我們在使用“validate structure”對索引進行分析後，透過查詢index_stats可以得到很多有關索引本身的屬性資訊。
透過這個實驗簡單展示一下幾個重要的資訊列。

1.建立測試表t，在t表的x列上存在主鍵。

2.對索引（主鍵）進行分析
sec@ora10g> analyze index pk_t validate structure;

Index analyzed.

3.查詢分析之後的結果
sec@ora10g> select lf_blks, br_blks, used_space, opt_cmpr_count, opt_cmpr_pctsave from index_stats;

LF_BLKS BR_BLKS USED_SPACE OPT_CMPR_COUNT OPT_CMPR_PCTSAVE
---------- ---------- ---------- -------------- ----------------
2088 4 15002737 0 0

4.所用該欄位註釋如下
LF_BLKS：索引使用的葉子塊（資料所在的塊）數目；
BR_BLKS：分支塊（索引結構中導航所用的塊）數目；
USED_SPACE：索引使用的空間，單位是位元組；
OPT_CMPR_COUNT：最優壓縮數；
OPT_CMPR_PCTSAVE：使用上面的最優壓縮數後最優節省壓縮百分比；

透過後面兩個引數的指示，對於使用索引壓縮技術來講有直接的指導和借鑑意義。

5.有關index_stats檢視的官方文件的解釋附在此處，方便大家參考。

INDEX_STATS

INDEX_STATS stores information from the last ANALYZE INDEX ... VALIDATE STRUCTURE statement.

Note:

The ANALYZE INDEX ... VALIDATE STRUCTURE OFFLINE statement must be used in order to collect statistics

Column	Datatype	NULL	Description
HEIGHT	NUMBER		Height of the B-Tree
BLOCKS	NUMBER	NOT NULL	Blocks allocated to the segment
NAME	VARCHAR2(30)	NOT NULL	Name of the index
PARTITION_NAME	VARCHAR2(30)		Name of the partition of the index which was analyzed. If the index is not partitioned, null is returned.
LF_ROWS	NUMBER		Number of leaf rows (values in the index)
LF_BLKS	NUMBER		Number of leaf blocks in the B-Tree
LF_ROWS_LEN	NUMBER		Sum of the lengths of all the leaf rows
LF_BLK_LEN	NUMBER		Usable space in a leaf block
BR_ROWS	NUMBER		Number of branch rows in the B-Tree
BR_BLKS	NUMBER		Number of branch blocks in the B-Tree
BR_ROWS_LEN	NUMBER		Sum of the lengths of all the branch blocks in the B-Tree
BR_BLK_LEN	NUMBER		Usable space in a branch block
DEL_LF_ROWS	NUMBER		Number of deleted leaf rows in the index
DEL_LF_ROWS_LEN	NUMBER		Total length of all deleted rows in the index
DISTINCT_KEYS	NUMBER		Number of distinct keys in the index (may include rows that have been deleted)
MOST_REPEATED_KEY	NUMBER		How many times the most repeated key is repeated (may include rows that have been deleted)
BTREE_SPACE	NUMBER		Total space currently allocated in the B-Tree
USED_SPACE	NUMBER		Total space that is currently being used in the B-Tree
PCT_USED	NUMBER		Percent of space allocated in the B-Tree that is being used
ROWS_PER_KEY	NUMBER		Average number of rows per distinct key (this figure is calculated without consideration of deleted rows)
BLKS_GETS_PER_ACCESS	NUMBER		Expected number of consistent mode block reads per row, assuming that a randomly chosen row is accessed using the index. Used to calculate the number of consistent reads that will occur during an index scan.
PRE_ROWS	NUMBER		Number of prefix rows (values in the index)
PRE_ROWS_LEN	NUMBER		Sum of lengths of all prefix rows
OPT_CMPR_COUNT	NUMBER		Optimal key compression length
OPT_CMPR_PCTSAVE	NUMBER		Corresponding space savings after an ANALYZE

6.小結
透過這種方法對索引進行分析後，可以快速的得到索引的基本情況，為進一步對其進行最佳化起到了非常大的幫助。
index_stats檢視的其他列也同樣很有意義，值得細細品味。

secooler
09.11.27

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【索引】使用索引分析快速得到索引的基本資訊

INDEX_STATS

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