Frank Joerdens <frank(at)joerdens(dot)de> writes:
> mpi=# update index set level = 2 where parentid = ( select id from
> index where level = 1 );
> ERROR: More than one tuple returned by a subselect used as an expression.
Apparently the subquery "select id from index where level = 1" is
returning more than one row. Perhaps you want WHERE parentid IN subselect
rather than WHERE parentid = subselect. "=" is a scalar operator, not
a set operator.
regards, tom lane