On Wed, 27 Apr 2005 00:28:18 -0700, John Barham <jbarham@xxxxxxxxx> wrote: > test=# create table tt (id serial unique, s varchar); > [populate tt w/ 100000 rows] > test=# insert into tt (s) values ('foo'); > test=# select currval('tt_id_seq'); > currval > --------- > 100002 > (1 row) > test=# explain select s from tt where id = 100002; > QUERY PLAN > --------------------------------------------------------------------- > Index Scan using tt_id_key on tt (cost=0.00..6.01 rows=1 width=32) > Index Cond: (id = 100002) > (2 rows) > test=# explain select s from tt where id = currval('tt_id_key'); > QUERY PLAN > ------------------------------------------------------ > Seq Scan on tt (cost=0.00..1734.42 rows=1 width=32) > Filter: (id = currval('tt_id_key'::text)) > (2 rows) > > Why is a sequential scan used when comparing id to currval() value vs. > index scan when compared to a constant? currval is volatile which means it can change from one row in a statement to the next. So the scan has to be sequential to check if the value of currval() has changed. klint. +---------------------------------------+-----------------+ : Klint Gore : "Non rhyming : : EMail : kg@xxxxxxxxxxxxxx : slang - the : : Snail : A.B.R.I. : possibilities : : Mail University of New England : are useless" : : Armidale NSW 2351 Australia : L.J.J. : : Fax : +61 2 6772 5376 : : +---------------------------------------+-----------------+ ---------------------------(end of broadcast)--------------------------- TIP 3: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to majordomo@xxxxxxxxxxxxxx so that your message can get through to the mailing list cleanly