1、索引结构。第一张图是索引的官方图解，右侧是存储方式的图解。
图中很清晰的展示了索引存储的状况。
在leaf 节点中存储了一列，索引所对应项的 ：值，rowId，长度，头信息（控制信息）
这样我们就能很清楚、如果通过索引查找数据，而只需要这个索引的值的时候，写上列名，就可以不需要回表。
2、索引在一般的数据量情况下，只有三层。leaf 是目录，branch 是目录的目录。可以做一个测试
1 drop table t1 purge; 2 drop table t2 purge; 3 drop table t3 purge; 4 drop table t4 purge; 5 drop table t5 purge; 6 drop table t6 purge; 7 drop table t7 purge; 8 9 10 create table t1 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=1; 11 create table t2 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=10; 12 create table t3 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=100; 13 create table t4 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=1000; 14 create table t5 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=10000; 15 create table t6 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=100000; 16 create table t7 as select rownum as id ,rownum+1 as id2,rpad('*',1000,'*') as contents from dual connect by level<=1000000; 17 18 19 create index idx_id_t1 on t1(id); 20 create index idx_id_t2 on t2(id); 21 create index idx_id_t3 on t3(id); 22 create index idx_id_t4 on t4(id); 23 create index idx_id_t5 on t5(id); 24 create index idx_id_t6 on t6(id); 25 create index idx_id_t7 on t7(id); 26 27 set linesize 1000 28 set autotrace off 29 select index_name, 30 blevel, 31 leaf_blocks, 32 num_rows, 33 distinct_keys, 34 clustering_factor 35 from user_ind_statistics 36 where table_name in( 'T1','T2','T3','T4','T5','T6','T7'); 37 38 索引的名字 层级 leaf 块 39 INDEX_NAME BLEVEL LEAF_BLOCKS NUM_ROWS DISTINCT_KEYS CLUSTERING_FACTOR 40 ------------------ ----------- ---------- ------------- ----------------- 41 IDX_ID_T1 0 1 1 1 1 42 IDX_ID_T2 0 1 10 10 2 43 IDX_ID_T3 0 1 100 100 15 44 IDX_ID_T4 1 3 1000 1000 143 45 IDX_ID_T5 1 21 10000 10000 1429 46 IDX_ID_T6 1 222 100000 100000 14286 47 IDX_ID_T7 2 2226 1000000 1000000 142858