sql-server - 什么是数据库中的书签列？

Question

MSDN documentation对于 SqlDataReader.GetSchemaTable 方法，返回的 ColumnOrdinal 声明如下:

"The ordinal of the column. This is zero for the bookmark column of the row, if any. Other columns are numbered starting with one. This column cannot contain a null value."

但是书签栏是什么意思呢？它像主键吗？

最后，我想知道的是 ColumnOrdinal 是否总是从零开始，或者是否存在它不会从零开始的情况。

Answer 1

来自 Bookmarks, MSDN :

Bookmarks let consumers quickly return to a row. With bookmarks, consumers can access rows randomly based on the bookmark value. The bookmark column is column 0 in the rowset. The consumer sets the dwFlag field value of the binding structure to DBCOLUMNSINFO_ISBOOKMARK to indicate that the column is used as a bookmark. The consumer also sets the rowset property DBPROP_BOOKMARKS to VARIANT_TRUE. This lets column 0 be present in the rowset. The IRowsetLocate::GetRowsAt method is then used to fetch rows, starting with the row specified as an offset from a bookmark.