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 ---------------------------(end of broadcast)--------------------------- TIP 2: Don't 'kill -9' the postmaster