Re: Wrong rows count in EXPLAIN

=?koi8-r?B?8MHO1MDbyc4g4czFy9PBzsTSIOnXwc7P18ne?= <AI(dot)Pantyushin(at)gaz-is(dot)ru> writes:
> When I create a new table, and then I evaluate the execution of the SELECT query, I see a strange rows count in EXPLAIN
> CREATE TABLE test1(f INTEGER PRIMARY KEY NOT NULL);
> ANALYZE test1;
> EXPLAIN SELECT * FROM test1;
> QUERY PLAN
> ---------------------------------------------------------
> Seq Scan on test1 (cost=0.00..35.50 rows=2550 width=4)
> (1 row)

> Table is empty but rows=2550.

This is intentional, arising from the planner's unwillingness to
assume that a table is empty. It assumes that such a table actually
contains (from memory) 10 pages, and then backs into a rowcount
estimate from that depending on the data-type-dependent width of
the table rows.

Without this provision, we'd produce very bad plans for cases
where a newly-populated table hasn't been analyzed yet.

regards, tom lane