func checkEquivExpr(a, b parser.TypedExpr, qvals qvalMap) error {
	// The expressions above only use the values 1 and 2. Verify that the
	// simplified expressions evaluate to the same value as the original
	// expression for interesting values.
	for _, v := range []parser.Datum{
		parser.NewDInt(0),
		parser.NewDInt(1),
		parser.NewDInt(2),
		parser.NewDInt(3),
		parser.DNull,
	} {
		for _, q := range qvals {
			q.datum = v
		}
		da, err := a.Eval(parser.EvalContext{})
		if err != nil {
			return fmt.Errorf("%s: %v", a, err)
		}
		db, err := b.Eval(parser.EvalContext{})
		if err != nil {
			return fmt.Errorf("%s: %v", b, err)
		}
		// This is tricky: we don't require the expressions to produce identical
		// results, but to either both return true or both return not true (either
		// false or NULL).
		if (da == parser.DBoolTrue) != (db == parser.DBoolTrue) {
			return fmt.Errorf("%s: %s: expected %s, but found %s", a, v, da, db)
		}
	}
	return nil
}

func sameTypeExprs(left, right parser.TypedExpr) bool {
	leftType := left.ReturnType()
	if leftType == parser.DNull {
		return true
	}
	rightType := right.ReturnType()
	if rightType == parser.DNull {
		return true
	}
	return leftType.TypeEqual(rightType)
}

// RunFilter runs a filter expression and returns whether the filter passes.
func RunFilter(filter parser.TypedExpr, evalCtx *parser.EvalContext) (bool, error) {
	if filter == nil {
		return true, nil
	}

	d, err := filter.Eval(evalCtx)
	if err != nil {
		return false, err
	}

	return d != parser.DNull && bool(*d.(*parser.DBool)), nil
}