SPI_prepare can be used with
SPI_cursor_open SPI_is_cursor_plan returns true
if a statement 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.
true or false to indicate if the plan can produce a cursor or not, with SPI_result set to zero. If it is not possible to determine the answer (for example, if the plan is NULL or invalid, or if called when not connected to SPI), then SPI_result is set to a suitable error code and false is returned.