On Fri, Jan 20, 2006 at 12:35:36PM +0800, K C Lau wrote: Here's the problem... the estimate for the backwards index scan is *way* off: > -> Limit (cost=0.00..1.26 rows=1 width=4) (actual > time=200032.928..200032.931 rows=1 loops=1) > -> Index Scan Backward using pk_log on > log (cost=0.00..108047.11 rows=86089 width=4) (actual > time=200032.920..200032.920 rows=1 loops=1) > Filter: (((create_time)::text < '2005/10/19'::text) AND > (logsn IS NOT NULL)) > Total runtime: 200051.701 ms BTW, these queries below are meaningless; they are not equivalent to min(logsn). > esdt=> explain analyze select LogSN from Log where create_time < > '2005/10/19' order by create_time limit 1; > > Limit (cost=0.00..0.98 rows=1 width=31) (actual time=0.071..0.073 rows=1 > loops=1) > -> Index Scan using idx_logtime on log (cost=0.00..84649.94 > rows=86089 width=31) (actual time=0.063..0.063 rows=1 loops=1) > Index Cond: ((create_time)::text < '2005/10/19'::text) > Total runtime: 0.182 ms > > esdt=> explain analyze select LogSN from Log where create_time < > '2005/10/19' order by create_time desc limit 1; > Limit (cost=0.00..0.98 rows=1 width=31) (actual time=0.058..0.061 rows=1 > loops=1) > -> Index Scan Backward using idx_logtime on log (cost=0.00..84649.94 > rows=86089 width=31) (actual time=0.051..0.051 rows=1 loops=1) > Index Cond: ((create_time)::text < '2005/10/19'::text) > Total runtime: 0.186 ms > > > ---------------------------(end of broadcast)--------------------------- > TIP 6: explain analyze is your friend > -- Jim C. Nasby, Sr. Engineering Consultant jnasby@xxxxxxxxxxxxx Pervasive Software http://pervasive.com work: 512-231-6117 vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461
