Re: Why sequential scan for currval?

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.

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