Re: 8.1 (win32): partial index not used?

hi list

i've noticed a problem with an index, where the index wouldn't be used
anymore as soon as a partial condition is added.

here's the setup:
table:
   "oldtables.movies" containing around 50'000 entries. a small percentage
(roughly 4000) have a value for the columns "mov_year" (int, default: 0) and
"mov_folder" (varchar, default: '')

indices:
   movies_mov_year_idx: USING btree ("mov_year");
   movies_folder_idx: USING btree (lower((mov_folder)::text));

query:
   SELECT * FROM oldtables.movies WHERE mov_year = 2007 AND
lower(mov_folder) = 'harrypotter5' LIMIT 1

queryplan1:

Bitmap Heap Scan on movies  (cost= 5.67..9.68 rows=1 width=232) (actual
time=0.197..0.198 rows=1 loops=1)
  Recheck Cond: ((lower((mov_folder)::text) = 'harrypotter5'::text) AND
(mov_year = 2007))
  ->  BitmapAnd  (cost=5.67..5.67 rows=1 width=0) (actual time= 0.191..0.191
rows=0 loops=1)
        ->  Bitmap Index Scan on movies_folder_idx  (cost=0.00..2.64
rows=183 width=0) (actual time=0.089..0.089 rows=1 loops=1)
              Index Cond: (lower((mov_folder)::text) = 'harrypotter5'::text)
        ->  Bitmap Index Scan on movies_mov_year_idx  (cost=0.00..2.78
rows=222 width=0) (actual time=0.097..0.097 rows=62 loops=1)
              Index Cond: (mov_year = 2007)
Total runtime: 0.274 ms


i thought of changing the movies_folder_idx to a partial index for speeding
up the query:

movies_folder_idx: USING btree ((lower((mov_folder)::text))) WHERE
((mov_folder)::text <> ''::text);

after adding the index, i've issued a reindex and ran the query again:

query:
   SELECT * FROM oldtables.movies WHERE mov_year = 2007 AND
lower(mov_folder) = 'harrypotter5' LIMIT 1

queryplan2:

Bitmap Heap Scan on movies  (cost=2.78..581.78 rows=1 width=232) (actual
time=0.440..0.630 rows=1 loops=1)
  Recheck Cond: (mov_year = 2007)
  Filter: (lower((mov_folder)::text) = 'harrypotter5'::text)
  ->  Bitmap Index Scan on movies_mov_year_idx  (cost=0.00..2.78 rows=222
width=0) (actual time= 0.182..0.182 rows=62 loops=1)
        Index Cond: (mov_year = 2007)
Total runtime: 0.768 ms


why isn't the index in queryplan2 used anymore?

on a sidenote, the query runtimes seem in both plans way to high for a
relatively small table. what settings would i have to tweak to get better
results?

thanks,
thomas



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