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Friday, December 08, 2017

Changing Physical Index Attributes without Down Time

Normally, we make an index invisible before we drop it, in order to see whether we can manage without it, and if not we can make it visible again.  In this blog, I will demonstrate how to use index invisibility to introduce an index change that I cannot do with an online operation.  I am also able to reverse it immediately, whereas an on-line operation would take time.

Problem Statement

I have a large partitioned table, but the primary key index on it was not partitioned.  Testing has shown that performance would improve if the index was locally partitioned.  It is not possible to introduce the partitioning by rebuilding the index online.  I cannot afford the downtime to drop and rebuild the index, and anyway I want an easy way back to the original state in case of a problem.

Demonstration

I encountered this problem in a PeopleSoft system with a unique index, but here I will demonstrate it with a primary key constraint also.  I will create and populate a simple range partitioned table with a primary key constraint.
DROP TABLEt PURGE;

CREATE TABLE t
(a number not null
,b number not null
,c number
,d number) 
partition by range (a) interval (42)
(partition values less than (42))
/
ALTER TABLE t ADD CONSTRAINT t_pk PRIMARY KEY (a,b)
/
TRUNCATE TABLE t
/
INSERT INTO t
WITH x AS (select rownum n from dual connect by level <= 100)
SELECT a.n, b.n, a.n*b.n, a.n+b.n
FROM x a, x b
/
Note that table is partitioned, but the unique index is not. I haven't had to explicitly build it without partitioning.
set long 5000
select dbms_metadata.get_ddl('INDEX','T_PK')
from dual
/
CREATE UNIQUE INDEX "SCOTT"."T_PK" ON "SCOTT"."T" ("A", "B")
PCTFREE 10 INITRANS 2 MAXTRANS 255 COMPUTE STATISTICS
TABLESPACE "USERS"
I cannot just create the new index because the column list is already indexed.
CREATE UNIQUE INDEX SCOTT.T_PK_NEW ON SCOTT.T (A, B)
                                               *
ERROR at line 1:
ORA-01408: such column list already indexed
So, now I will create a new unique index on the same columns, but I will create it invisible. I will also create it online so that I do not block the application from performing DML on the table while the index is built.
CREATE UNIQUE INDEX SCOTT.T_PK_NEW ON SCOTT.T (A, B)
LOCAL INVISIBLE ONLINE
/
So now I have two unique indexes, one visible, one invisible
set autotrace off
column constraint_name format a20
column table_name format a10
column index_name format a10
SELECT index_name, visibility FROM user_indexes WHERE table_name = 'T'
/

INDEX_NAME VISIBILIT
---------- ---------
T_PK       VISIBLE
T_PK_NEW   INVISIBLE
I cannot make the new index visible while the original index is also visible because I cannot have two unique indexes
ALTER INDEX SCOTT.T_PK_NEW VISIBLE
/
*
ERROR at line 1:
ORA-14147: There is an existing VISIBLE index defined on the same set of columns.
Instead, I have to make the original index invisible first. However, even if both indexes are invisible the unique constraint is still enforced.
ALTER INDEX SCOTT.T_PK INVISIBLE
/
INSERT INTO t VALUES (1,2,3,4)
/
*
ERROR at line 1:
ORA-00001: unique constraint (SCOTT.T_PK) violated

ALTER INDEX SCOTT.T_PK_NEW VISIBLE
/
Now my new index is visible.
SELECT index_name, visibility FROM user_indexes WHERE table_name = 'T'
/

INDEX_NAME VISIBILIT
---------- ---------
T_PK       INVISIBLE
T_PK_NEW   VISIBLE
The constraint still points to the original index.
INSERT INTO t VALUES(1,2,3,4)
/
*
ERROR at line 1:
ORA-00001: unique constraint (SCOTT.T_PK) violated

SELECT constraint_name, table_name, index_name
FROM   user_constraints
WHERE  constraint_name = 'T_PK'
/

CONSTRAINT_NAME      TABLE_NAME INDEX_NAME
-------------------- ---------- ----------
T_PK                 T          T_PK
However, queries now use the new index.
set autotrace on lines 160
SELECT * FROM t WHERE a = 1 and b=2
/
Plan hash value: 3024084754
---------------------------------------------------------------------------------------------------------------
| Id  | Operation                          | Name     | Rows  | Bytes | Cost (%CPU)| Time     | Pstart| Pstop |
---------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                   |          |     1 |    52 |     2   (0)| 00:00:01 |       |       |
|   1 |  PARTITION RANGE SINGLE            |          |     1 |    52 |     2   (0)| 00:00:01 |     1 |     1 |
|   2 |   TABLE ACCESS BY LOCAL INDEX ROWID| T        |     1 |    52 |     2   (0)| 00:00:01 |     1 |     1 |
|*  3 |    INDEX UNIQUE SCAN               | T_PK_NEW |     1 |       |     1   (0)| 00:00:01 |     1 |     1 |
---------------------------------------------------------------------------------------------------------------
The original index can now be removed. However, unique indexes used by primary key constraints cannot be dropped directly. Instead the modifying the primary key constraint drops the original index.
ALTER TABLE scott.t 
MODIFY CONSTRAINT t_pk USING INDEX t_pk_new
/

SELECT constraint_name, table_name, index_name
FROM   user_constraints
WHERE  constraint_name = 'T_PK'
/
CONSTRAINT_NAME      TABLE_NAME INDEX_NAME
-------------------- ---------- ----------
T_PK                 T          T_PK_NEW

SELECT index_name, visibility FROM user_indexes WHERE table_name = 'T'
/

INDEX_NAME VISIBILIT
---------- ---------
T_PK_NEW   VISIBLE
Finally, the new index can be renamed.
ALTER INDEX t_pk_new RENAME TO t_pk
/

2 comments :

Grzegorz Goryszewski said...

Hi,
thank You for the informative post.
Would You consider DBMS_REDEFINITION as an alternative ?
Regards .
G

David Kurtz said...

On-line redefinition is certainly an option for altering a table. For an index, you can make some changes with an online rebuild, but reversing it out also takes time. The point here is that you can switch it back and forth, and then when you are satisfied drop the unnecessary index.