When to Gather Statistics
在10g、11g里面，搜集statistics信息分自动和手工方式两种.
Table Statistics
- Number of rows
- Number of blocks and empty blocks
- Average available free space
- Number of chained or migrated rows
- Average row length
- Last analyze date and smaple size
- Data dictionary view:dba_tables
查看表的统计信息，主要依据数据字典dba_tables
Index Statistics
- Index level(height)
- Number of leaf blocks and distinct keys
- Average number of leaf blocks per key
- Average number of data blocks per key
- Number of index entries
- Clustering factor:反应了表的排序程度.
- Data dictionary view:dba_indexes
Index Tuning Wizard
