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Flink built-in functions should precede catalog functions, because 1)  it always give a deterministic resolution order on ambiguous reference by invoking the built-in functions 2) catalog functions can always be precisely referenced with fully/partially qualified names. In contrary, if catalog functions precede built-in functions, built-in functions can never be referenced.

4. Code Changes

FunctionIdentifier

Class FunctionIdentifier {
	// empty for system functions
	Optional<ObjectIdentifier> oi;

	String name;
}

Changes to CallExpression and UnresolvedCallExpression

We should replace ObjectIdentifier with FunctionIdentifier in CallExpression and UnresolvedCallExpression. This would be a breaking change for these public evolving classes.

public final class CallExpression implements Expression {
	private final @Nullable FunctionIdentifier functionIdentifier;

    ...

    public Optional<FunctionIdentifier> getFunctionIdentifier() {
         return Optional.ofNullable(functionIdentifier);
    }
}

public final class UnresolvedCallExpression implements Expression {
	private final @Nullable FunctionIdentifier functionIdentifier;

    ...

	public Optional<FunctionIdentifier> getFunctionIdentifier() {
         return Optional.ofNullable(functionIdentifier);
    }
}

Changes to FunctionLookup interface

In case of a partially qualified name (<db_name>.<function_name>) in SQL, planner/parser should recognize it with context be padding name of the current catalog of the session first to build an ObjectIdentifier.

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/**
 * Resolves a function.
 *
 * @param fi identifier of function
 * @return An optional result of FunctionLookup
 */
public Optional<FunctionLookup.Result> lookupFunction(FunctionIdentifier fi);

5. Changes to FunctionCatalog class

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