當你想評估一個索引對你SQL查詢語句的影響時，恰巧你的數據庫是11g時候，可以快速將索引設置成VISIBLE或INVISIBLE。值得稱贊的時，當索引被設置成INVISIBLE時候，DML語句發生時候照常會維護索引，也就是說處於INVISIBLE狀態下的索引並不失效，只是優化器不選擇索引路徑而已。
下面通過實驗來驗證一下：
SQL> select * from v$version; BANNER -------------------------------------------------------------------------------- Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production PL/SQL Release 11.2.0.4.0 - Production CORE 11.2.0.4.0 Production TNS for Linux: Version 11.2.0.4.0 - Production NLSRTL Version 11.2.0.4.0 - Production SQL> select * from t; ID ---------- 2 11 22 1 33 SQL> select count(id) from t; COUNT(ID) ---------- 5 Execution Plan ---------------------------------------------------------- Plan hash value: 2966233522 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 3 | 2 (0)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | 3 | | | | 2 | <strong>TABLE ACCESS FULL</strong>| T | 5 | 15 | 2 (0)| 00:00:01 |--可以看到此時是全表掃描
在T表ID列創建唯一索引
<p>SQL> create unique index idx_t_id on t(id) invisible;</p><p>Index created.</p>SQL> select count(id) from t; COUNT(ID) ---------- 5 Execution Plan ---------------------------------------------------------- Plan hash value: 4168287108 ----------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ----------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 3 | 1 (0)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | 3 | | | | 2 | <strong>INDEX FULL SCAN</strong>| IDX_T_ID | 5 | 15 | 1 (0)| 00:00:01 | ----------------------------------------------------------------------------- --發生索引全掃描
將索引設置成INVISIBLE
SQL> alter index idx_t_id invisible; Index altered<p>SQL> select index_name,status,visibility from dba_indexes where table_name='T'; INDEX_NAME STATUS VISIBILITY ------------------------------ -------- ---------- IDX_T_ID <span style="color:#ff0000;"><strong>VALID</strong></span> <strong>INVISIBLE</strong></p><p>SQL> select count(id) from t;</p><p> COUNT(ID) ---------- 5</p><p> Execution Plan ---------------------------------------------------------- Plan hash value: 2966233522</p><p>--------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 3 | 2 (0)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | 3 | | | | 2 | <strong>TABLE ACCESS FULL</strong>| T | 5 | 15 | 2 (0)| 00:00:01 | --此時執行計劃裡又是走全表掃描路徑 ---------------------------------------------------------------------------</p><p>SQL> alter index idx_t_id visible; Index altered</p>
索引的不可見可以避免了索引重建，尤其是大表的索引，這個新特性更有利於數據庫的優化
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