Query plan degradation 8.2 --> 8.3

All,

I now have a simple test case which shows significant performance
degradation on 8.3devel for a specific query, apparenly due to an
unnecessary call to Top-N sort. I've tried to forward the test case to
the lists but the package is 3.5m, so I'm putting it on pgFoundry instead:

If you run the example query (badsql.sql), you get the following for 8.2,
which is a Good Plan:

Limit (cost=0.00..24.04 rows=20 width=64) (actual time=0.091..0.129
rows=20 loops=1)
-> Index Scan using abc_idx_t_s_symb_dts on abc (cost=0.00..908.55
rows=756 width=64) (actual time=0.089..0.123 rows=20 loops=1)
Index Cond: (((t_s_symb)::text = 'SYMPRA'::text) AND (t_dts >=
'2006-02-15 00:00:00'::timestamp without time zone) AND (t_dts <=
'2007-06-28 00:00:00'::timestamp without time zone))
Filter: ((t_st_id = 'CMPT'::bpchar) AND (t_ca_id <=
43000050000::bigint))
Total runtime: 0.222 ms

However, running the same against 8.3-snapshot-5-28 gives you:

Limit (cost=631.85..631.90 rows=20 width=55) (actual time=2.325..2.336
rows=20 loops=1)
-> Sort (cost=631.85..633.61 rows=704 width=55) (actual
time=2.323..2.326 rows=20 loops=1)
Sort Key: t_dts
Sort Method: top-N heapsort Memory: 27kB
-> Bitmap Heap Scan on abc (cost=25.23..613.12 rows=704
width=55) (actual time=0.537..1.477 rows=843 loops=1)
Recheck Cond: (((t_s_symb)::text = 'SYMPRA'::text) AND
(t_dts >= '2006-02-15 00:00:00'::timestamp without time zone) AND (t_dts
<= '2007-06-28 00:00:00'::timestamp without time zone))
Filter: ((t_ca_id <= 43000050000::bigint) AND (t_st_id =
'CMPT'::bpchar))
-> Bitmap Index Scan on abc_idx_t_s_symb_dts
(cost=0.00..25.06 rows=704 width=0) (actual time=0.506..0.506 rows=843
loops=1)
Index Cond: (((t_s_symb)::text = 'SYMPRA'::text) AND
(t_dts >= '2006-02-15 00:00:00'::timestamp without time zone) AND (t_dts
<= '2007-06-28 00:00:00'::timestamp without time zone))
Total runtime: 2.460 ms

.... and if you disable bitmap:

Limit (cost=812.35..812.40 rows=20 width=55) (actual time=2.363..2.372
rows=20 loops=1)
-> Sort (cost=812.35..814.11 rows=704 width=55) (actual
time=2.360..2.363 rows=20 loops=1)
Sort Key: t_dts
Sort Method: top-N heapsort Memory: 27kB
-> Index Scan using abc_idx_t_s_symb_dts on abc
(cost=0.00..793.62 rows=704 width=55) (actual time=0.080..1.567 rows=843
loops=1)
Index Cond: (((t_s_symb)::text = 'SYMPRA'::text) AND (t_dts
>= '2006-02-15 00:00:00'::timestamp without time zone) AND (t_dts <=
'2007-06-28 00:00:00'::timestamp without time zone))
Filter: ((t_ca_id <= 43000050000::bigint) AND (t_st_id =
'CMPT'::bpchar))
Total runtime: 2.475 ms

The problem appears to be that top-N heapsort is being called even when
it's not needed, such as immediately after an indexscan.

Is my assessment correct?

--
--Josh

Josh Berkus
PostgreSQL @ Sun
San Francisco