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

On May 21, 3:04=A0am, alexei_nova...@yahoo.com ("Alexei") wrote:
> The following bug has been logged online:
>
> Bug reference: =A0 =A0 =A04819
> Logged by: =A0 =A0 =A0 =A0 =A0Alexei
> Email address: =A0 =A0 =A0alexei_nova...@yahoo.com
> PostgreSQL version: 8.3.7
> Operating system: =A0 Open SuSE 11.1 AMD Athlon 64 X2
> Description: =A0 =A0 =A0 =A0Ordering big tables by indexed columns is ver=
y 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 sta=
rts
> 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 =A0(cost=3D522779.47..530279.47 rows=3D3000000 width=3D12)
> =A0 Sort Key: field1, field2
> =A0 -> =A0Seq Scan on tmp1 =A0(cost=3D0.00..46217.00 rows=3D3000000 width=
=3D12)
>
> Index is not used for the sorting here. But if I add "limit 1000" in the =
end
> I get the following:
> Limit =A0(cost=3D0.00..75.33 rows=3D100 width=3D12)
> =A0 -> =A0Index Scan using tmp1_idx on tmp1 =A0(cost=3D0.00..2259857.96 r=
ows=3D3000000
> width=3D12)
>
> 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.

The planner cannot estimate the speed of your disks and thinks, that
doing a seqscan will be faster for so many columns, then scanning the
index, and then do a random lookup on the table.

Have a look on the documentation for the following planner
configuration parameters: effective_cache_size, seq_page_cost and
random_page_cost

With best regards,

-- Valentine Gogichashvili