“大于”查询的索引

Question

我有几个查询，其中大多数是：

select * from Blah where col > 0

和

select * from Blah where date > current_date

由于它们都是一个范围，因此在col和date上使用未聚类的b +树索引会是加快查询速度的一个好主意吗？还是哈希索引？还是没有索引会更好？

Answer 1

在 过滤谓词上 用作 日期范围条件 的列上创建 INDEX 应该很有用，因为它将执行 INDEX RANGE SCAN 。

这是有关如何在Oracle中创建，显示和阅读EXPLAIN
PLAN
的演示。

让我们看一下两种情况的测试用例：

测试＃1：无索引

SQL> EXPLAIN PLAN FOR
  2  SELECT * FROM emp WHERE hiredate > to_date('01/04/1981','mm/dd/yyyy');

Explained.

SQL>
SQL> SELECT * FROM TABLE(dbms_xplan.display);

PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------
Plan hash value: 3956160932

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |    14 |   518 |     3   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| EMP  |    14 |   518 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------

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

PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------

   1 - filter("HIREDATE">TO_DATE(' 1981-01-04 00:00:00', 'syyyy-mm-dd
              hh24:mi:ss'))

14 rows selected.

SQL>

测试＃1：带索引

SQL> CREATE INDEX emp_idx ON emp(hiredate);

Index created.

SQL>
SQL> EXPLAIN PLAN FOR
  2  SELECT * FROM emp WHERE hiredate > to_date('01/04/1981','mm/dd/yyyy');

Explained.

SQL>
SQL> SELECT * FROM TABLE(dbms_xplan.display);

PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------
Plan hash value: 3589413211

-----------------------------------------------------------------------------------------------
| Id  | Operation                           | Name    | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                    |         |    14 |   518 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID BATCHED| EMP     |    14 |   518 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN                  | EMP_IDX |    14 |       |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------
---------------------------------------------------

   2 - access("HIREDATE">TO_DATE(' 1981-01-04 00:00:00', 'syyyy-mm-dd hh24:mi:ss'))

14 rows selected.

SQL>

因此，在第二个测试用例中，您将看到索引范围扫描。我建议您也对您的环境进行类似的测试。