---oracle 如何監控索引的使用狀況?
在開發應用程序時，可能會建立很多索引，那么這些索引的使用到底怎么樣，是否有些索引一直都沒有用到過，在這種情況下就需要對這些索引進行監控，
以便確定它們的使用情況，並為是否可以清除它們給出依據。
監控索引有兩種方式：
1、直接監控索引的使用情況
（1）設置所要監控的索引：ALTER INDEX IDX_T_XX MONITORING USAGE;
（2）查看該索引有沒有被使用：SELECT * FROM V$OBJECT_USAGE;
（3）關閉監控：ALTER INDEX IDX_T_XX NOMONITORING USAGE;
2.schema級別索引監控
如果我們想在系統中監控所有的索引，那么我們可以通過下面腳本實現監控數據庫所有的索引。注意我們要排除一些系統表的索引、以及LOB indexes。原因有下面兩個：
1：LOB indexes不能修改，否則會報ORA-22864錯誤（ORA-22864: cannot ALTER or DROP LOB indexes）。
2：ORA-00701: object necessary for warmstarting database cannot be altered
ORA-00701: object necessary for warmstarting database cannot be altered
00701. 00000 - "object necessary for warmstarting database cannot be altered"
*Cause: Attempt to alter or drop a database object (table, cluster, or
index) which are needed for warmstarting the database.
*Action: None.
直接執行腳本來開啟索引監控，當然監控索引時長非常重要，太短的話有可能導致查詢出來的數據有問題，一般建議監控一周后即可，OLAP系統則需要適當延長監控的時間。
SELECT 'ALTER INDEX ' || owner || '.' || index_name || ' MONITORING USAGE;' enable_monitor,
'ALTER INDEX ' || owner || '.' || index_name ||
' NOMONITORING USAGE;' disable_monitor
FROM dba_indexes
WHERE INDEX_TYPE != 'LOB'
and owner IN
(SELECT username FROM dba_users WHERE account_status = 'OPEN')
AND owner NOT IN ('SYS',
'SYSTEM',
'PERFSTAT',
'MGMT_VIEW',
'MONITOR',
'SYSMAN',
'DBSNMP')
AND owner not like '%SYS%';
監控一個月就大概可以知道那些是無用的索引了。
雖然v$object_usage表能記錄索引監控和使用的狀態，但它不能統計索引被使用的次數和頻率，只記錄了在開啟索引監控的時間段索引是否被使用過，這一點要值的注意。
另外需要注意的2點：
① 10g在收集統計信息時會導致索引被監控、這並非SQL語句產生、而在11g則不會出現這種情況了
② 外鍵索引不會因為主表的DML操作而被監控到、不要因為該索引沒用而將它給刪了
查詢V$OBJECT_USAGE就可以知道數據庫對索引的使用情況了。通過一段時間的監控，就可以確定哪些是無用的索引。
另外，為了避免使用V$OBJECT_USAGE只能查詢到當前用戶下索引的監控情況，可以使用如下語句查詢數據庫中所有被監控索引的使用情況：
SELECT U.NAME OWNER,
IO.NAME INDEX_NAME,
T.NAME TABLE_NAME,
DECODE(BITAND(I.FLAGS, 65536), 0, 'NO', 'YES') MONITORING,
DECODE(BITAND(OU.FLAGS, 1), 0, 'NO', 'YES') USED,
OU.START_MONITORING START_MONITORING,
OU.END_MONITORING END_MONITORING
FROM SYS.USER$ U,
SYS.OBJ$ IO,
SYS.OBJ$ T,
SYS.IND$ I,
SYS.OBJECT_USAGE OU
WHERE I.OBJ# = OU.OBJ#
AND IO.OBJ# = OU.OBJ#
AND T.OBJ# = I.BO#
AND U.USER# = IO.OWNER#;
2、通過查看歷史的執行計划，分析索引的使用情況
