jueves, mayo 29, 2008

Reorganización de tablas en Oracle

En Oracle los datos e índices se encuentran físicamente en tablespaces. En algunos modelos de almacenamiento tiene sentido tener tablespaces separados para índices y datos. Sin embargo con las cabinas tipo EVA5000, EVA8100, etc., debido al concepto de virtualización la separación no ofrece ventajas en cuanto a rendimiento.

Un tablespace es uno, o varios ficheros físicos para el sistema operativo. Dentro de un tablespace tenemos las tablas de datos y/o índices. Esa información se encuentra almacenada en bloques. En nuestra instalación el tamaño del bloque es de 8 Kbytes. También existen las extensiones, pero ya hablaremos de ello otro día.

El que los datos de los bloques se encuentren fragmentados significa que los bloque no se encuentran llenos del todo y queda espacio libre en ellos. Ello supone que si se hace un full scan de la tabla por ejemplo se tienen que recorrer más bloques de lo que haría falta si no hubiera fragmentación.

La fragmentación aumenta no sólo el número de bloques que hay que leer de disco, sino también el movimiento de bloques en memoria. Por ello la reorganización de las tablas e índices puede mejorar el rendimiento.

Formas de reorganizar las tablas:

Hay 3 formas de reorganizar las tablas:

  1. Export e Import del esquema Oracle. El inconveniente es que lo tiene que hacer el Administrador, que es un proceso pesado y que se interrumpe el servicio.

  1. Alter table move y alter index rebuild. Esta método produce bloqueos. Es más selectivo que el anterior por lo que el tiempo de interrupción es menor. El inconveniente es que los índices se quedan en mal estado y hay que reconstruirlos.

ALTER TABLE MOVE;

ALTER INDEX REBUILD;

La pega es que una tabla puede tener 15 índices por ejemplo. Si hacemos el MOVE de la tabla, ésta no se encuentra disponible hasta que hagamos el REBUILD de los índices. Para facilitar el trabajo podemos hacer lo siguiente:

ALTER TABLE TABLA-UNO MOVE;

ALTER TABLE TABLA-DOS MOVE;

SELECT 'ALTER INDEX',index_name,'REBUILD;'

from user_indexes

where table_name in ('TABLA-UNO','TABLA-DOS');

  1. Utilización de DBMS_REDEFINITION. La ventaja es que no hay ningún bloqueo. La reorganización se hace on-line. Sin embargo no se puede utilizar para todo tipo de tablas. Hay una nota Oracle que explica su uso. Es la 177407.1.

NOTA 177407.1 DE ORACLE

PURPOSE
Prior to Oracle9i, any table re-organizations and redefinitions forced the table to be offlined for the duration of the operation.
From Oracle9i, table re-organizations and redefinitions can be performed 
  online.  
 
  This note explains how to reorganize a table online.
 
  NOTE:  This note does not cover the use of row ids.  As of 9.2.0.5,
         additional parameters were added to the DBMS_REDEFINITION 
         packages which allow redefinition to use a row id instead 
         of a primary key.
 
 
SCOPE & APPLICATION
  Oracle8, Oracle8i, and Oracle9i.
 
 
How to Re-Organize a Table Online:
==================================
 
What can be Redefined ONLINE on a Table?
----------------------------------------
 
 => A non-partitioned table can be converted into a partitioned table, and 
    vice versa
 => The organization of a table can be changed from a heap based to IOTs (Index
    Organized Tables), and vice versa
 => Non-primary key columns can be dropped
 => New columns can be added to a table
 => Existing columns can be renamed 
 => Parallel support can be added or removed
 => Storage parameters can be modified
 
Renaming columns is possible without using Online Redefinition in RDBMS version
9.2 with the new RENAME COLUMN clause of the ALTER TABLE statement.
 
Restrictions
------------
The table to be re-organized:
  * Must have a primary key (restriction lifted in 9.2.0.5)
  * Cannot have User-defined data types
  * Cannot have FILE or LONG columns
  * Cannot be clustered
  * Cannot be in the SYS or SYSTEM schema
  * Cannot have materialized view logs and/or 
                materialized views defined on them
  * Cannot be an horizontal subsetting of data
  * Must be re-organized within the same schema
  * Looses its snapshot logs
  * Can get new columns as part of the re-organization, but the
    new columns must be declared NULL until the re-organization 
    is complete
 
Process
-------
 
   ------  5' Table INT_EMP becomes EMP  -------
   |             and vice-versa                 |
    
   1                                         2
 Source Table           3              Create interim Table 
  EMP     ---> Start redefinition --->      INT_EMP 
               3' data Source to Target   ^      ^          
   ^                                     /       |
   |                                    /        |
   |                                   /         4
   |                                  /    Create constraints 
                                     /     Create indexes
   3''                              /      Create triggers
DML on source table                /
updates/deletes/inserts         5 /Finish redifinition
stored into MLOG$_EMP       ------
 
Step 1: Verify that the source table is able to undergo an ONLINE 
        redefinition using the dbms_redefinition.can_redef_table
        procedure.
 
