Expression could return Value directly and don't need to set the eval result to the private data member result_(

[jievince]

Currently, eval() of expression returns a const value&. And most expressions store the result that evaled to a result_ and then return it to the caller. The caller most often picks it up with a copy of the value.

virtual const Value& eval(ExpressionContext& ctx) = 0;

 Value val = col->expr()->eval(ctx(iter.get()));

So this caused two copy overhead.
The reason why most of the callers use copy of value to pick the evaled result is some of the expressions don't store the evaled result to result_:

const Value& InputPropertyExpression::eval(ExpressionContext& ctx) {
    return ctx.getInputProp(prop_); // The value returned comes from the valueMap of ExecutionContext, so it shouldn't be moved by the caller
}

Moreover, it is a bit strange to store the evaled result that is only valid for the current row to the expression class.

We could let eval() return a Value and don't need to store the evaled result to result_.
Meanwhile, AggregateExpression is special. It's not stateless, so it should store the evaled result to result_.

[CPWstatic]

We had a benchmark for these two situation.