 <p>I can't speak for all RDBMS systems, but Postgres specifically uses estimated table sizes as part of its efforts to construct query plans. As an example, if a table has two rows, it may choose a sequential table scan for the portion of the JOIN that uses that table, whereas if it has 10000+ rows, it may choose to use an index or hash scan (if either of those are available.) Incidentally, it used to be possible to trigger poor query plans in Postgres by joining VIEWs instead of actual tables, since there were no estimated sizes for VIEWs.</p> <p>Part of how Postgres constructs its query plans depend on tunable parameters in its configuration file. More information on how Postgres constructs its query plans can be found on the Postgres website.</p>