performance question

Reinoud van Leeuwen writes:

radius=# select count (radiuspk) from radius ;
count
--------
398543
(1 row)

This query takes about 3 seconds. But the query plan *already* knows the
number of rows ("rows=398543").

This is only an estimate which is only updated by VACUUM. Presumably you
didn't add or remove any rows since your last VACUUM.

--
Peter Eisentraut peter_e@gmx.net http://funkturm.homeip.net/~peter

On Tue, 28 Aug 2001, Reinoud van Leeuwen wrote:

Can somebody explain to me:

radius=# explain select count (radiuspk) from radius ;
NOTICE: QUERY PLAN:

Aggregate (cost=12839.79..12839.79 rows=1 width=8)
-> Seq Scan on radius (cost=0.00..11843.43 rows=398543 width=8)

EXPLAIN

This query answers me *instantly* after hitting return

radius=# select count (radiuspk) from radius ;
count
--------
398543
(1 row)

This query takes about 3 seconds. But the query plan *already* knows the
number of rows ("rows=398543"). So why does it take 3 seconds. Is my
assumption correct that the optimiser still can be optimized a little? :-)

Not in this case. The row numbers from explain are just estimates
from the last vacuum. As you modify the table, the estimated rows
will be off.

For example:
sszabo=> create table a (a int);
CREATE
sszabo=> insert into a values (100);
INSERT 808899 1
sszabo=> insert into a values (101);
INSERT 808900 1
sszabo=> explain select count(a) from a;
NOTICE: QUERY PLAN:

Aggregate (cost=22.50..22.50 rows=1 width=4)
-> Seq Scan on a (cost=0.00..20.00 rows=1000 width=4)

EXPLAIN
sszabo=> vacuum analyze a;
VACUUM
sszabo=> explain select count(a) from a;
NOTICE: QUERY PLAN:

Aggregate (cost=1.02..1.02 rows=1 width=4)
-> Seq Scan on a (cost=0.00..1.02 rows=2 width=4)

EXPLAIN
sszabo=> insert into a values (102);
INSERT 808902 1
sszabo=> explain select count(a) from a;
NOTICE: QUERY PLAN:

Aggregate (cost=1.02..1.02 rows=1 width=4)
-> Seq Scan on a (cost=0.00..1.02 rows=2 width=4)

EXPLAIN
sszabo=> vacuum analyze a;
VACUUM
sszabo=> explain select count(a) from a;
NOTICE: QUERY PLAN:

Aggregate (cost=1.04..1.04 rows=1 width=4)
-> Seq Scan on a (cost=0.00..1.03 rows=3 width=4)

EXPLAIN

On Tue, 28 Aug 2001, Reinoud van Leeuwen wrote:

Can somebody explain to me:

radius=# explain select count (radiuspk) from radius ;
NOTICE: QUERY PLAN:

Aggregate (cost=12839.79..12839.79 rows=1 width=8)
-> Seq Scan on radius (cost=0.00..11843.43 rows=398543 width=8)

EXPLAIN

This query answers me *instantly* after hitting return

radius=# select count (radiuspk) from radius ;
count
--------
398543
(1 row)

This query takes about 3 seconds. But the query plan *already* knows
the number of rows ("rows=398543"). So why does it take 3 seconds. Is
my assumption correct that the optimiser still can be optimized a
little? :-)

Not in this case. The row numbers from explain are just estimates
from the last vacuum. As you modify the table, the estimated rows will
be off.

Yes, I just found out that somebody else is running a script on our test
server that vacuums all databases each night. That explains a lot.

Thanx for thinking with me

Reinoud