Hi,
Here is an example showing the problem:
Welcome to psql 8.3.6, the PostgreSQL interactive terminal.
manu=# create table foo (x int);
CREATE TABLE
manu=# explain select * from foo;
QUERY PLAN
-------------------------------------------------------
Seq Scan on foo (cost=0.00..34.00 rows=2400 width=4)
(1 row)
manu=# analyze foo;
ANALYZE
manu=# explain select * from foo;
QUERY PLAN
-------------------------------------------------------
Seq Scan on foo (cost=0.00..34.00 rows=2400 width=4)
(1 row)
manu=# insert into foo values (1);
INSERT 0 1
manu=# analyze foo;
ANALYZE
manu=# explain select * from foo;
QUERY PLAN
---------------------------------------------------
Seq Scan on foo (cost=0.00..1.01 rows=1 width=4)
(1 row)
Now a possible cause for this might be the relpages attribute in pg_class (the
default value 0 does not seem to be interpreted correctly):
manu=# create table bar(x int);
CREATE TABLE
manu=# explain select * from bar;
QUERY PLAN
-------------------------------------------------------
Seq Scan on bar (cost=0.00..34.00 rows=2400 width=4)
(1 row)
manu=# select relpages from pg_class where relname='bar';
relpages
----------
0
(1 row)
manu=# update pg_class set relpages=1 where relname='bar';
UPDATE 1
manu=# explain select * from bar;
QUERY PLAN
---------------------------------------------------
Seq Scan on bar (cost=0.00..0.00 rows=1 width=4)
(1 row)
This is a real problem if you have a lot of empty child tables. Postgres will
not optimize correctly queries in the presence of empty child tables.
Is this a bug?
Thanks for your help,
Emmanuel
--
Emmanuel Cecchet
Aster Data Systems
Web: http://www.asterdata.com
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