I would like clarification on the first paragraph of section 9.20.2 on
the page at

http://www.postgresql.org/docs/9.1/static/functions-subquery.html

regarding the syntax of the 'expression in (subquery)' clause.

What is meant by

The right-hand side is a parenthesized subquery, which must return
exactly one column.

? Appears to me that the subquery must only contain the same number of
columns as the left hand side, not just 1 column.

thanks-j