一个用 Oracle 函数索引进行优化的例子

表中有500万条记录，原来没有索引：

set timing on set autotrace traceonly SQL> select count(*), to_char(time,'hh24') from userloginlog 2 where trunc(time) = trunc(sysdate) - 1 3 group by to_char(time,'hh24') 4 order by to_char(time,'hh24');

24 rows selected. Elapsed: 00:00:06.70 Execution Plan ----------------------------------------------------------    0      SELECT STATEMENT Optimizer=ALL_ROWS (Cost=6179 Card=37215 Bytes=297720)    1    0   SORT (GROUP BY) (Cost=6179 Card=37215 Bytes=297720)    2    1     TABLE ACCESS (FULL) OF 'USERLOGINLOG' (TABLE) (Cost=6039 Card=37257 Bytes=298056) Statistics ----------------------------------------------------------           1  recursive calls           0  db block gets       25154  consistent gets       24470  physical reads           0  redo size         763  bytes sent via SQL*Net to client         514  bytes received via SQL*Net from client           3  SQL*Net roundtrips to/from client           1  sorts (memory)           0  sorts (disk)          24  rows processed

查询使用全表扫描，执行需6.7秒。

下面建立函数索引：

create index idx_time on userloginlog (to_char(time,'hh24')) tablespace indexes;  create index idx_time2 on userloginlog (trunc(time)) tablespace indexes;

执行同样的查询：

SQL> select count(*), to_char(time,'hh24') from userloginlog 2 where trunc(time) = trunc(sysdate) - 1 3 group by to_char(time,'hh24') 4 order by to_char(time,'hh24');

24 rows selected. Elapsed: 00:00:00.34 Execution Plan ----------------------------------------------------------    0      SELECT STATEMENT Optimizer=ALL_ROWS (Cost=323 Card=37215 Bytes=297720)    1    0   SORT (GROUP BY) (Cost=323 Card=37215 Bytes=297720)    2    1     TABLE ACCESS (BY INDEX ROWID) OF 'USERLOGINLOG' (TABLE) (Cost=183 Card=37257 Bytes=298056)    3    2       INDEX (RANGE SCAN) OF 'IDX_TIME2' (INDEX) (Cost=64 Card=16143)  Statistics ----------------------------------------------------------         197  recursive calls           0  db block gets         341  consistent gets           1  physical reads           0  redo size         763  bytes sent via SQL*Net to client         514  bytes received via SQL*Net from client           3  SQL*Net roundtrips to/from client           6  sorts (memory)           0  sorts (disk)          24  rows processed

查询使用索引扫描，执行需0.34秒，快了20倍。

还有一点，建立索引后并没有执行 analyze table userloginlog compute statistics; 进行分析，索引就生效了，这是10g的改进吧。