SQL 语句访问路径的方式
全表扫描（Full Table Scans）
sql">select * from t_Vio_Violation tPlan Hash Value : 1218663174 -----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost | Time |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 7670989 | 4042611203 | 103263 | 00:00:09 |
| 1 | TABLE ACCESS FULL | T_VIO_VIOLATION | 7670989 | 4042611203 | 103263 | 00:00:09 |
-----------------------------------------------------------------------------------------
rowid 访问
单条件查询，rowid最快的
sql">select rowid from t_Vio_Violation t where t.violation_id ='3321474226' -- AAAQ9lAAFAAAMSzAAH
select * from t_Vio_Violation t where rowid='AAAQ9lAAFAAAMSzAAH';Plan Hash Value : 3532886438 ----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost | Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 527 | 1 | 00:00:01 |
| 1 | TABLE ACCESS BY USER ROWID | T_VIO_VIOLATION | 1 | 527 | 1 | 00:00:01 |
----------------------------------------------------------------------------------------
索引扫描（Index Scan 或 index lookup）
索引唯一扫描(index unique scan)
- 特点：
- 唯一性：索引唯一扫描只能用于唯一索引，确保每次查询返回的行数是唯一的。
- 高效性：由于索引键值是唯一的，Oracle 可以直接定位到目标行，无需进一步搜索，因此效率非常高。
- 这里的violation_id 唯一索引unique。
sql">ITMS5_1@hfzcdb> explain plan for select * from t_Vio_Violation t where t.violation_id='3321474226';Explained.ITMS5_1@hfzcdb> select * from table(dbms_xplan.display);PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 4183191119-----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 527 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T_VIO_VIOLATION | 1 | 527 | 3 (0)| 00:00:01 |
|* 2 | INDEX UNIQUE SCAN | SYS_C0013148 | 1 | | 2 (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------Predicate Information (identified by operation id):
---------------------------------------------------2 - access("T"."VIOLATION_ID"='3321474226')14 rows selected.ITMS5_1@hfzcdb>
索引范围扫描(index range scan)
- 特点：
- 非唯一性：索引范围扫描使用非唯一索引，允许索引键值有重复。
- 范围查询：适用于范围查询条件，如 >、<、BETWEEN、LIKE 等。
- 顺序访问：按索引顺序扫描数据，通常是从最小值到最大值。
sql">#select rowid from t_Vio_Violation t where t.violation_id <'3321474226';Plan Hash Value : 94343750 ----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost | Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 23 | 3 | 00:00:01 |
| * 1 | INDEX RANGE SCAN | SYS_C0013148 | 1 | 23 | 3 | 00:00:01 |
----------------------------------------------------------------------------Predicate Information (identified by operation id):
------------------------------------------
* 1 - access("T"."VIOLATION_ID"<'3321474226')#select rowid from t_Vio_Violation t where t.violation_id <'3321474226' order by violation_id desc;Plan Hash Value : 1205653834 ---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost | Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 23 | 3 | 00:00:01 |
| * 1 | INDEX RANGE SCAN DESCENDING | SYS_C0013148 | 1 | 23 | 3 | 00:00:01 |
---------------------------------------------------------------------------------------Predicate Information (identified by operation id):
------------------------------------------
* 1 - access("T"."VIOLATION_ID"<'3321474226')
索引全扫描(index full scan)
-
特点：
-
仅访问索引：索引全扫描只读取索引数据块，不访问表的数据块。
-
顺序访问：按索引顺序访问数据，通常是从最小值到最大值。
-
不需要过滤条件：适用于没有过滤条件或过滤条件较弱的情况。
-
与全表扫描（Full Table Scan）不同，索引全扫描只访问索引数据块，而不需要访问整个表的数据块，因此在某些情况下可以显著提高查询性能。
-
sql">#select max(t.violation_time) from t_Vio_Violation t Plan Hash Value : 2284200782 ----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost | Time |
----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 8 | 3 | 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 8 | | |
| 2 | INDEX FULL SCAN (MIN/MAX) | IDX_VIOLATION_TIME_01 | 1 | 8 | 3 | 00:00:01 |
----------------------------------------------------------------------------------------------
索引快速扫描(index fast full scan)
-
特点：
-
并行访问：索引快速全扫描并行访问所有可用的索引块，不按索引键值的顺序访问数据。
-
仅访问索引：只读取索引数据块，不访问表的数据块。
-
不排序：返回的数据不按索引键值排序。
-
sql">#select t.violation_type from t_Vio_Violation t
Plan Hash Value : 2392038657 -----------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost | Time |
-----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 7670989 | 23012967 | 13074 | 00:00:02 |
| 1 | INDEX FAST FULL SCAN | IDX_VIOLATION_TIME_01 | 7670989 | 23012967 | 13074 | 00:00:02 |
-----------------------------------------------------------------------------------------------
索引跳跃扫描(index skip scan)
-
特点：
-
复合索引：索引跳跃扫描仅适用于复合索引，即具有多个列的索引。
-
跳过前导列：在查询中未使用的前导列（索引定义中的第一列或前几列）会被跳过，直接使用后续列进行索引扫描。
-
提高选择性：通过跳过选择性较低的前导列，可以提高后续列的选择性，从而提高查询性能
-
sql"> #select t.violation_type from t_Vio_Violation t where t.violation_type='1625A' and t.address_desc='%路口%'Plan Hash Value : 2819657385 ----------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost | Time |
----------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 44 | 53690 | 00:00:05 |
| * 1 | TABLE ACCESS BY INDEX ROWID BATCHED | T_VIO_VIOLATION | 1 | 44 | 53690 | 00:00:05 |
| * 2 | INDEX SKIP SCAN | IDX_VIOLATION_TIME_01 | 25394 | | 34768 | 00:00:03 |
----------------------------------------------------------------------------------------------------------Predicate Information (identified by operation id):
------------------------------------------
* 1 - filter("T"."ADDRESS_DESC"='%路口%')
* 2 - access("T"."VIOLATION_TYPE"='1625A')
* 2 - filter("T"."VIOLATION_TYPE"='1625A')/*Access：表示对应的谓词条件会影响数据的访问路径（是按照索引还是表）
Filter：表示谓词条件只会起到过滤作用，不会影响数据的访问路径。
因此，需要着重关注filter部分是否可创建索引
