Friday, March 23

Understanding Oracle pseudo-columns

All the cool people I meet at Oracle events keep referring to "function based indexes" as pseudo-columns. I never really understood the whole thing about it, sure I figures one could consider it a pseudo-column as it was sort of a calculated value that was stored in the database. Never thought much about investigating it either.
However, when playing around with some composite indexes including functions today I finally got it. They are indeed pseudo-columns, or calculated columns as I think Oracle wants to call them in 11g.

As always,
an example:
SQL> create table d as select * from all_objects;

Table created.

SQL> insert into d select * from d;

66242 rows created.

SQL> update d set object_id=rownum;

132484 rows updated.

-- Ok, got a test table. Let's index it with some pseudo stuff.SQL> create index d_1 on d(upper(status));

Index created.

SQL> select index_name,column_name from user_ind_columns where table_name='D';

INDEX_NAME COLUMN_NAME
------------------------------ ----------------------------------------
D_1 SYS_NC00014$

SQL> select index_name,column_expression from user_ind_expressions
2 where table_name='D';

INDEX_NAME COLUMN_EXPRESSION
------------------------------ ------------------------------
D_1 UPPER("STATUS")

SQL> create index d_2 on d(object_id,upper(status),timestamp);

Index created.

SQL> select index_name,column_name from user_ind_columns where table_name='D';

INDEX_NAME COLUMN_NAME
------------------------------ ------------------------------
D_1 SYS_NC00014$
D_2 OBJECT_ID
D_2 SYS_NC00014$
D_2 TIMESTAMP

SQL> select index_name,column_expression from user_ind_expressions
2 where table_name='D';

INDEX_NAME COLUMN_EXPRESSION
------------------------------ ------------------------------
D_1 UPPER("STATUS")
D_2 UPPER("STATUS")

-- Thats the index bit, we got two indexes with the same "function",
-- so Oracle will index the same pseudo column! Eureka!
-- Some tests then...SQL> set autotrace traceonly exp
SQL> select object_name from d where upper(status)='T';

Execution Plan
----------------------------------------------------------
Plan hash value: 2452139598

------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 21 | 462 | 165 (0)| 00:00:02 |
| 1 | TABLE ACCESS BY INDEX ROWID| D | 21 | 462 | 165 (0)| 00:00:02 |
|* 2 | INDEX RANGE SCAN | D_1 | 418 | | 157 (0)| 00:00:02 |
------------------------------------------------------------------------------------

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

2 - access(UPPER("STATUS")='T')

Note
-----
- dynamic sampling used for this statement

-- A column can go in the where clause, even pseudo-columns.SQL> select object_name from d where sys_nc00014$='T';

Execution Plan
----------------------------------------------------------
Plan hash value: 2452139598

------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 21 | 462 | 165 (0)| 00:00:02 |
| 1 | TABLE ACCESS BY INDEX ROWID| D | 21 | 462 | 165 (0)| 00:00:02 |
|* 2 | INDEX RANGE SCAN | D_1 | 418 | | 157 (0)| 00:00:02 |
------------------------------------------------------------------------------------

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

2 - access(UPPER("STATUS")='T')

Note
-----
- dynamic sampling used for this statement


-- Since it is a column we can obviously select it as well.SQL> select SYS_NC00014$ from d where object_id=500;

SYS_NC0
-------
VALID

Execution Plan
----------------------------------------------------------
Plan hash value: 2591304836

-------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 18 | 5 (0)| 00:00:01 |
|* 1 | INDEX RANGE SCAN| D_2 | 1 | | 3 (0)| 00:00:01 |
-------------------------------------------------------------------------

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

1 - access("OBJECT_ID"=500)

Note
-----
- dynamic sampling used for this statement

-- You can even rename the pseudo-column.
-- Oracle did actually dump core after I did that so better not.SQL> alter table d rename column SYS_NC00014$ to dog;

Table altered.

SQL> alter table d rename column dog to SYS_NC00014$;

Table altered.

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