Step 2: Create an empty interim table reflecting the final 
        structure 
 
Step 3: Start the redefinition of the source table using the 
        dbms_redefinition.start_redef_table procedure, defining: 
         -> the source table to be reorganized
         -> the interim table
         -> the columns mapping for each column undergoing a change
         
        The start_redef_table procedure automatically: 
         -> inserts all rows from the source table into the 
            interim table 
         -> creates a snapshot table MLOG$_EMP and a snapshot log
            to store DML changes temporarily until the final step
            of the redefinition
 
Step 4: Create the constraints, indexes and triggers on the interim
        table as they are defined on the source table.
        Any referential constraint involving the interim table
        should be created disabled.
 
Step 5: Finish the redefinition of the source table using the 
        dbms_redefinition.finish_redef_table procedure.
        The finish_redef_table procedure automatically:       
         -> applies all DML changes stored in the snapshot table
            into the interim table 
         -> exchanges the names of the 2 tables:
            the interim table becomes the source table and vice versa
 
        Be aware that the names of the constraints, indexes, and 
        triggers do not have the names they had on the source table.
 
Step 6: Drop the interim table
 
Step 7: From RDBMS version 9.2, the constraints and indexes can be 
        renamed with the ALTER TABLE ... RENAME CONSTRAINT ... 
        statement.
        
        In RDBMS version 8.x, and 9.0.x, if you want the restructured 
        table to have the initial names for the constraints and indexes, 
        you have to rebuild them, and for the triggers, you have
        to recreate them.
  
 
Other Useful Procedures
-----------------------
 
a. Intermediate synchronization
   ----------------------------
It is recommended to periodically synchronize the interim table with the source
one when a large amount of DML is executed on the table while the 
re-organization is taking place by calling the 
dbms_redefinition.sync_interim_table() procedure. 
Calling this procedure reduces the time taken by 
dbms_redefinition.finish_redef_table() to complete the re-organization process. 
 
The small amount of time that the original table is locked during 
dbms_redefinition.finish_redef_table() is independent of whether 
dbms_redefinition.sync_interim_table() has been called.
 
b. Abort and cleanup after errors
   ------------------------------
The user can call dbms_redefinition.abort_redef_table() in the event that an
error is raised during the re-organization process or the user wants to abort 
the re-organization process. This procedure allows to user to specify if the 
interim table should be dropped as part of the cleanup operations.
 
Example
-------
 
The example below re-organizes the EMP table (assumes use of primary key):
 
--> whose current structure is:
 
    SQL> desc emp
    Name                                  Null?    Type
    ------------------------------------- -------- ---------------
    EMPNO                                 NOT NULL NUMBER
    NAME                                           VARCHAR2(20)
    DEPTNO                                         NUMBER
 
--> with the following constraints:
 
    EMP_PK : a primary key with a UNIQUE index EMP_PK
    EMP_FK : a foreign key referencing the DEPT_PK (PK of the DEPT table)
 
--> with the following indexes:
 
    EMP_PK on the primary key column EMPNO
    I_EMP_DEPTNO non-unique index on column DEPTNO 
 
--> with the following trigger:
 
    T_EMP that increments a counter in the AUDIT_EMP table by 1 each time an
          INSERT/UPDATE/DELETE occurs on a row in the EMP table
 
--> The table contains 100 000 rows before the re-organization:
 
    SQL> select count(*) from emp;
 
      COUNT(*)
    ----------
        100000
 
--> The counter in the audit_emp table corresponds to the number
    of rows in the emp table:
 
    SQL> select * from audit_emp;
 
             C
    ----------
        100000
 
 
   ----------------------------------------------------------------
1. Determine if the table is a candidate for online re-organization 
   ----------------------------------------------------------------
   SQL> connect sys/x as sysdba
   Connected.
   SQL> grant execute on dbms_redefinition to test;
   Grant succeeded.
 
   SQL> connect test/test
   Connected.
 
