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Home > Archive > PostgreSQL Discussion > April 2005 > Why sequential scan for currval?
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Why sequential scan for currval?
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| John Barham 2005-04-27, 3:23 am |
| 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?
TIA,
John
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| John Barham 2005-04-27, 3:23 am |
| > 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)
should be:
test=# explain select s from tt where id = currval('tt_id_seq')
; --
tt_id_seq vs. tt_id_key
QUERY PLAN
------------------------------------------------------
Seq Scan on tt (cost=0.00..1734.42 rows=1 width=32)
Filter: (id = currval('tt_id_seq':
:text))
(2 rows)
but the question still holds.
John
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| Klint Gore 2005-04-27, 3:23 am |
| 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 : :
+---------------------------------------+-----------------+
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