oracle - Oracle 何时为空列值建立索引？

Question

我曾经认为当其中一个列值为空时，Oracle 不会索引一行。

一些简单的实验表明情况并非如此。我能够运行一些查询，意外地仅访问索引，即使某些列可以为空(这当然是一个令人愉快的惊喜)。

谷歌搜索导致一些博客的答案相互矛盾:我读到，除非所有索引列都为空，否则行会被索引，并且除非前导列，否则行会被索引索引的值为 null。

那么，什么情况下一行不会进入索引呢？这个 Oracle 版本是否特定？

Answer 1

如果任何索引列包含非空值，则该行将被索引。正如您在下面的示例中看到的，只有一行没有被索引，并且该行在两个索引列中都有 NULL。您还可以看到，当前导索引列具有 NULL 值时，Oracle 肯定会对行建立索引。

SQL> create table big_table as
  2       select object_id as pk_col
  3               , object_name as col_1
  4               , object_name as col_2
  5  from all_objects
  6  /

Table created.

SQL> select count(*) from big_table
  2  /

  COUNT(*)
----------
     69238

SQL> insert into big_table values (9999990, null, null)
  2  /

1 row created.

SQL> insert into big_table values (9999991, 'NEW COL 1', null)
  2  /

1 row created.

SQL> insert into big_table values (9999992, null, 'NEW COL 2')
  2  /

1 row created.

SQL> select count(*) from big_table
  2  /

  COUNT(*)
----------
     69241

SQL> create index big_i on big_table(col_1, col_2)
  2  /

Index created.

SQL> exec dbms_stats.gather_table_stats(user, 'BIG_TABLE', cascade=>TRUE)

PL/SQL procedure successfully completed.


SQL> select num_rows from user_indexes where index_name = 'BIG_I'
  2  /

  NUM_ROWS
----------
     69240

SQL> set autotrace traceonly exp
SQL>
SQL> select pk_col from big_table
  2  where col_1 = 'NEW COL 1'
  3  /

Execution Plan
----------------------------------------------------------
Plan hash value: 1387873879

-----------------------------------------------------------------------------------------
| Id  | Operation                   | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |           |     2 |    60 |     4   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| BIG_TABLE |     2 |    60 |     4   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | BIG_I     |     2 |       |     3   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------

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

   2 - access("COL_1"='NEW COL 1')

SQL> select pk_col from big_table
  2  where col_2 = 'NEW COL 2'
  3  /

Execution Plan
----------------------------------------------------------
Plan hash value: 3993303771

-------------------------------------------------------------------------------
| Id  | Operation         | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |           |     2 |    60 |   176   (1)| 00:00:03 |
|*  1 |  TABLE ACCESS FULL| BIG_TABLE |     2 |    60 |   176   (1)| 00:00:03 |
-------------------------------------------------------------------------------

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

   1 - filter("COL_2"='NEW COL 2')

SQL> select pk_col from big_table
  2  where col_1 is null
  3  and col_2 = 'NEW COL 2'
  4  /

Execution Plan
----------------------------------------------------------
Plan hash value: 1387873879

-----------------------------------------------------------------------------------------
| Id  | Operation                   | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |           |     1 |    53 |     4   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| BIG_TABLE |     1 |    53 |     4   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | BIG_I     |     2 |       |     3   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------

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

   2 - access("COL_1" IS NULL AND "COL_2"='NEW COL 2')
       filter("COL_2"='NEW COL 2')

SQL> select pk_col from big_table
  2  where col_1 is null
  3  and col_2 is null
  4  /

Execution Plan
----------------------------------------------------------
Plan hash value: 3993303771

-------------------------------------------------------------------------------
| Id  | Operation         | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |           |     1 |    53 |   176   (1)| 00:00:03 |
|*  1 |  TABLE ACCESS FULL| BIG_TABLE |     1 |    53 |   176   (1)| 00:00:03 |
-------------------------------------------------------------------------------

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

   1 - filter("COL_1" IS NULL AND "COL_2" IS NULL)

SQL>

此示例在 Oracle 11.1.0.6 上运行。但我非常有信心它适用于所有版本。