   SQL> exec dbms_redefinition.can_redef_table('TEST', 'EMP');
   BEGIN dbms_redefinition.can_redef_table('TEST', 'EMP'); END;
   *
   ERROR at line 1:
   ORA-12089: cannot online redefine table "TEST"."EMP" with no primary key
   ORA-06512: at "SYS.DBMS_REDEFINITION", line 8
   ORA-06512: at "SYS.DBMS_REDEFINITION", line 247
   ORA-06512: at line 1
 
A primary key is mandatory since materialized views and logs are 
created during the start of redefinition.
 
   SQL> alter table emp add constraint emp_pk primary key(empno);
   Table altered.
 
   SQL> execute DBMS_REDEFINITION.CAN_REDEF_TABLE('TEST','EMP');
   PL/SQL procedure successfully completed.
 
 
   ------------------------------------------------------------
2. Create an empty interim table reflecting the final structure
   ------------------------------------------------------------
   For example, the EMP table undergoes the following transformations:
   * non partitioned --> partitioned
   * column NAME     --> LAST_NAME
   * a new colum SAL is added
 
   SQL> create table int_emp(empno number, last_name varchar2(20),
     2                       deptno number, sal number)
     3  partition by list (deptno)
     4  (partition p10 values (10),
     5   partition p20 values (20),
     6   partition p30 values (30),
     7   partition p40 values (40));
 
   SQL> select object_name,object_type,status,object_id,data_object_id
     2  from user_objects;
 
   OBJECT_NAME  OBJECT_TYPE        STATUS   OBJECT_ID DATA_OBJECT_ID
   ------------ ------------------ ------- ---------- --------------
   AUDIT_EMP    TABLE              VALID         5806           5806
   DEPT         TABLE              VALID         5793           5793
   DEPT_PK      INDEX              VALID         5794           5794
   EMP          TABLE              VALID         5803           5803
   EMP_PK       INDEX              VALID         5808           5808
   I_EMP_DEPTNO INDEX              VALID         5805           5805
   T_EMP        TRIGGER            VALID         5807
   INT_EMP      TABLE PARTITION    VALID         5831           5831
   INT_EMP      TABLE PARTITION    VALID         5832           5832
   INT_EMP      TABLE PARTITION    VALID         5833           5833
   INT_EMP      TABLE PARTITION    VALID         5834           5834
   INT_EMP      TABLE              VALID         5830
 
   ---------------------------------
3. Start the re-organization process 
   ---------------------------------
 
   SQL> execute SYS.DBMS_REDEFINITION.START_REDEF_TABLE('TEST', -
   >                                                  'EMP', -
   >                                                  'INT_EMP', -
   >                                               'name last_name');
   BEGIN SYS.DBMS_REDEFINITION.START_REDEF_TABLE('TEST',                          ;
 
   *
   ERROR at line 1:
   ORA-01031: insufficient privileges
   ORA-06512: at "SYS.DBMS_REDEFINITION", line 8
   ORA-06512: at "SYS.DBMS_REDEFINITION", line 146
   ORA-06512: at line 1
 
The user performing the re-organization requires the following 
privileges:
 
     * CREATE ANY TABLE 
     * ALTER ANY TABLE 
     * DROP ANY TABLE 
     * LOCK ANY TABLE 
     * SELECT ANY TABLE
     * CREATE ANY INDEX
     * CREATE ANY TRIGGER 
 
   SQL> execute DBMS_REDEFINITION.START_REDEF_TABLE('TEST', -
   >                                                'EMP', -
   >                                                'INT_EMP', -
   >              'empno empno, name last_name, deptno deptno');
 
   PL/SQL procedure successfully completed.
 
   SQL> select sql_text from v$sqlarea where sql_text like '%INT_EMP%';
 
   SQL_TEXT
   --------------------------------------------------------------------------------
   truncate table "TEST"."INT_EMP" purge snapshot log
 
   DELETE FROM "TEST"."INT_EMP" SNAP$ WHERE "EMPNO" = :1
 
   INSERT INTO "TEST"."INT_EMP"  ("EMPNO","LAST_NAME","DEPTNO") VALUES (:1,:2,:3)
 
   UPDATE "TEST"."INT_EMP" 
   SET "EMPNO" = :1,"LAST_NAME" = :2,"DEPTNO" = :3 WHERE "EMPNO" = :1
 
   INSERT INTO "TEST"."INT_EMP"("EMPNO","LAST_NAME","DEPTNO")
      SELECT "EMP"."EMPNO","EMP"."NAME","EMP"."DEPTNO" 
      FROM "TEST"."EMP" "EMP"
 
   INSERT /*+ APPEND */ INTO "TEST"."INT_EMP"
     ("EMPNO","LAST_NAME","DEPTNO")
      SELECT "EMP"."EMPNO","EMP"."NAME","EMP"."DEPTNO" 
      FROM "TEST"."EMP" "EMP"
 
