can better deploy it on a cluster of postgres machines. I have also
> changed it to a score/rating style system. However, it is still a word ->
> oid relationship and is not scaling as well as I had hoped.
> 
> > How should it look?
> 
> In terms of interface to SQL, the function call which activates your FTI
> search is much neater than the way I do it -- build a query based on the
> number of search terms and the boolean operator used.
> 
> In terms of how it is interfaced to Postgres backend, I think it should be
> an index type which one can apply to any character orientated
> columns(s). It would be important that the index capable of handling
> multiple columns so that many textual fields in a single row could be
> indexed.
> 
> The index itself is where troubles would be found. People expect a lot
> from full text searches. My own implementations allow chronological
> searching, score based searching and searching on similar words. It would
> be hard to interface this to CREATE INDEX as well as select. So, if a
> native full text index was to be build it would have to be able to:
> 
> a) Index multiple columns
> b) Be configurable: score/frequency based sorting, sorting in terms of a
> column in an index row?
> c) Be interfaced to a user level fti() function
> d) Be ignored by the planner (if we want searches to occur only through
> fti())
> e) honour insert/delete.
> 
> Something else which is an issue is the size of the index. Indices on text
> columns are generally very large. In my applications I have managed to
> reduce this through segmenting the indices along the following lines: case
> sensitivity/insensitivty, leading characters. This dramatically reduces
> the IO load of an index scan -- but it would be quite difficult to build
> this into a dynamic framework for the backend. For one, a VACUUM or some
> equivalent would need to evaluate the size of a given index and, based on
> other configuration information (is the user allowing index
> segmentation?) segment the index based on some criteria. The problem then
> is that for large indices, this could take quite some time.
> 
> I've probably over looked a fair number of other things which would need
> to be considered. However, it's safe to say that such a feature native to
> Postgres would be greatly appreciated by many.
> 

Actually, I was sort of thinking.....

We could implement bitmap handling functions based on one dimentional arrays of
integers. That's how my stuff deals with them, and postgres already manages
them. 

create table fubar_words (char string, bitmap int4 []);

Store various info in the bitmap.

Then we can create another table:

create table fubar_oids(bit integer, id oid);


It would work much like my system, but would use postgres to manage all the
data. It would be less efficient than the stand alone external system, but
perform much better than the fti package.

Hmmm. thinking. 



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