Re: Flexible configuration for full-text search

> The patch introduces way to configure FTS based on CASE/WHEN/THEN/ELSE
> construction.

Interesting feature. I needed this flexibility before when I was
implementing text search for a Turkish private listing application.
Aleksandr and Arthur were kind enough to discuss it with me off-list
today.

> 1) Multilingual search. Can be used for FTS on a set of documents in
> different languages (example for German and English languages).
>
> ALTER TEXT SEARCH CONFIGURATION multi
> ALTER MAPPING FOR asciiword, asciihword, hword_asciipart,
> word, hword, hword_part WITH CASE
> WHEN english_hunspell AND german_hunspell THEN
> english_hunspell UNION german_hunspell
> WHEN english_hunspell THEN english_hunspell
> WHEN german_hunspell THEN german_hunspell
> ELSE german_stem UNION english_stem
> END;

I understand the need to support branching, but this syntax is overly
complicated. I don't think there is any need to support different set
of dictionaries as condition and action. Something like this might
work better:

ALTER TEXT SEARCH CONFIGURATION multi
ALTER MAPPING FOR asciiword, asciihword, hword_asciipart,
word, hword, hword_part WITH
CASE english_hunspell UNION german_hunspell
WHEN MATCH THEN KEEP
ELSE german_stem UNION english_stem
END;

To put it formally:

ALTER TEXT SEARCH CONFIGURATION name
ADD MAPPING FOR token_type [, ... ] WITH config

where config is one of:

dictionary_name
config { UNION | INTERSECT | EXCEPT } config
CASE config WHEN [ NO ] MATCH THEN [ KEEP ELSE ] config END

> 2) Combination of exact search with morphological one. This patch not
> fully solve the problem but it is a step toward solution. Currently, we
> should split exact and morphological search in query manually and use
> separate index for each part. With new way to configure FTS we can use
> following configuration:
>
> ALTER TEXT SEARCH CONFIGURATION exact_and_morph
> ALTER MAPPING FOR asciiword, asciihword, hword_asciipart,
> word, hword, hword_part WITH CASE
> WHEN english_hunspell THEN english_hunspell UNION simple
> ELSE english_stem UNION simple
> END

This could be:

CASE english_hunspell
THEN KEEP
ELSE english_stem
END
UNION
simple

> 3) Using different dictionaries for recognizing and output generation.
> As I mentioned before, in new syntax condition and command are separate
> and we can use it for some more complex text processing. Here an
> example for processing only nouns:
>
> ALTER TEXT SEARCH CONFIGURATION nouns_only
> ALTER MAPPING FOR asciiword, asciihword, hword_asciipart,
> word, hword, hword_part WITH CASE
> WHEN english_noun THEN english_hunspell
> END

This would also still work with the simpler syntax because
"english_noun", still being a dictionary, would pass the tokens to the
next one.

> 4) Special stopword processing allows us to discard stopwords even if
> the main dictionary doesn't support such feature (in example pl_ispell
> dictionary keeps stopwords in text):
>
> ALTER TEXT SEARCH CONFIGURATION pl_without_stops
> ALTER MAPPING FOR asciiword, asciihword, hword_asciipart,
> word, hword, hword_part WITH CASE
> WHEN simple_pl IS NOT STOPWORD THEN pl_ispell
> END

Instead of supporting old way of putting stopwords on dictionaries, we
can make them dictionaries on their own. This would then become
something like:

CASE polish_stopword
WHEN NO MATCH THEN polish_isspell
END