   BEGIN DBMS_REDEFINITION.START_REDEF_TABLE('TEST','EMP','INT_EMP',
         'empno empno, name last_name, deptno deptno'); END;
 
 
   SQL> select object_name, object_type, status, object_id, data_object_id 
     2  from user_objects order by 4;
 
   OBJECT_NAME  OBJECT_TYPE        STATUS   OBJECT_ID DATA_OBJECT_ID
   ------------ ------------------ ------- ---------- --------------
   DEPT         TABLE              VALID         5793           5793
   DEPT_PK      INDEX              VALID         5794           5794
   EMP          TABLE              VALID         5803           5803
   I_EMP_DEPTNO INDEX              VALID         5805           5805
   AUDIT_EMP    TABLE              VALID         5806           5806
   T_EMP        TRIGGER            VALID         5807
   EMP_PK       INDEX              VALID         5808           5808
   INT_EMP      TABLE              VALID         5830
   INT_EMP      TABLE PARTITION    VALID         5831           5857
   INT_EMP      TABLE PARTITION    VALID         5832           5858
   INT_EMP      TABLE PARTITION    VALID         5833           5859
   INT_EMP      TABLE PARTITION    VALID         5834           5860
   MLOG$_EMP    TABLE              VALID         5855           5855
   RUPD$_EMP    TABLE              VALID         5856
 
   14 rows selected.
 
=> 2 tables are created: 
     --> a permanent table MLOG$_EMP which is a snapshot log on EMP
         to store all DML performed on EMP table once the 
         START_REDEF_TABLE is launched
     --> a temporary table RUPD$_EMP (of SESSION duration)
 
 
   SQL> select count(*) from int_emp;
 
     COUNT(*)
   ----------
       100000
 
   SQL> select count(*) from mlog$_emp;
 
     COUNT(*)
   ----------
            0
 
   SQL> select count(*) from rupd$_emp;
 
     COUNT(*)
   ----------
            0
 
   SQL> select count(*) from emp;
 
     COUNT(*)
   ----------
       100000
 
   SQL> select count(*) from audit_emp;
 
     COUNT(*)
   ----------
       100000
 
 
   SQL> select master,log_table from user_mview_logs;
 
   MASTER                         LOG_TABLE
   ------------------------------ ------------------------------
   EMP                            MLOG$_EMP
 
   SQL> select mview_name,container_name, build_mode
     2  from user_mviews;
 
   MVIEW_NAME            CONTAINER_NAME           BUILD_MOD
   --------------------- ------------------------ ---------
   INT_EMP               INT_EMP                  PREBUILT
 
   SQL> select query from user_mviews;
 
   QUERY
   -------------------------------------------------------------------
   select empno empno, name last_name, deptno deptno from "TEST"."EMP"
 
   ---------------------------------------
4. Create constraints on the interim table
   ---------------------------------------
 
   SQL> alter table int_emp 
     2  add constraint int_emp_pk primary key(empno);
   Table altered.
 
   SQL> alter table int_emp add constraint 
     2  int_emp_fk foreign key(deptno) references dept(deptno);
   Table altered.
 
   SQL> alter table int_emp MODIFY CONSTRAINT int_emp_fk 
     2  DISABLE KEEP INDEX;
 
   SQL> select constraint_name,constraint_type, status 
     2  from user_constraints;
 
   CONSTRAINT_NAME                C STATUS
   ------------------------------ - --------
   DEPT_PK                        P ENABLED
   EMP_PK                         P ENABLED
   EMP_FK                         R ENABLED
   INT_EMP_PK                     P ENABLED
   INT_EMP_FK                     R DISABLED
 
   ---------------------------------------
   Create triggers on the interim table
   ---------------------------------------
   We deliberately create a trigger which increments the C column
   by 2 instead of 1 to show that after the re-organization completion
   the appropriate triggge is used.
 
   SQL> create or replace trigger t_int_emp
     2  before insert or update or delete on int_emp
     3  for each row
     4  declare
     5   PRAGMA AUTONOMOUS_TRANSACTION;
     6  begin
     7    update audit_emp set c=c+2;
     8    commit;
     9  end;
    10  /
   Trigger created.
 
   SQL> select trigger_name, status from user_triggers;
 
   TRIGGER_NAME                   STATUS
   ------------------------------ --------
   T_EMP                          ENABLED
   T_INT_EMP                      ENABLED
 
   -----------------------------------------
5. Simulate DML activity on the source table
   -----------------------------------------
 
   SQL> delete emp where empno=1;
   1 row deleted.
 
