// VisitBinaryOperation swaps the right side identifier to left side if left side expression is static.
// So it can be used in index plan.
func (iev *IdentEvalVisitor) VisitBinaryOperation(binop *expression.BinaryOperation) (expression.Expression, error) {
	var err error
	binop.L, err = binop.L.Accept(iev)
	if err != nil {
		return binop, errors.Trace(err)
	}
	binop.R, err = binop.R.Accept(iev)
	if err != nil {
		return binop, errors.Trace(err)
	}

	if binop.L.IsStatic() {
		if _, ok := binop.R.(*expression.Ident); ok {
			switch binop.Op {
			case opcode.EQ:
			case opcode.NE:
			case opcode.NullEQ:
			case opcode.LT:
				binop.Op = opcode.GE
			case opcode.LE:
				binop.Op = opcode.GT
			case opcode.GE:
				binop.Op = opcode.LT
			case opcode.GT:
				binop.Op = opcode.LE
			default:
				// unsupported opcode
				return binop, nil
			}
			binop.L, binop.R = binop.R, binop.L
		}
	}
	return binop, nil
}

func (r *TableDefaultPlan) filterBinOp(ctx context.Context, x *expression.BinaryOperation) (plan.Plan, bool, error) {
	ok, name, rval, err := x.IsIdentCompareVal()
	if err != nil {
		return r, false, errors.Trace(err)
	}
	if !ok {
		return r, false, nil
	}
	if rval == nil {
		// if nil, any <, <=, >, >=, =, != operator will do nothing
		// any value compared null returns null
		// TODO: if we support <=> later, we must handle null
		return &NullPlan{r.GetFields()}, true, nil
	}

	_, tn, cn := field.SplitQualifiedName(name)
	t := r.T
	if tn != "" && tn != t.TableName().L {
		return r, false, nil
	}
	c := column.FindCol(t.Cols(), cn)
	if c == nil {
		return nil, false, errors.Errorf("No such column: %s", cn)
	}
	var seekVal interface{}
	if seekVal, err = types.Convert(rval, &c.FieldType); err != nil {
		return nil, false, errors.Trace(err)
	}
	spans := toSpans(x.Op, rval, seekVal)
	if c.IsPKHandleColumn(r.T.Meta()) {
		if r.rangeScan {
			spans = filterSpans(r.spans, spans)
		}
		return &TableDefaultPlan{
			T:         r.T,
			Fields:    r.Fields,
			rangeScan: true,
			spans:     spans,
		}, true, nil
	} else if r.rangeScan {
		// Already filtered on PK handle column, should not switch to index plan.
		return r, false, nil
	}

	ix := t.FindIndexByColName(cn)
	if ix == nil { // Column cn has no index.
		return r, false, nil
	}
	return &indexPlan{
		src:     t,
		col:     c,
		unique:  ix.Unique,
		idxName: ix.Name.O,
		idx:     ix.X,
		spans:   spans,
	}, true, nil
}

func (r *TableDefaultPlan) filterBinOp(ctx context.Context, x *expression.BinaryOperation) (plan.Plan, bool, error) {
	ok, name, rval, err := x.IsIdentCompareVal()
	if err != nil {
		return r, false, err
	}
	if !ok {
		return r, false, nil
	}
	if rval == nil {
		// if nil, any <, <=, >, >=, =, != operator will do nothing
		// any value compared null returns null
		// TODO: if we support <=> later, we must handle null
		return &NullPlan{r.GetFields()}, true, nil
	}

	_, tn, cn := field.SplitQualifiedName(name)
	t := r.T
	if tn != "" && tn != t.TableName().L {
		return r, false, nil
	}
	c := column.FindCol(t.Cols(), cn)
	if c == nil {
		return nil, false, errors.Errorf("No such column: %s", cn)
	}

	ix := t.FindIndexByColName(cn)
	if ix == nil { // Column cn has no index.
		return r, false, nil
	}

	var seekVal interface{}
	if seekVal, err = types.Convert(rval, &c.FieldType); err != nil {
		return nil, false, err
	}
	return &indexPlan{
		src:     t,
		col:     c,
		idxName: ix.Name.O,
		idx:     ix.X,
		spans:   toSpans(x.Op, rval, seekVal),
	}, true, nil
}