When to create statistics?

Question

在SQL Server中，何时仅在表列而不是索引上创建统计信息，何时创建索引（也会自动创建统计信息）？

Answer 1

According to the documentation when the automatic create statistics option, AUTO_CREATE_STATISTICS is ON :

The query optimizer creates statistics on single columns in query predicates, as necessary, to improve query plans and query performance. These single-column statistics are created when the query optimizer compiles queries.

Hence you need to create statistics manually in the following cases at least:

• AUTO_CREATE_STATISTICS is OFF
• multi-column statistics is required
• filtered statistics is required

Index and statistics are not comparable. In general, index is used to access data the more efficient way than a table scan. Statistics is required for optimizer to estimate the number of returned rows.

Answer 2

仅在向SQL引擎告知数据的分布/基数模式可以帮助其制定更好的执行计划时，统计信息才有用。在这种情况下，当告诉数据模式不足以及特殊的数据访问结构（即b树）时，特别需要使用索引需要减少数据扫描和滚动。