   SQL> commit;
   Commit complete.
 
   SQL> select count(*) from emp;
 
     COUNT(*)
   ----------
        99999
 
   SQL> select count(*) from int_emp;
 
     COUNT(*)
   ----------
       100000
 
   SQL> select count(*) from mlog$_emp;
 
     COUNT(*)
   ----------
            1
 
   SQL> select * from audit_emp;
 
            C
   ----------
       100001
 
   ------------------------------------------------------
6. You can resync the Interim table with the Source table
   ------------------------------------------------------
 
   SQL> execute DBMS_REDEFINITION.SYNC_INTERIM_TABLE('TEST', -
   >                                      'EMP', 'INT_EMP');
 
   PL/SQL procedure successfully completed.
 
   SQL> select count(*) from emp;
 
     COUNT(*)
   ----------
        99999
 
   SQL> select count(*) from int_emp;
 
     COUNT(*)
   ----------
        99999
 
   SQL> select count(*) from mlog$_emp;
 
     COUNT(*)
   ----------
            0
 
   SQL> select * from audit_emp;
 
            C
   ----------
       100001
 
   -------------------------------------
7. Grant privileges on the interim table
   -------------------------------------
 
   SQL> select * from USER_TAB_PRIVS_MADE;
 
   GRANTEE    TABLE_NAME  GRANTOR   PRIVILEGE  GRA HIE                              
   ---------- ----------- --------- ---------- --- ---
   SCOTT      EMP         TEST      SELECT     NO  NO
   SCOTT      EMP         TEST      UPDATE     NO  NO
 
   SQL> grant select, update on INT_EMP to SCOTT;
   Grant succeeded.
 
   ----------------------------------
8. Finish the re-organization process
   ----------------------------------
 
    SQL> execute DBMS_REDEFINITION.FINISH_REDEF_TABLE('TEST',-
    >                                     'EMP', 'INT_EMP');
 
    PL/SQL procedure successfully completed.
 
    SQL> delete from emp where empno=2;
    1 row deleted.
 
    SQL> commit;
    Commit complete.
 
    SQL> select * from audit_emp;
 
             C 
    ----------
        100003
 
    The new trigger is used: 100001 is incremented by 2.
 
    SQL> desc emp
     Name                            Null?    Type
     ------------------------------- -------- ----------------------------
     EMPNO                           NOT NULL NUMBER
     LAST_NAME                                VARCHAR2(20)
     DEPTNO                                   NUMBER
     SAL                                      NUMBER
 
   ----------------------------------
9. Clean up
   ----------------------------------
 
    SQL> drop table int_emp;
    Table dropped.
 
-- If you receive an ORA-02449 error while trying to drop the interim table, then 
issue:
    SQL> drop table int_emp cascade constraints;
    Table dropped.
 
    ----------------------------------
10. Rename the constraints
    ----------------------------------
In RDBMS version 9.2, you can rename the constraints:
 
    SQL> alter table emp rename constraint INT_EMP_PK to EMP_PK ;                        
    Table altered.
 
    SQL>  alter table emp rename constraint INT_EMP_FK to EMP_FK;
    Table altered.
 
    SQL> select constraint_name, index_name, status from user_constraints;
 
    CONSTRAINT_NAME          INDEX_NAME                  STATUS
    ------------------------ --------------------------- --------
    EMP_PK                   INT_EMP_PK                  ENABLED
    EMP_FK                                               ENABLED
 
Be aware that the index associated to the primary key keeps the former name.    
Before RDBMS version 9.2, if you want to rename the constraints, drop and 
recreate them:
    SQL> alter table emp drop constraint int_emp_pk;
    Table altered.
 
    SQL> alter table emp add constraint emp_pk primary key (empno);
    Table altered.
 
    SQL> alter table emp drop constraint int_emp_fk;
    Table altered.
 
    SQL> alter table emp add constraint emp_fk       
      2  foreign key (deptno) references dept (deptno);
    Table altered.

Note
----
Once the START_REDEF_TABLE procedure is started, you can interrumpt the process 
using the following procedure:
SQL>  execute dbms_redefinition.abort_redef_table('TEST','EMP','INT_EMP');
PL/SQL procedure successfully completed.
 
Related Documents
-----------------
Oracle9i Database Administrator's Guide Chapter Managing Tables
Note 177408.1 ORA-12091: cannot online redefine table with materialized views
Note 139335.1 Drop View with Referencing Foreign Keys Requires CASCADE CONSTRAINTS Clause

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Muchas Gracias por este aporte!