执行过程先分析select语句的解析过程：

    select语句的解析集中在ParserDQL类的XreadQueryExpression方法中，其中XreadQueryExpressionBody select除了limit,offset,fetch关键字外的其他条件解析，XreadOrderByExpression是limit,offset,fetch，ordre by的解析，我们分析简单的select * from user where * order by * limit 2 offset 5 这种语句，对于嵌套子查询和联合查询待会简单过下。

  QueryExpression queryExpression = XreadQueryExpressionBody();
        SortAndSlice    sortAndSlice    = XreadOrderByExpression();

        if (queryExpression.sortAndSlice == null) {
            queryExpression.addSortAndSlice(sortAndSlice);
        } else {
            if (queryExpression.sortAndSlice.hasLimit()) {
                if (sortAndSlice.hasLimit()) {
                    throw Error.error(ErrorCode.X_42549);
                }

                for (int i = 0; i < sortAndSlice.exprList.size(); i++) {
                    Expression e = (Expression) sortAndSlice.exprList.get(i);

                    queryExpression.sortAndSlice.addOrderExpression(e);
                }
            } else {
                queryExpression.addSortAndSlice(sortAndSlice);
            }
        }

   selelct top 20 和limit 20这种条件先查询，XreadTopOrLimit读取top和limit的值并作为一个条件放到SortAndSlice对象中，XreadFromClause是读取嵌套查询或者联合查询的，readWhereGroupHaving是处理where条件和group by，having的

    QuerySpecification XreadQuerySpecification() {

        QuerySpecification select = XreadSelect();

        if (!select.isValueList) {
            XreadTableExpression(select);
        }

        return select;
    }

    void XreadTableExpression(QuerySpecification select) {
        XreadFromClause(select);
        readWhereGroupHaving(select);
    }