"George Pavlov" <gpavlov(at)mynewplace(dot)com> writes:
> select count(*) from t2 where t2.name not in (
> select t1.name from t1 limit 261683)
> --> 13
> select count(*) from t2 where t2.name not in (
> select t1.name from t1 limit 261684)
> --> 0
> What is so magical about 261683?
Most likely, the 261684'th row of t1 has a NULL value of name.
Many people find the behavior of NOT IN with nulls unintuitive,
but it's per SQL spec ...
regards, tom lane