Note: Deletes perform normal DML.
That is, they take locks on rows, they generate redo (lots of it), and they require segments in the UNDO tablespace.
Deletes clear records out of blocks carefully. If a mistake is made a rollback can be issued to restore the records prior to a commit.
A delete does not relinquish segment space thus a table in which all records have been deleted retains all of its original blocks.
Truncates are DDL and, in a sense, cheat. A truncate moves the High Water Mark of the table back to zero.
No row-level locks are taken, no redo or rollback is generated.
All extents bar the initial are de-allocated from the table (if you have MINEXTENTS set to anything other than 1,
then that number of extents is retained rather than just the initial). By re-positioning the high water mark, they prevent reading of any table data,
so they have the same effect as a delete, but without all the overhead.
Just one slight problem: a truncate is a DDL command, so you can't roll it back if you decide you made a mistake.
(It's also true that you can't selectively truncate -no "WHERE" clause is permitted, unlike with deletes, of course).
By resetting the High Water Mark, the truncate prevents reading of any table's data, so they it has the same effect as a delete, but without the overhead.
There is, however, one aspect of a Truncate that must be kept in mind. Because a Truncate is DDL it issues a COMMIT before it acts and another COMMIT
afterward so no rollback of the transaction is possible. |
| |
| Truncate Table |
| Simple Truncate |
TRUNCATE TABLE <table_name> [<PRESERVE | PURGE>]
[<DROP [ALL] | REUSE> STORAGE];
By default drops storage even if DROP STORAGE is not specified.
The PURGE option is for truncating materialized views and purges the log. |
CREATE TABLE test AS
SELECT * FROM all_objects;
SELECT COUNT(*) FROM test;
TRUNCATE TABLE test;
or more explicitly
TRUNCATE TABLE test DROP ALL STORAGE;
SELECT COUNT(*) FROM test; |
| Empty Table And Move Highwater Mark |
CREATE TABLE test (
testcol VARCHAR2(20));
BEGIN
FOR i IN 1..10000
LOOP
INSERT INTO test
(testcol)
VALUES
('ABCDEFGHIJKLMNOPQRST');
END LOOP;
COMMIT;
END;
/
exec dbms_stats.gather_table_stats(OWNNAME=>'UWCLASS', TABNAME=>'TEST');
SELECT SUM(blocks)
FROM user_extents
WHERE segment_name = 'TEST';
TRUNCATE TABLE test;
SELECT SUM(blocks)
FROM user_extents
WHERE segment_name = 'TEST'; |
| Empty Table And Do Not Move Highwater Mark |
TRUNCATE TABLE <table_name>
REUSE STORAGE; |
BEGIN
FOR i IN 1..10000
LOOP
INSERT INTO test
(testcol)
VALUES
('ABCDEFGHIJKLMNOPQRST');
END LOOP;
COMMIT;
END;
/
exec dbms_stats.gather_table_stats(OWNNAME=>'UWCLASS', TABNAME=>'TEST');
SELECT SUM(blocks)
FROM user_extents
WHERE segment_name = 'TEST';
TRUNCATE TABLE test REUSE STORAGE;
SELECT SUM(blocks)
FROM user_extents
WHERE segment_name = 'TEST'; |
| |
| Truncate Partitions & Subpartitions |
| Truncate A Single Partition In A Partitioned Table |
ALTER TABLE <table_name>
TRUNCATE PARTITION <partition_name>; |
CREATE TABLE parttab (
state VARCHAR2(2),
sales NUMBER(10,2))
PARTITION BY LIST (state) (
PARTITION northwest VALUES ('OR', 'WA') TABLESPACE uwdata,
PARTITION southwest VALUES ('AZ', 'CA') TABLESPACE uwdata);
INSERT INTO parttab VALUES ('OR', 100000);
INSERT INTO parttab VALUES ('WA', 200000);
INSERT INTO parttab VALUES ('AZ', 300000);
INSERT INTO parttab VALUES ('CA', 400000);
COMMIT;
SELECT * FROM parttab PARTITION(northwest);
SELECT * FROM parttab PARTITION(southwest);
ALTER TABLE parttab
TRUNCATE PARTITION southwest;
SELECT * FROM parttab PARTITION(northwest);
SELECT * FROM parttab PARTITION(southwest); |
| Truncate A Subpartition In A Composite Partitioned Table |
ALTER TABLE <table_name>
TRUNCATE PARTITION <partition_name>; |
CREATE TABLE demo_list_list (
cust_id NUMBER(5),
dotw NUMBER(1),
testdata VARCHAR2(20))
PARTITION BY LIST(cust_id)
SUBPARTITION BY LIST (dotw)
SUBPARTITION TEMPLATE(
SUBPARTITION sunday VALUES (1),
SUBPARTITION monday VALUES(2),
SUBPARTITION tuesday VALUES(3),
SUBPARTITION wednesday VALUES(4),
SUBPARTITION thursday VALUES(5),
SUBPARTITION friday VALUES(6),
SUBPARTITION saturday VALUES (7)) (
PARTITION airbus VALUES (2),
PARTITION boeing VALUES (1));
INSERT INTO demo_list_list VALUES (1, 1, 'Test Data1');
INSERT INTO demo_list_list VALUES (2, 1, 'Test Data2');
