Re: Full Text Index Scanning

[Matt Warner <matt@xxxxxxxxxxxxxxxxxxxx>]

Reverse isn't a built-in Postgres function, so I found one and installed it. However, attempting to use it in creating an index gets me the message "ERROR:  functions in index _expression_ must be marked IMMUTABLE", even though the function declaration already has the immutable argument.

Is there a specific version of the reverse function you're using? Or am I just missing something obvious? This is Postgres 9, BTW.

Thanks,

Matt

On Sat, Jan 29, 2011 at 6:46 AM, Matt Warner <matt@xxxxxxxxxxxxxxxxxxxx> wrote:

Thanks Oleg. I'm going to have to experiment with this so that I understand it better.

Matt

On Fri, Jan 28, 2011 at 1:12 PM, Oleg Bartunov <oleg@xxxxxxxxxx> wrote:

Matt, I'd try to use prefix search on original string concatenated with reverse string:

Just tried on some spare table

knn=# \d spot_toulouse
           Table "public.spot_toulouse"
      Column        |       Type        | Modifiers ---------------------+-------------------+-----------
 clean_name          | character varying |


1. create index knn=# create index clean_name_tlz_idx on spot_toulouse using gin(to_tsvector('french', clean_name || ' ' || reverse(clean_name)));
2.
select clean_name from spot_toulouse where to_tsvector('french', clean_name|| ' ' ||  reverse(clean_name) ) @@ to_tsquery('french','the:* | et:*');

Select looks cumbersome, but you can always write wrapper functions. The only drawback I see for now is that ranking function will a bit confused,
since coordinates of original and reversed words will be not the same, but again, it's possible to obtain tsvector by custom function, which aware about reversing.

Good luck and let me know if this help you.

Oleg

On Fri, 28 Jan 2011, Matt Warner wrote:

I'm in the process of migrating a project from Oracle to Postgres and have
run into a feature question. I know that Postgres has a full-text search
feature, but it does not allow scanning the index (as opposed to the data).
Specifically, in Oracle you can do "select * from table where
contains(colname,'%part_of_word%')>1". While this isn't terribly efficient,
it's much faster than full-scanning the raw data and is relatively quick.

It doesn't seem that Postgres works this way. Attempting to do this returns
no rows: "select * from table where to_tsvector(colname) @@
to_tsquery('%part_of_word%')"

The reason I want to do this is that the partial word search does not
involve dictionary words (it's scanning names).

Is this something Postgres can do? Or is there a different way to do scan
the index?

TIA,

Matt

       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