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A heap is a table without a clustered index. One or more nonclustered indexes can be created on tables stored as a heap. Data is stored in the heap without specifying an order. Usually data is initially stored in the order in which is the rows are inserted into the table, but the Database Engine can move data around in the heap to store the rows efficiently; so the data order cannot be predicted. To guarantee the order of rows returned from a heap, you must use the ORDER BY clause. To specify a permanent logical order for storing the rows, create a clustered index on the table, so that the table is not a heap.
堆表就是含有一列或者多列无聚簇索引的表，其中的数据存储无序。通常数据是按照初始化的顺序存储，但数据库引擎会为了更高效的数据插入，将原有数据移动到别处来释放空间给新的数据。因此，数据的顺序是不确定的，为了保证数据的顺序，可以在查询的时候使用 ORDER BY 关键字进行排序。要保证数据物理存储的顺序，就需要建立聚簇索引，这个时候就不是堆表了。
When a table is stored as a heap, individual rows are identified by reference to an 8-byte row identifier (RID) consisting of the file number, data page number, and slot on the page (FileID:PageID:SlotID). The row ID is a small and efficient structure.
ROWID 是文件编号、数据页编号以及页面上的卡槽组成。(FileID:PageID:SlotID)