From: "jobapply" <jobapply@nextmail.ru>
To: <postgresql-performance@postgresql.org>
Subject: Indexing Function called on VACUUM and sorting ?
Date: Thu, 14 Jul 2005 03:46:23 +0400
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Resent-From: "Marc G. Fournier" <scrappy@hub.org>
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ReSent-Subject: Indexing Function called on VACUUM and sorting ?
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The question appeared because of strange issues with functional indexes.
It seems they are recalculated even where it is obviously not needed.

\d+ test:

 i              | integer |                          |
 t              | text    |                          |
 x              | text    |                          |
    "i_i" btree (i)
    "x_i" btree (xpath_string(x, 'data'::text))
    "x_ii" btree (xpath_string(x, 'movie/characters/character'::text))
    "x_iii" btree (xpath_string(x, 'movie/rating'::text))


1) 
When I run
VACUUM FULL ANALYZE VERBOSE 
OR
VACUUM ANALYZE

After text

INFO:  analyzing "public.test"
INFO:  "test": scanned 733 of 733 pages, containing 10000 live rows and 0
dead rows; 3000 rows in sample, 10000 estimated total rows

a lot of xpath_string calls occur. 
Does VACUUM rebuild indexes ? What for to recalculate that all?
It makes VACUUMing very slow.

Simple VACUUM call does not lead to such function calls.

2)
When I do 
select * from test order by xpath_string(x, 'movie/rating'::text) limit 1000
offset 10;

Planner uses index x_iii (as it should, ok here): 	Limit -> Index scan.
But many of calls to xpath_string occur in execution time. 
Why ? Index is calculated already and everything is so immutable..


Please answer if you have any ideas.. Functional indexes seemed so great
first, but now I uncover weird issues I can't understand..