Screwy Statistics...

First, shouldn't statistics have some clue how many rows
might be in the table, at least right after an analyze?

Second, if the planner believes it will find 6700 rows in a
35000 row table, shouldn't it choose an index? I have a
unique index on "branch". What percentage of expected
rows/possible rows is the usual cutoff, anyone know?
Whatever it is, it seems too low to me.


oms=# select version();
version
-------------------------------------------------------------
PostgreSQL 7.2.1 on i686-pc-linux-gnu, compiled by GCC 2.96
(1 row)

oms=# analyze items;
ANALYZE

oms=# select count(*) from items;
count
-------
34865
(1 row)

oms=# explain select * from items where branch='10';
NOTICE: QUERY PLAN:

Seq Scan on items (cost=0.00..3588.70 rows=46546 width=520)

EXPLAIN

oms=# explain select * from items where branch='30';
NOTICE: QUERY PLAN:

Seq Scan on items (cost=0.00..3588.70 rows=6710 width=520)

EXPLAIN

Glen Parker
glenebob(at)nwlink(dot)com