Portal SPI_cursor_open(const char * name, void * plan, Datum * values, const char * nulls, bool read_only)
SPI_cursor_open sets up a cursor (internally, a portal) that will execute a plan prepared by SPI_prepare. The parameters have the same meanings as the corresponding parameters to SPI_execute_plan.
Using a cursor instead of executing the plan directly has two benefits. First, the result rows can be retrieved a few at a time, avoiding memory overrun for queries that return many rows. Second, a portal can outlive the current procedure (it can, in fact, live to the end of the current transaction). Returning the portal name to the procedure's caller provides a way of returning a row set as result.
If nulls is NULL then SPI_cursor_open assumes that no parameters are null.
pointer to portal containing the cursor, or NULL on error