Oracle优化——LIKE与索引（以%开头的LIKE会不走索引或走索引全扫描）

这样会走索引范围扫描，因为这个表达式有前导性。
SH@ prod> set autotrace on
SH@ prod> select max(cust_credit_limit) , count(*) from customers_ne where cust_last_name like 'Vaugh%' ;

MAX(CUST_CREDIT_LIMIT)   COUNT(*)
---------------------- ----------
                 11000         81


Execution Plan
----------------------------------------------------------
Plan hash value: 3473995898

---------------------------------------------------------------------------------------------
| Id  | Operation                    | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |              |     1 |    16 |    13   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE              |              |     1 |    16 |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID| CUSTOMERS_NE |    61 |   976 |    13   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN          | LAST_IDX1    |    61 |       |     2   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - access("CUST_LAST_NAME" LIKE 'Vaugh%')
       filter("CUST_LAST_NAME" LIKE 'Vaugh%')


Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
         14  consistent gets
          0  physical reads
          0  redo size
        614  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

这样不会走索引，因为这个表达式没有前导性，无法找到索引的开始点。
SH@ prod> select max(cust_credit_limit) , count(*) from customers_ne where cust_last_name like '%aughn' ;

MAX(CUST_CREDIT_LIMIT)   COUNT(*)
---------------------- ----------
                 11000         81


Execution Plan
----------------------------------------------------------
Plan hash value: 3963802310

-----------------------------------------------------------------------------------
| Id  | Operation          | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |              |     1 |    16 |   405   (1)| 00:00:05 |
|   1 |  SORT AGGREGATE    |              |     1 |    16 |            |          |
|*  2 |   TABLE ACCESS FULL| CUSTOMERS_NE |  2775 | 44400 |   405   (1)| 00:00:05 |
-----------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter("CUST_LAST_NAME" LIKE '%aughn')


Statistics
----------------------------------------------------------
          8  recursive calls
          0  db block gets
       1460  consistent gets
          0  physical reads
          0  redo size
        614  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

强制使用索引，走的时索引全扫描，还远不如全表扫描。
SH@ prod> select /*+ index(customers_ne) */ max(cust_credit_limit) , count(*) from customers_ne where cust_last_name like '%aughn' ;

MAX(CUST_CREDIT_LIMIT)   COUNT(*)
---------------------- ----------
                 11000         81


Execution Plan
----------------------------------------------------------
Plan hash value: 359032907

---------------------------------------------------------------------------------------------
| Id  | Operation                    | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |              |     1 |    16 |   636   (1)| 00:00:08 |
|   1 |  SORT AGGREGATE              |              |     1 |    16 |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID| CUSTOMERS_NE |  2775 | 44400 |   636   (1)| 00:00:08 |
|*  3 |    INDEX FULL SCAN           | LAST_IDX1    |  2775 |       |   143   (1)| 00:00:02 |
---------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - filter("CUST_LAST_NAME" LIKE '%aughn')


Statistics
----------------------------------------------------------
          8  recursive calls
          0  db block gets
        155  consistent gets
        139  physical reads
          0  redo size
        614  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