SPI_preparecan be used with
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.
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.