Support transform functions in query preprocessing if all the arguments are all literals

[shenyu0127]

@npawar and I found this feature gap.

The Broker preprocess the query by invoking functions if all the function arguments are literals, and then passes the preprocessed query to the Server for query execution.

When we preprocess the query, we only support scalar functions. We should also support transform functions.

[Jackie-Jiang]

Transform function works on columnar data, and we should always have a scalar function counterpart for every transform function so that we can do row based transform. Which transform function do you find that doesn't have the scalar function counterpart?

[shenyu0127]

We didn't find any transform function missing the scalar function counterpart. We did not assume every transform function has a scalar function counterpart and assumed the feature gap.

If we can guarantee that every transform function has a scalar function counterpart, then:

• There is no feature gap.
• Why don't we make the transform functions depend on the scalar functions? The performance overhead should be minimal. The benefit is we ensure the two function won't diverge.

[Jackie-Jiang]

• Why don't we make the transform functions depend on the scalar functions? The performance overhead should be minimal. The benefit is we ensure the two function won't diverge.

Think of TransformFunction as a specialized implementation for better performance. There is a ScalarTransformFunctionWrapper which leverages the scalar functions so that if there is no specialized implementation for a scalar function, we fall back to using the scalar function. The performance overhead is not trivial because it always involves boxing the value, casting the type, and method invocation on a per record base.

[shenyu0127]

I am not proposing to use the combination ofScalarTransformFunctionWrapper and scalar functions to replace transform functions. I am proposing to make the transform functions call their corresponding scalar functions, e.g. make DateTruncTransformFunction call DateTimeFunctions::datetrunc.