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一 反向索引
1.1 反向索引的定義
- 反向索引作為B-tree索引的一個分支，主要是在創建索引時，針對索引列的索引鍵值進行字節反轉，進而實現分散存放到不同葉子節點塊的目的。
1.2 反向索引針對的問題
- 使用傳統的B-tree索引，當索引的列是按順序產生時，相應的索引鍵值會基本分布在同一個葉塊中。當用戶對該列進行操作時，難免會發生索引塊的爭用。
- 使用反向索引，將索引列的鍵值進行反轉，實現順序的鍵值分散到不同的葉塊中，從而減少索引塊的爭用。
- 例如：鍵值1001、1002、1003，反轉后1001、2001、3001，進而分散到不用的葉子節點塊中。
1.3 反向索引應用場景
- 索引塊成為熱點塊
- rac環境
- rac環境下中多節點訪問訪問數據呈現密集且集中的特點，索引熱塊的產生較高。
- 在范圍檢索不高的rac環境中使用反向索引可有效提高性能。
1.4 反向索引的優點與缺點
- 優點：降低索引葉子塊的爭用問題，提升系統性能。
- 缺點：對於范圍檢索，例如：between，>，<時，反向索引無法引用，進而導致全表掃面的產生，降低系統性能。
1.5 反向索引示例說明
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-- 創建兩張相同結構的表，內部結構及數據均引用scott用戶下的emp表
SQL> select count(*) from test01; COUNT(*) ---------- 14
SQL> select count(*) from test02; COUNT(*) ---------- 14 --針對表TEST01的empno列，添加B-tree索引 SQL> create index PK_TEST01 on TEST01(EMPNO); Index created.
--針對表TEST02的empno列，添加反向索引 SQL> create index PK_REV_TEST02 on TEST02(EMPNO) REVERSE; Index created.
--驗證上面的索引，NORMAL/REV表明為反向索引 SQL> select TABLE_NAME,INDEX_NAME,INDEX_TYPE from user_indexes where INDEX_NAME like '%TEST%'; TABLE_NAME INDEX_NAME INDEX_TYPE -------------------- -------------------- -------------------- TEST01 PK_TEST01 NORMAL TEST02 PK_REV_TEST02 NORMAL/REV
--打開會話追蹤 SQL> set autotrace traceonly
--相同條件查詢，觀察兩表的執行計划 SQL> select * from TEST01 where empno=7369;
Execution Plan ---------------------------------------------------------- Plan hash value: 515586510 ----------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ----------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 87 | 2 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| TEST01 | 1 | 87 | 2 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | PK_TEST01 | 1 | | 1 (0)| 00:00:01 | ----------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("EMPNO"=7369) Note ----- - dynamic sampling used for this statement (level=2) Statistics ---------------------------------------------------------- 152 recursive calls 0 db block gets 23 consistent gets 0 physical reads 0 redo size 1025 bytes sent via SQL*Net to client 523 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed SQL> select * from TEST02 where empno=7369; Execution Plan ---------------------------------------------------------- Plan hash value: 1053012716 --------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 87 | 2 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| TEST02 | 1 | 87 | 2 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | PK_REV_TEST02 | 1 | | 1 (0)| 00:00:01 | --------------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("EMPNO"=7369) Note ----- - dynamic sampling used for this statement (level=2) Statistics ---------------------------------------------------------- 148 recursive calls 0 db block gets 21 consistent gets 0 physical reads 0 redo size 1025 bytes sent via SQL*Net to client 523 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 1 rows processed -- 相同范圍條件查詢，觀察兩表的執行計划 SQL> select * from TEST01 where empno between 7350 and 7500; Execution Plan ---------------------------------------------------------- Plan hash value: 515586510 ----------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ----------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 2 | 174 | 2 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| TEST01 | 2 | 174 | 2 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | PK_TEST01 | 2 | | 1 (0)| 00:00:01 | ----------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("EMPNO">=7350 AND "EMPNO"<=7500) Note ----- - dynamic sampling used for this statement (level=2) Statistics ---------------------------------------------------------- 9 recursive calls 0 db block gets 10 consistent gets 0 physical reads 0 redo size 1120 bytes sent via SQL*Net to client 523 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 2 rows processed SQL> select * from TEST02 where empno between 7350 and 7500; Execution Plan ---------------------------------------------------------- Plan hash value: 3294238222 ---------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ---------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 2 | 174 | 3 (0)| 00:00:01 | |* 1 | TABLE ACCESS FULL| TEST02 | 2 | 174 | 3 (0)| 00:00:01 | ---------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("EMPNO">=7350 AND "EMPNO"<=7500) Note ----- - dynamic sampling used for this statement (level=2) Statistics ---------------------------------------------------------- 5 recursive calls 0 db block gets 8 consistent gets0 redo size 1112 bytes sent via SQL*Net to client 523 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 2 rows processed通過上面的示例可以看到，當使用between條件進行范圍查詢時，采用反向索引的表，並沒有使用索引，而是采用了全表掃面的方式進行檢索。