Can someone tell me, how I can compare two datum fields, when I do not know the data type in advance inside an executor function?
In a nutshell, there is no way to compare Datums.
Datum is an abstact data type. It's the backend internal representation of a single value of any SQL data type.
The code using Datum has to know which type it is, since the Datum itself doesn't contain that information.
For example, "x less than y" where x and y are of various types that form intervals. I have found the method ExecTuplesMatch, but it is only for equality comparison, I think. Another one is ApplySortComparator... maybe that's the correct way to go?
Some code to make things clearer...
Datum x = heap_getattr(out->tts_tuple, node->xpos, out->tts_tupleDescriptor, &isNull1); Datum y = slot_getattr(curr, node->ypos, &isNull2);
if (compareDatumWithCorrectMethod(x,y) < 0) { /* do something */ }