6b8a139f2d
Support function registry.
The classes:
- BuiltinFunctions: contains the built-in functions list
- FunctionRegistry: used to register scalar functions and aggregate functions, it can find the function by name
- FunctionBuilder: used to resolve a BoundFunction class, extract the constructor, and build to a BoundFunction by arguments(`List<Expression>`)
Register example: you can add built-in functions in the list for simplicity
```java
public class BuiltinFunctions implements FunctionHelper {
public final List<ScalarFunc> scalarFunctions = ImmutableList.of(
scalar(Substring.class, "substr", "substring"),
scalar(WeekOfYear.class),
scalar(Year.class)
);
public final ImmutableList<AggregateFunc> aggregateFunctions = ImmutableList.of(
agg(Avg.class),
agg(Count.class),
agg(Max.class),
agg(Min.class),
agg(Sum.class)
);
}
```
Note:
- Currently, we only support register scalar functions add aggregate functions, we will support register table functions.
- Currently, we only support resolve function by function name and difference arity, but can not resolve the same arity override function, e.g. `some_function(Expression)` and `some_function(Literal)`
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