Oracle Database collects the following statistics for an index. Statistics
marked with an asterisk are always computed exactly. For conventional indexes,
when you compute or estimate statistics, the statistics appear in the data
dictionary views USER_INDEXES,ALL_INDEXES, and DBA_INDEXES in the columns shown
in parentheses.
■ * Depth of the index from its root block to its leaf blocks (BLEVEL)
■ Number of leaf blocks (LEAF_BLOCKS)
■ Number of distinct index values (DISTINCT_KEYS)
■ Average number of leaf blocks for each index value (AVG_LEAF_BLOCKS_PER_KEY)
■ Average number of data blocks for each index value (for an index on a table)
(AVG_DATA_BLOCKS_PER_KEY)
■ Clustering factor (how well ordered the rows are about the indexed values)
(CLUSTERING_FACTOR)
使用ANALYZE INDEX 分析索引的root块至叶子块的深度 ，叶子块数目 ， distinct索引值 等信息 ，将分析数据保存在*INDEXES等视图的BLEVEL,LEAF_BLOCKS等字段
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