[Feature Request][Kernel] Support implicit cast for `DefaultExpressionEvaluator` output.

[vkorukanti]

Feature request

Which Delta project/connector is this regarding?

• Spark
• Standalone
• Flink
• Kernel
• Other (fill in here)

Overview

Currently ExpressionHandler.getEvaluator takes an expected output type as argument. The comment here is to handle the implicit cast incase the expression output can be safely cast to the asked output type.

[irfanghat]

Hi @vkorukanti and @allisonport-db.
I took a look at the ImplicitCastExpression implementation and the logic in transformBinaryComparator.

if (canCastTo(leftResult.outputType, rightResult.outputType)) {
    left = new ImplicitCastExpression(left, rightResult.outputType);
} else if (canCastTo(rightResult.outputType, leftResult.outputType)) {
    right = new ImplicitCastExpression(right, leftResult.outputType);
} else {
       ...

and came up with this, if it's accurate, I'd like to help out anyway I can:

  public DefaultExpressionEvaluator(
      StructType inputSchema, Expression expression, DataType outputType) {
    ExpressionTransformResult transformResult =
        new ExpressionTransformer(inputSchema).visit(expression);
    if (!transformResult.outputType.equivalent(outputType)) {
      if (canCastTo(transformResult.outputType, outputType)) {
        transformResult =
            new ExpressionTransformResult(
                new ImplicitCastExpression(transformResult.expression, outputType), outputType);
      } else {
        throw unsupportedExpressionException(
            expression,
            String.format("Expression %s cannot be implicitly cast to %s", expression, outputType));
      }
    }
    this.expression = transformResult.expression;
  }