SPI_is_cursor_plan — return true if a plan
prepared by SPI_prepare can be used with
SPI_cursor_open
bool SPI_is_cursor_plan(void * plan)
SPI_is_cursor_plan returns true
if a plan prepared by SPI_prepare can be passed
as an argument to SPI_cursor_open, or
false if that is not the case. The criteria are that the
plan represents one single command and that this
command returns tuples to the caller; for example, SELECT
is allowed unless it contains an INTO clause, and
UPDATE is allowed only if it contains a RETURNING
clause.