Thread: turning a tsvector without position in a weighted tsvector

If I convert a string to a tsvector just casting (::tsvector) I
obtain a vector without positions.
tsvectors without positions don't have weights too.

I haven't found a way to turn a vector without weight/pos, into a
vector with weight/pos.

Is there a way to apply weight/add positions to tsvectors without
positions?
Is there any use-case?

--
Ivan Sergio Borgonovo
http://www.webthatworks.it

Ivan,

what's wrong with:

postgres=# select 'abc:1'::tsvector;
  tsvector
----------
  'abc':1

postgres=# select setweight('abc:1'::tsvector,'a');
  setweight
-----------
  'abc':1A

or just use to_tsvector() instead of casting?

Oleg
On Mon, 8 Feb 2010, Ivan Sergio Borgonovo wrote:

> If I convert a string to a tsvector just casting (::tsvector) I
> obtain a vector without positions.
> tsvectors without positions don't have weights too.
>
> I haven't found a way to turn a vector without weight/pos, into a
> vector with weight/pos.
>
> Is there a way to apply weight/add positions to tsvectors without
> positions?
> Is there any use-case?
>
>

     Regards,
         Oleg
_____________________________________________________________
Oleg Bartunov, Research Scientist, Head of AstroNet (www.astronet.ru),
Sternberg Astronomical Institute, Moscow University, Russia
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(495)939-16-83, +007(495)939-23-83

On Mon, 8 Feb 2010 23:01:45 +0300 (MSK)
Oleg Bartunov <oleg@sai.msu.su> wrote:

> Ivan,
>
> what's wrong with:
>
> postgres=# select 'abc:1'::tsvector;
>   tsvector
> ----------
>   'abc':1

Yes you're right. I think I misplaced some quotes.
But still, once a vector has no position, I can't add the weights.

test=# select setweight('tano'::tsvector, 'A');
 setweight
-----------
 'tano'
(1 row)

test=# select setweight('tano:1'::tsvector, 'A');
 setweight
-----------
 'tano':1A
(1 row)

Since I'm writing some helper to manipulate tsvectors I was
wondering if
a) there is any reasonable use case of adding weights to
vectors with no position
b) I missed any obvious way to add weights to tsvectors that were
initially without positions

thanks

--
Ivan Sergio Borgonovo
http://www.webthatworks.it

This was what I was after:

test=# select version();
                                            version
------------------------------------------------------------------------------------------------
 PostgreSQL 8.3.9 on x86_64-pc-linux-gnu, compiled by GCC
 gcc-4.3.real (Debian 4.3.2-1.1) 4.3.2

test=# select to_tsvector('pino gino');
    to_tsvector
-------------------
 'gino':2 'pino':1
(1 row)

test=# select 'pino gino'::tsvector;
   tsvector
---------------
 'gino' 'pino'
(1 row)

test=# select to_tsvector('pino gino') @@ 'gino:B'::tsquery;
 ?column?
----------
 f
(1 row)

test=# select to_tsvector('pino gino') @@ 'gino:D'::tsquery;
 ?column?
----------
 t
(1 row)

test=# select ('pino gino'::tsvector) @@ 'gino:B'::tsquery;
 ?column?
----------
 t
(1 row)

test=# select to_tsvector('pino:1B gino') @@ 'pino'::tsquery;
 ?column?
----------
 t
(1 row)

test=# select 'pino gino'::tsvector || to_tsvector('pino gino');
     ?column?
-------------------
 'gino':2 'pino':1
(1 row)

test=# select 'pino gino'::tsvector || 'pino gino'::tsvector;
   ?column?
---------------
 'gino' 'pino'
(1 row)

test=# select to_tsvector('pino gino') || to_tsvector('pino gino');
       ?column?
-----------------------
 'gino':2,4 'pino':1,3
(1 row)

test=# select 'pino gino'::tsvector || to_tsvector('gino tano');
         ?column?
--------------------------
 'gino':1 'pino' 'tano':2

test=# select
  setweight('pino gino'::tsvector || to_tsvector('gino tano'), 'A');
         setweight
----------------------------
 'gino':1A 'pino' 'tano':2A
(1 row)


So (even if it may sound obvious to many):
- tsvectors may be a mix of lexemes with and without weights
- a lexeme without a weight (=! default D weight) is a lexeme with
  ALL weights
- you can't assign a weight to a lexeme without a position and it
  would be hard to assign a position after a document is parsed into
  a tsvector, so while in theory it could be reasonable to have
  lexemes with weight and no position, in practice you'll have to
  assign not meaningful positions if you'd like to assign a weight
  to a tsvector with no positions.

I still wonder if it would be reasonable to write a function that
forcefully assign a position and a weight to vectors to be used with
ts_rank.
I've some ideas about possible use cases but I'm still unsure if they
are reasonable.
eg. someone would be willing to save storage and CPU cycles storing
part of documents in precomputed tsvectors with no weight and then
build up a search with merged tsvectors with weights using ts_rank.

OK.. trying to finish up my tsvector_to_tsquery function in a
reasonable way first.

--
Ivan Sergio Borgonovo
http://www.webthatworks.it