Re: Using GIN indexes on 8.2

Alexander Staubo wrote:
> Two questions about GIN on 8.2. There's not much documentation about
> GIN, but this should be possible:
>
> create table foo (values text[]);
> create index foo_values_index on foo using gin (text);
>
> However, this then fails saying the operator "@" does not exist:
>
> select * from foo where values @ '{hello, world}'

Use @>, <@ operations instead of @ and ~
Look for discussions in -hackers for reasons of changing names

> Do I need to reference a specific opclass when creating the index? From
> the documentation I got the impression that GIN bundled operators for
> most built-in types.
if there is a default opclass for your datatype - you may do not specify.

>
> Secondly, are GIN indexes immutable and (unlike Tsearch2) non-lossy and
> therefore useful with functional indexes? I would like to do this:
>
> create table bar (value text);
> create index bar_value_index on bar using gin (analyze(value));
>
> where analyze() is a function of my own that tokenizes, stems and
> filters the text into a text[] array.

Be careful -
select
pg_opclass.opcname,
pg_operator.oprname,
pg_amop.amopreqcheck
from
pg_opclass,
pg_operator,
pg_amop,
pg_am
where
pg_operator.oid = pg_amop.amopopr and
pg_opclass.oid = pg_amop.amopclaid and
pg_opclass.opcamid = pg_am.oid and
pg_am.amname='gin' and pg_opclass.opcname='_text_ops';
opcname | oprname | amopreqcheck
-----------+---------+--------------
_text_ops | && | f
_text_ops | @> | f
_text_ops | <@ | t
_text_ops | = | t
(4 rows)

So, operations <@ and = will recheck result with table's row.

Pls, why don't you use tsearch2 with GIN?

--
Teodor Sigaev E-mail: teodor(at)sigaev(dot)ru
WWW: http://www.sigaev.ru/