No docs or regression tests yet, but it seems to work ... please check
against Oracle and DB2 behavior. Supported cases are
RANGE UNBOUNDED PRECEDING -- same as AND CURRENT ROW
RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
RANGE BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
ROWS UNBOUNDED PRECEDING -- same as AND CURRENT ROW
ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
regards, tom lane