SPI_is_cursor_plan man page
SPI_is_cursor_plan — return true if a statement prepared by SPI_prepare can be used with SPI_cursor_open
bool SPI_is_cursor_plan(SPIPlanPtr plan)
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.
prepared statement (returned by SPI_prepare)
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.