[Coral-Trino] Avoid accidental translation of Trino from_unixtime SQL call

[findinpath]

What changes are proposed in this pull request, and why are they necessary?

Avoid the accidental translation of the Trino from_unixtime introduced through #426

Context #459 (comment)

Fixes #459

How was this patch tested?

Regular unit tests

[wmoustafa]

I think you would want to implement an operator in coral-trino similar to TrinoElementAtFunction and have the transformer convert to that one as opposed to creating a Hive function (operator). Operators currently part of "Hive Function Registry" will end up in Coral common as they capture Coral IR, and not a language specific set of operators.

[findinpath]

@wmoustafa

I created an alternative PR #465 to follow-up on your request

think you would want to implement an operator in coral-trino similar to TrinoElementAtFunction

However, I failed in getting it to work exactly because the function was not mentioned in StaticHiveFunctionRegistry.

StaticHiveFunctionRegistry contains already Trino (not Hive) specific content:

coral/coral-hive/src/main/java/com/linkedin/coral/hive/hive2rel/functions/StaticHiveFunctionRegistry.java

Lines 306 to 307 in a7e33e6

	createAddUserDefinedFunction("from_unixtime_nanos", explicit(SqlTypeName.TIMESTAMP), NUMERIC);
	createAddUserDefinedFunction("$canonicalize_hive_timezone_id", explicit(SqlTypeName.VARCHAR), STRING);

I believe that, in the current state of the coral library, my current PR is the right approach to fix the from_unixtime regresssion reported on #459.

[wmoustafa]

Is it possible to create an operator (e.g., TimestampFromUnixtime) and have its unparse method return to_unixtimestamp and add it to the HiveFunctionRegistry instead of sticking to
the standard way of creating operators through createAddUserDefinedFunction? I think this can save us from having to create TrinoFromUnixtimeOperatorTransformer?

[findinpath]

I've tried adding the operator TimestampFromUnixtime and register it in StaticHiveFunctionRegistry, but still failed to get the tests 🟢 .

The not so good thing about using an operator is that we'd need to define it in coral-hive, so that we can make use of it in StaticHiveFunctionRegistry . This is definitely not the right place for a Trino specific operator.

I still think that, in the current circumstances, using the transformer TimestampFromUnixtimeTransformer is a viable option.

[findinpath]

Superseded by #467