I found a few problems with GetOperatorFromCompareType and fixed them here.
First of all, the comment was out of date: we never return InvalidStrategy; instead we ereport.
You're right.
Second, we were potentially using uninitialized Oids to build error messages (if get_opclass_opfamily_and_input_type failed). If that function fails, we should just die. In fact since get_opclass_method just succeeded, which makes the same lookup, how could it ever fail? I don't think we need to try very hard to build a fancy message.
Failing right away simplifies the logic, because we only reach the bottom of the function one way. And I think we can make things ever clearer by inverting the conditional, so it acts like a guard, and we can avoid some nesting.
+1.
After your patch changes, the line '*opid = InvalidOid;' seems removable.
Also, if the second validation check of opclass after 'get_opclass_method' feels a bit odd, moving 'get_opclass_opfamily_and_input_type' to the very top would work -- purely for visual clarity. ：）
