Problem with bitmap-index-scan plan

Hi,

If I run the query

explain analyze select * from ind_uni_100 where a=1 and b=1 and c=1

I get the following plan:

Bitmap Heap Scan on ind_uni_100 (cost=942.50..1411.12 rows=125 width=104)
(actual time=72.556..72.934 rows=116 loops=1)
  Recheck Cond: ((c = 1) AND (a = 1) AND (b = 1))
  ->  BitmapAnd  (cost=942.50..942.50 rows=125 width=0) (actual
time=72.421..72.421 rows=0 loops=1)
        ->  Bitmap Index Scan on index_c_ind_uni_100  (cost=0.00..314.00
rows=50000 width=0) (actual time=21.854..21.854 rows=49832 loops=1)
              Index Cond: (c = 1)
        ->  Bitmap Index Scan on index_a_ind_uni_100  (cost=0.00..314.00
rows=50000 width=0) (actual time=22.371..22.371 rows=50319 loops=1)
              Index Cond: (a = 1)
        ->  Bitmap Index Scan on index_b_ind_uni_100  (cost=0.00..314.00
rows=50000 width=0) (actual time=14.226..14.226 rows=49758 loops=1)
              Index Cond: (b = 1)
Total runtime: 73.395 ms

Which is quite reasonable.The table has 1.000.000 rows (17.242 pages). From
pg_stat_get_blocks_fetched I can see that there were 102 page requests for
table. So all things seem to work great here!

But if I multiply the size of the table ten-times (10.000.000 rows - 172.414
pages) and run the same query I get:

explain analyze select * from ind_uni_1000 where a=1 and b=1 and c=1

Bitmap Heap Scan on ind_uni_1000  (cost=9369.50..14055.74 rows=1250 width=104)
(actual time=18111.415..176747.937 rows=1251 loops=1)
  Recheck Cond: ((c = 1) AND (a = 1) AND (b = 1))
  ->  BitmapAnd  (cost=9369.50..9369.50 rows=1250 width=0) (actual
time=17684.587..17684.587 rows=0 loops=1)
        ->  Bitmap Index Scan on index_c_ind_uni_1000  (cost=0.00..3123.00
rows=500000 width=0) (actual time=5704.624..5704.624 rows=500910 loops=1)
              Index Cond: (c = 1)
        ->  Bitmap Index Scan on index_a_ind_uni_1000  (cost=0.00..3123.00
rows=500000 width=0) (actual time=6147.962..6147.962 rows=500080 loops=1)
              Index Cond: (a = 1)
        ->  Bitmap Index Scan on index_b_ind_uni_1000  (cost=0.00..3123.00
rows=500000 width=0) (actual time=5767.754..5767.754 rows=500329 loops=1)
              Index Cond: (b = 1)
Total runtime: 176753.200 ms

which is slower even than a seq scan. Now I get that there were 131.398 page
requests for table in order to retrieve almost 1250 tuples!Can someone explain
why this is happening? All memory parameters are set to default.

Thanks!