Row estimates on empty table

Hello all,

I'm seeing something strange with the row-estimates on an empty table.
The table in question is merely a template-table that specialised
tables inherit from, it will never contain any data. Nevertheless,
after importing my creation script and vacuum analyse the result I see
is this:

dalroi=# SELECT * FROM ONLY unit;
unit | format | scales_up | scales_down
------+--------+-----------+-------------
(0 rows)

dalroi=# EXPLAIN ANALYZE SELECT * FROM ONLY unit;
QUERY PLAN
--------------------------------------------------------------------------------------------------
Seq Scan on unit (cost=0.00..18.50 rows=850 width=66) (actual
time=0.001..0.001 rows=0 loops=1)
Total runtime: 0.025 ms
(2 rows)

As you see, estimated rows 850, actual rows 0!

Now 25 �s doesn't sound like much, but this data is going to be joined
to another small table and it's throwing the estimated number of rows
WAY off. See here: http://explain-analyze.info/query_plans/3956-Alban-s-unit-normalization-query-1

(Yes, 4 ms still isn't bad, but these queries are likely going to be
at the basis of many other queries so they need to be snap-snap-snap!
The more joins the worse the estimate will get, right?)

So what's going on here?

For the record, this is PG 8.4 compiled from macports on Snow Leopard.
I've seen a few odd reports with that combination so I thought I'd
mention it. To be exact: PostgreSQL 8.4.0 on i386-apple-darwin10.0.0,
compiled by GCC i686-apple-darwin10-gcc-4.2.1 (GCC) 4.2.1 (Apple Inc.
build 5646), 64-bit

Alban Hertroys

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