Hi community, In order to support TUMBLE as a TABLE function (see CALCITE-3272), the first step is to add a new operator in SqlStdOperatorTable(which is CALCITE-3340). Note that new operator will need a name, which is the syntax used in a query. For example, GROUP BY TUMBE(...) will lead to a TUMBLE operator created with a name "TUMBLE".
Thus, for the new operator for TUMBLE as built-in table function, we also need to specify a name for it. And there are options: still use "TUMBLE", or use a new name, e.g. "TUMBLE_TABLE". For readers' convenience, start from here I will name GROUP BY TUMBLE as the old operator, and new table function one as the new operator. Option 1 that reuse "TUMBLE" as new operator's name: This is called operator overloading in Calcite because Calcite seems two operators are overloading by checking their names. It turns out, unlike other built-in operators, old TUMBLE is not recognized by Parser directly: Parser leaves it as UnresolvedFunction to validator. Then it also turns out validator is not favor of overloaded built-in operators: overloaded built-in operators will lead to "function not found" because more than one built-in operators are found from operator table for TUMBLE based on an operator name lookup approach. If more than one built-in operator is found by validator, validator will just not match either one. I am now convinced by many of my exploration, testing, code reading and prototyping that the resolution for option 1, is to code *both* old and new TUMBLE direclty in Parser, and overrite "SqlOperator,deriveType" for *both* old and new operators. You could check one of my prototype: https://github.com/apache/calcite/pull/1457 For option 2 that use a new name for new operator's name, e.g. "TUMBLE_TALBE" Because validator treats two operators as operator overloading if they have the same name, using another name is ok. It would be very straighforward to let the new operator go through parse, validate, rel steps if using a new name (I have proven it in an old verrsion of PR1457 <https://github.com/apache/calcite/pull/1457> but unfortunetly I did a force push so that version was overwritten and it cannot be seen now). For me it seems like option 2 makes sense and require less work. However I do want to hear more opinions from community. Please let me know if you have any thought. -Rui
