On Wed, 27 Apr 2005 00:28:18 -0700, John Barham <jbarham@gmail.com> 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@kgb.une.edu.au : 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 : :
+---------------------------------------+-----------------+
