Re: Differences between to_tsquery() and plainto_tsquery() when it comes to applying an index?

Denis,

we need examples of your explain analyze. I don't want to waste my time
reading theoretical reasoning :)

btw, Be sure you use the same search configuration as in create index or
index will not be used at all.

Oleg

On Wed, 25 Aug 2010, Denis Papathanasiou wrote:

> As a follow-up to my question from this past Saturday
> (http://archives.postgresql.org/pgsql-general/2010-08/msg00624.php), I
> experimented with adding two types of indices to the same text column:
>
> CREATE INDEX item_eng_searchable_text_idx ON item USING
> gin(to_tsvector('english', searchable_text));
>
> and
>
> CREATE INDEX item_searchable_text_idx ON item (searchable_text);
>
> Running my queries under explain, I noticed that queries of the form:
>
> select pk from item where searchable_text @@ plainto_tsquery('search
> phrase');
>
> Actually run *slower* with the item_eng_searchable_text_idx index applied.
>
> But when I switch the query to use to_tsquery() instead, e.g. something
> like this:
>
> select pk from item where searchable_text @@ to_tsquery('phrase');
>
> The performance is better.
>
> Is this because the gin/to_tsvector() index works differently for
> to_tsquery() compared to plainto_ts_query() ?
>
> If so, how can I create an effective index for queries that will use
> plainto_tsquery() ?
>
> Note that I need the plainto_tsquery() form b/c my search phrases will
> correspond to exact blocks of text, and therefore they will contain
> spaces and punctuation, etc.
>
>
>

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