可以從視圖DBA_HIST_SQL_PLAN中獲取到數據庫中所有索引的掃描次數情況，然后根據掃描次數和開發人員溝通是否需要保留索引。
WITH TMP1 AS
(SELECT I.OWNER INDEX_OWNER,
I.TABLE_OWNER,
TABLE_NAME,
INDEX_NAME,
INDEX_TYPE,
(SELECT NB.CREATED
FROM DBA_OBJECTS NB
WHERE NB.OWNER = I.OWNER
AND NB.OBJECT_NAME = I.INDEX_NAME
AND NB.SUBOBJECT_NAME IS NULL
AND NB.OBJECT_TYPE = 'INDEX') CREATED,
(SUM(S.BYTES) / 1024 / 1024) INDEX_MB,
(SELECT COUNT(1)
FROM DBA_IND_COLUMNS DIC
WHERE DIC.INDEX_NAME = I.INDEX_NAME
AND DIC.TABLE_NAME = I.TABLE_NAME
AND DIC.INDEX_OWNER = I.OWNER) COUNT_INDEX_COLS
FROM DBA_SEGMENTS S, DBA_INDEXES I
WHERE I.INDEX_NAME = S.SEGMENT_NAME
AND I.OWNER = S.OWNER
AND S.OWNER NOT LIKE '%SYS%'
GROUP BY I.OWNER, I.TABLE_OWNER, TABLE_NAME, INDEX_NAME, INDEX_TYPE
HAVING SUM(S.BYTES) > 1024 * 1024),
TMP2 AS
(SELECT INDEX_OWNER,
INDEX_NAME,
PLAN_OPERATION,
(SELECT MIN(TO_CHAR(NB.BEGIN_INTERVAL_TIME, 'YYYY-MM-DD HH24:MI:SS'))
FROM DBA_HIST_SNAPSHOT NB
WHERE NB.SNAP_ID = V.MIN_SNAP_ID) MIN_DATE,
(SELECT MAX(TO_CHAR(NB.END_INTERVAL_TIME, 'YYYY-MM-DD HH24:MI:SS'))
FROM DBA_HIST_SNAPSHOT NB
WHERE NB.SNAP_ID = V.MAX_SNAP_ID) MAX_DATE,
COUNTS
FROM (SELECT D.OBJECT_OWNER INDEX_OWNER,
D.OBJECT_NAME INDEX_NAME,
D.OPERATION || ' ' || D.OPTIONS PLAN_OPERATION,
MIN(H.SNAP_ID) MIN_SNAP_ID,
MAX(H.SNAP_ID) MAX_SNAP_ID,
COUNT(1) COUNTS
FROM DBA_HIST_SQL_PLAN D, DBA_HIST_SQLSTAT H
WHERE D.OPERATION LIKE '%INDEX%'
AND D.SQL_ID = H.SQL_ID
GROUP BY D.OBJECT_OWNER, D.OBJECT_NAME, D.OPERATION, D.OPTIONS) V)
SELECT A.TABLE_OWNER,
A.TABLE_NAME,
A.INDEX_OWNER,
A.INDEX_NAME,
A.CREATED,
A.INDEX_TYPE,
A.INDEX_MB,
A.COUNT_INDEX_COLS,
B.PLAN_OPERATION,
CASE
WHEN MIN_DATE IS NULL THEN
(SELECT MIN(TO_CHAR(NB.BEGIN_INTERVAL_TIME, 'YYYY-MM-DD HH24:MI:SS'))
FROM DBA_HIST_SNAPSHOT NB)
ELSE
MIN_DATE
END AS MIN_DATE,
CASE
WHEN MAX_DATE IS NULL THEN
(SELECT MAX(TO_CHAR(NB.BEGIN_INTERVAL_TIME, 'YYYY-MM-DD HH24:MI:SS'))
FROM DBA_HIST_SNAPSHOT NB)
ELSE
MAX_DATE
END AS MAX_DATE,
COUNTS
FROM TMP1 A
LEFT OUTER JOIN TMP2 B
ON (A.INDEX_OWNER = B.INDEX_OWNER AND A.INDEX_NAME = B.INDEX_NAME);
--原文：http://blog.itpub.net/26736162/viewspace-2120752/