BUG #4819: Ordering big tables by indexed columns is very slow.

The following bug has been logged online:

Bug reference:      4819
Logged by:          Alexei
Email address:      alexei_novakov@yahoo.com
PostgreSQL version: 8.3.7
Operating system:   Open SuSE 11.1 AMD Athlon 64 X2
Description:        Ordering big tables by indexed columns is very slow.
Details:

Hello.

I have very simple query, which runs very long when has "order by" clause,
even though all columns in "order by" are indexed. Here is the simplified
testcase.

1) Table:
create table tmp1
(
field1 bigint not null,
field2 integer not null
)

2) Data:
I generated some test data for this table: field2 is always 2; field1 starts
from 1242865824484 and every next one is incremented by 1. I generated 3
million records.

3) Index:
create index tmp1_idx on tmp1 (field1, field2)

4) Query:
select field1, field2 from tmp1 order by 1, 2

The query plan for this query is:
Sort  (cost=522779.47..530279.47 rows=3000000 width=12)
  Sort Key: field1, field2
  ->  Seq Scan on tmp1  (cost=0.00..46217.00 rows=3000000 width=12)


Index is not used for the sorting here. But if I add "limit 1000" in the end
I get the following:
Limit  (cost=0.00..75.33 rows=100 width=12)
  ->  Index Scan using tmp1_idx on tmp1  (cost=0.00..2259857.96 rows=3000000
width=12)

If I increase limit to 700000 index is not used again and the difference in
execution time is very noticeable:
1 millisecond for "limit 600000"; and 6 seconds for "limit 700000"

Is there anything what can be configured to make it use the index for the
ordering?

Best Regards,
Alexei Novakov.