code: func (pc PbConverter) ExprToPB(expr Expression) *tipb.Expr {
  	switch x := expr.(type) {
  	case *Constant, *CorrelatedColumn:
  		return pc.conOrCorColToPBExpr(expr)
  	case *Column:
  		return pc.columnToPBExpr(x)
  	case *ScalarFunction:
  		return pc.scalarFuncToPBExpr(x)
  	}
  	return nil
  }
docs: // ExprToPB converts Expression to TiPB.