INSERT INTO demo_list_list VALUES (1, 1, 'Test Data3');
INSERT INTO demo_list_list VALUES (2, 2, 'Test Data4');
INSERT INTO demo_list_list VALUES (1, 1, 'Test Data5');
col high_value format a50
select partition_name, subpartition_name, high_value
from user_tab_subpartitions
where table_name = 'DEMO_LIST_LIST';
SELECT COUNT(*) FROM demo_list_list SUBPARTITION(BOEING_SUNDAY);
ALTER TABLE demo_list_list TRUNCATE SUBPARTITION boeing_sunday;
SELECT COUNT(*) FROM demo_list_list SUBPARTITION(BOEING_SUNDAY); |
| |
| Truncate Cluster |
| Demo with Hash and Index Clusters |
TRUNCATE CLUSTER <schema_name.cluster_name> [<DROP [ALL] | REUSE> STORAGE]; |
CREATE CLUSTER uw_cluster (
rid NUMBER(10))
PCTFREE 0
TABLESPACE uwdata
HASHKEYS 5
ROWDEPENDENCIES;
CREATE TABLE cservers (
srvr_id NUMBER(10),
network_id NUMBER(10))
CLUSTER uw_cluster (srvr_id);
CREATE TABLE cserv_inst (
custacct_id VARCHAR2(10),
srvr_id NUMBER(10),
ws_id NUMBER(10))
CLUSTER uw_cluster (srvr_id);
INSERT INTO cservers (srvr_id, network_id) VALUES (1,2);
INSERT INTO cserv_inst (custacct_id, srvr_id, ws_id) VALUES ('TEST', 1, 4);
-- this does not work
TRUNCATE CLUSTER uw_cluster;
DROP CLUSTER uw_cluster INCLUDING TABLES;
CREATE CLUSTER uw_cluster (
srvr_id NUMBER(10))
SIZE 1024;
CREATE TABLE cservers (
srvr_id NUMBER(10),
network_id NUMBER(10))
CLUSTER uw_cluster (srvr_id);
CREATE TABLE cserv_inst (
custacct_id VARCHAR2(10),
srvr_id NUMBER(10),
ws_id NUMBER(10))
CLUSTER uw_cluster (srvr_id);
CREATE INDEX idx_sc_srvr_id ON CLUSTER uw_cluster;
INSERT INTO cservers (srvr_id, network_id) VALUES (1,2);
INSERT INTO cserv_inst (custacct_id, srvr_id, ws_id) VALUES ('TEST', 1, 4);
TRUNCATE CLUSTER uw_cluster; |
| |
| Truncate Materialized View Log |
|
Truncating materialized view and materialized view logs. Read the docs before doing this as there are restrictions on what can and can not be preserved. |
TRUNCATE TABLE <table_name> [<PRESERVE | PURGE>] [MATERIALIZED VIEW LOG]
[<DROP [ALL] | REUSE> STORAGE]; |
conn uwclass/uwclass
CREATE MATERIALIZED VIEW LOG ON servers
PCTFREE 0
PCTUSED 99
TABLESPACE uwdata
WITH PRIMARY KEY, ROWID;
CREATE MATERIALIZED VIEW mv_simple
TABLESPACE uwdata
BUILD IMMEDIATE
REFRESH FAST ON COMMIT AS
SELECT *
FROM servers;
SELECT COUNT(*) FROM mv_simple;
INSERT INTO servers (srvr_id) VALUES (9999);
COMMIT;
SELECT COUNT(*) FROM mv_simple;
TRUNCATE TABLE mv_simple PRESERVE MATERIALIZED VIEW LOG; |
| |
| Truncate In Other Schemas |
| This procedure was provided to the library by JP Vijaykumar |
/*
In applications, users need truncate privileges on third party tables. The owner of the table can truncate his/her own tables.
A user with DBA role granted can truncate a third party table. Any user with "drop any table" system privilege can truncate a third party table.
It is not recommended in Production environments to grant powerful roles like DBA role or privileges like "drop any table" privilege to accomplish this purpose.
This procedure need to be created in each schema. The schema owner need to grant execute permission to the user, and the delete permission on specified tables.
Then the user can execute the procedure to truncate tables User A owns the table temp_jp. */
conn uwclass/uwclass
CREATE TABLE t1 AS
SELECT * FROM all_tables;
CREATE OR REPLACE PROCEDURE trunc_tab(p_tabname IN VARCHAR2) AUTHID DEFINER AS
/**********************************
AUTHOR JP Vijaykumar
ORACLE DBA
**********************************/
v_num NUMBER(10):=0;
v_owner VARCHAR2(30);
v_user VARCHAR2(30);
sql_stmt VARCHAR2(2000);
BEGIN
SELECT username
INTO v_owner
FROM user_users;
SELECT sys_context('USERENV','SESSION_USER')
INTO v_user
FROM dual;
sql_stmt:='TRUNCATE TABLE ' || v_owner || '.' || p_tabname;
IF (v_owner = v_user) THEN
execute immediate sql_stmt;
ELSE
SELECT COUNT(*)
INTO v_num
FROM all_tab_privs
WHERE table_name = UPPER(p_tabname)
AND table_schema = v_owner
AND grantee = v_user
AND privilege in 'DELETE';
IF (v_num > 0) THEN
execute immediate sql_stmt;
ELSE
RAISE_APPLICATION_ERROR(-20001,'Insufficient privileges.');
END IF;
END IF;
EXCEPTION
WHEN OTHERS THEN
RAISE_APPLICATION_ERROR(-20001,'Insufficient privileges');
END trunc_tab;
/
GRANT execute ON t1 TO abc;
conn abc/abc
exec uwclass.trunc_tab('T1');
conn uwclass/uwclass
GRANT delete ON t1 TO abc;
conn abc/abc
exec uwclass.trunc_tab('T1');
conn uwclass/uwclass
SELECT * FROM t1; |
