Monthly Archives: March 2014

Oracle12c : Truncate Table Cascade

Prior to Oracle 12c truncating tables with dependent children was a  pain. We had to disable the constraint before truncating the parent table followed  by enabling the constraint.  The performance gain achieved using TRUNCATE instead of DELETE  almost disappeared if the constraints were enabled with VALIDATE option.  If enabling the constraint with NOVALIDATE option was not acceptable, then DELETE seemed  to be the only way to go.

Starting with Oracle 12c , Oracle supports truncating tables with CASCADE option similar to other RDBMS like MySQL & PostgreSQL. Well  It took some time for Oracle to introduce this option but glad we finally have this option.

Similar to DELETE , the constraints have to be defined with ON DELETE CASCADE. Otherwise just like  DELETE , TRUNCATE also will fail. Also the cascading effect will impact all children , grand children and so on.

Let us look at an example. See scripts to create the table towards end of this blog.

In t his example we have 3 tables DEPT (PARENT) , EMP(CHILD) & EMPPROJ (GRANDCHILD) . The child/grandchild tables are created without the “ON DELETE CASCADE” , So obviously the TRUNCATE command with CASCADE option will fail. Please do note the difference in the error message for with and without CASCADE option.

SQL> truncate table emp;

truncate table emp
*
ERROR at line 1:
ORA-02266: unique/primary keys in table referenced by enabled foreign keys
SQL> truncate table emp cascade ;
truncate table emp cascade
*
ERROR at line 1:
ORA-14705: unique or primary keys referenced by enabled foreign keys in table
“SHAN”.”EMPPROJ”

 

Now lets go ahead modify the table to add the ON DELETE CASCADE option; To do this, we have to drop existing constraint and add the constraint with ON DELETE CASCADE option.

 

ALTER TABLE EMP DROP CONSTRAINT fk_deptno_dept ;

ALTER TABLE EMPPROJ DROP CONSTRAINT fk_empno_emp ;

ALTER TABLE EMP ADD CONSTRAINT fk_deptno_dept FOREIGN KEY (DEPTNO) REFERENCES dept(deptno) ON DELETE CASCADE;

ALTER TABLE EMPPROJ ADD CONSTRAINT fk_empno_emp foreign key (empno) REFERENCES emp(empno) ON DELETE CASCADE;

Before we truncate the table, lets take the count of emp & empproj table.

SQL>

SQL> select count(*) from emp;

COUNT(*)
———-
5

SQL> select count(*) from empproj;

COUNT(*)
———-
5

 

Now let us truncate the table with and without t he CASACADE option and then take a count of emp and empproj tables. The key thing to remember is that the CASCADE option will truncate all  data from empproj when emp table is truncated because of child & grandchild option.

SQL> truncate table emp;

truncate table emp
*
ERROR at line 1:
ORA-02266: unique/primary keys in table referenced by enabled foreign keys
SQL> truncate table emp cascade;

Table truncated.

SQL> select count(*) from emp;

COUNT(*)
———-
0

SQL> select count(*) from empproj;

COUNT(*)
———-
0

 

x==x==x==x==x==x==x==x==x==x==x=x=x==x==x==x==x==x==x==x==x==x==x==x=x=x==x

 

Scripts to create the tables 
create table dept
(
deptno number(4) constraint pk_dept primary key,
dname varchar2(20) not null,
loc varchar2(30)
);

 

create table emp
(
empno number(4) constraint pk_emp primary key,
ename varchar2(20) not null,
salary number(10,2) ,
deptno number(4) constraint fk_deptno_dept references dept(deptno)
);

 

create table empproj
(
empno number(4) ,
proj varchar2(20) not null,
hours number(4) ,
constraint pk_empproj primary key (empno, proj),
constraint fk_empno_emp foreign key (empno) references emp(empno)
);

INSERT INTO DEPT VALUES (10,’FINANCE’,’RESTON’);
INSERT INTO DEPT VALUES (20,’ACCOUNTING’,’CHICAGO’);
INSERT INTO DEPT VALUES (30,’RESEARCH’,’DALLAS’);

INSERT INTO EMP VALUES ( 1001,’TIM’, 5800, 10);
INSERT INTO EMP VALUES ( 1002,’MIKE’, 5800, 30);
INSERT INTO EMP VALUES ( 1003,’SAM’, 5800, 30);
INSERT INTO EMP VALUES ( 1004,’JOE’, 5800, 20);
INSERT INTO EMP VALUES ( 1005,’SEAN’, 5800, 10);
INSERT INTO EMPPROJ VALUES (1001,’RED’,20);
INSERT INTO EMPPROJ VALUES (1001,’PINK’,30);
INSERT INTO EMPPROJ VALUES (1001,’BLUE’,10);
INSERT INTO EMPPROJ VALUES (1002,’PINK’,10);
INSERT INTO EMPPROJ VALUES (1002,’BLUE’,40);

 

 

 

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Oracle 12c RMAN New Features

In Oracle 12c , there are lot of new great RMAN features  focused on reducing recovery time; Some of them to provide better DBA 🙂 experience too .Let us  glance at some of them in this blog.

  1. Support for point in time recovery for tables  and partitions. I would rate this as one of best options of Oracle 12c.
  2. Option to duplicate database with NOOPEN option so that duplicated/cloned database remains in mount state. Prior to 12c, the cloned database is automatically opened in RESETLOGS mode. The NOOPEN option is very useful when you want to clone a database  as part of upgrade.
  3. In Oracle 12c , duplicate database supports pull based restore from backup sets; Prior to Oracle 12c , the support was only for push based restore from image copies.
  4. Support for recovering the database from snapshots taken using 3rd party vendors like EMC,Hitachi etc. The recover command now supports SNAPSHOT  TIME clause to implement this feature.
  5. Transporting database across platforms using backupsets. Prior to 12c, only image copies was supported.
  6. Transporting database across platforms using inconsistent backups i.e.. backups taken without putting the database in READONLY mode. New clause ALLOW INCONSISTENT is introduced to implement this.
  7. Support for execution of SQL commands from RMAN prompt without SQL prefix.
  8. Support for SQL*Plus command DESC  from RMAN prompt.