SPI_execp is the same as SPI_execute_plan, with the latter's read_only parameter always taken as false.
prepared statement (returned by SPI_prepare)
An array of actual parameter values. Must have same length as the statement's number of arguments.
An array describing which parameters are null. Must have same length as the statement's number of arguments.
If nulls is NULL then SPI_execp assumes that no parameters are null. Otherwise, each entry of the nulls array should be ' ' if the corresponding parameter value is non-null, or 'n' if the corresponding parameter value is null. (In the latter case, the actual value in the corresponding values entry doesn't matter.) Note that nulls is not a text string, just an array: it does not need a '\0' terminator.
maximum number of rows to return, or 0 for no limit