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  • 11g新特性:Note raised when explain plan for create index

    这是一个挺实用的小特性,在11g r2中使用explain plan for create index时Oracle会提示评估的索引大小(estimated index size)了:
    SQL> set linesize 200 pagesize 1400;
    SQL> explain plan for create index ind_t23 on maclean(t2,t3);
    Explained.
    
    SQL> select * from table(dbms_xplan.display());
    
    PLAN_TABLE_OUTPUT
    ----------------------------------------------------------------------------------
    Plan hash value: 2510282917
    ----------------------------------------------------------------------------------
    | Id  | Operation              | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
    ----------------------------------------------------------------------------------
    |   0 | CREATE INDEX STATEMENT |         |  1000K|  9765K|  1267   (1)| 00:00:16 |
    |   1 |  INDEX BUILD NON UNIQUE| IND_T23 |       |       |            |          |
    |   2 |   SORT CREATE INDEX    |         |  1000K|  9765K|            |          |
    |   3 |    TABLE ACCESS FULL   | MACLEAN |  1000K|  9765K|   760   (2)| 00:00:10 |
    ----------------------------------------------------------------------------------
    
    Note
    -----
       - estimated index size: 24M bytes
    
    14 rows selected.
    
    SQL> create index ind_t23 on maclean(t2,t3);
    Index created.
    
    SQL> select bytes/1024/1024 from dba_segments where segment_name='IND_T23';
    BYTES/1024/1024
    ---------------
                 23
    
    SQL> analyze index ind_t23 validate structure;
    Index analyzed.
    
    SQL> select btree_space/1024/1024,used_space/1024/1024 from index_stats;
    BTREE_SPACE/1024/1024 USED_SPACE/1024/1024
    --------------------- --------------------
               22.3849487           20.0912952
    
    /* 可以看到 explain plan给出的索引评估大小与实际索引占用空间差别不大 */
    
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  • 原文地址:https://www.cnblogs.com/macleanoracle/p/2967602.html
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