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