rows in explain

Hi all,

I want to find out how PG gets rows when it explains a query.

For example, when I issue:

explain select count(*) from pg_class where relname='pg_class';

                          QUERY PLAN

--------------------------------------------------------------

 Aggregate  (cost=4.55..4.55 rows=1 width=0)

   ->  Seq Scan on pg_class  (cost=0.00..4.55 rows=1 width=0)

         Filter: (relname = 'pg_class'::name)

(3 rows)

Where and how PG get “rows=1”?

Thank you!