Oracle Pseudocolumns ORA_ROWSCN ROWNUM ROWID


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Oracle Pseudocolumns

Version 11.1

General

Pseudocolumns are data associated with table data, as though columns, but not columns stored in the database. Information on the heirarchical pseudocolumns used with CONNECT BY are on the CONNECT BY page. Information on the NEXTVAL and CURRVAL pseudocolumns are on the SEQUENCES page. The XMLDATA pseudocolumn is documented on the XML Tables page.

COLUMN_VALUE

When referring to an XMLTable construct without the COLUMNS clause, or when using a TABLE function to refer to a scalar nested table type, the database returns a virtual table with a single column. This name of this pseudocolumn is COLUMN_VALUE.

COLUMN_VALUE

CREATE TYPE phone AS TABLE OF NUMBER;
/

CREATE TYPE phone_list AS TABLE OF phone;
/

SELECT t.COLUMN_VALUE
FROM TABLE(phone(1,2,3)) t;

SELECT t.COLUMN_VALUE FROM
TABLE(phone_list(phone(1,2,3))) p, TABLE(p.COLUMN_VALUE) t;
COLUMN_VALUE

SELECT t.COLUMN_VALUE
FROM TABLE(phone_list(phone(1,2,3))) p, TABLE(p.COLUMN_VALUE) t;

CREATE TABLE my_customers (
cust_id NUMBER,
name VARCHAR2(25),
phone_numbers phone_list,
credit_limit NUMBER)
NESTED TABLE phone_numbers STORE AS outer_ntab
(NESTED TABLE COLUMN_VALUE STORE AS inner_ntab);

OBJECT_ID

Returns the object identifier of a column of an object table or view. Oracle uses this pseudocolumn as the primary key of an object table.

OBJECT_ID

In earlier releases, this pseudocolumn was called SYS_NC_OID$.

CREATE OR REPLACE TYPE person_typ AS OBJECT (
ssn NUMBER(9), name VARCHAR2(30), address VARCHAR2(100))
NOT FINAL;
/

CREATE TABLE person_obj_table OF person_typ;

INSERT INTO person_obj_table
VALUES (person_typ(20, 'Bob Jones', '111-555-1212'));

set linesize 121
col address format a30

SELECT * FROM person_obj_table;

SELECT p.object_id, p.object_value FROM person_obj_table p;

OBJECT_VALUE

Returns system-generated names for the columns of an object table, XMLType table, object view, or XMLType view.

OBJECT_VALUE

In earlier releases, this pseudocolumn was called SYS_NC_ROWINFO$.

See OBJECT_ID demo above.

ORA_ROWSCN

ORA_ROWSCN returns, for each row, the conservative upper bound system change number (SCN) of the most recent change to the row. This pseudocolumn is useful for determining approximately when a row was last updated. It is not absolutely precise, because Oracle tracks SCNs by transaction committed for the block in which the row resides.

Warning: The mapping of SCN to times is kept in bitmaps in sys.smon_scn_time - this table normally has about 1440 rows (more or less are possible) representing 5 minute windows of time over 5 days - 5 days of uptime (so you you start your database for 1 hour every day and shutdown for the other 23 - you'll have 5*24 days of history in this table).

If you select the minimum scn from this table (mine was 26447476 - representing 5 days ago for me) you'll get an answer from scn-to-timestamp, if you go back just ONE scn from that - it fails:

sys%ORA10GR2> select scn_to_timestamp( 26447475 ) from DUAL;
select scn_to_timestamp( 26447475 ) from DUAL
*
ERROR at line 1:
ORA-08181: specified number is not a valid system change number
ORA-06512: at "SYS.SCN_TO_TIMESTAMP", line 1

sys%ORA10GR2> select scn_to_timestamp( 26447476 ) from DUAL;

SCN_TO_TIMESTAMP(26447476)
---------------------------------------------------------------------------
13-AUG-07 02.05.23.000000000 PM

sys%ORA10GR2> select systimestamp - scn_to_timestamp( 26447476 ) from DUAL;

SYSTIMESTAMP-SCN_TO_TIMESTAMP(26447476)
---------------------------------------------------------------------------
+000000005 01:00:23.881952000

that last bit just shows it is about 5 days in the past - my max...

Tom Kyte

In 11g this behavior changes when a Flashback Archive has been created as you can see from the following note from a member of the 11g development team.

The association between a system change number and a timestamp when the number is generated is remembered by the database for a limited period of time. This period is the maximum of the auto-tuned undo retention period, if the database runs in the Automatic Undo Management mode, and the retention times of all flashback archives in the database, but no less than 120 hours. The time for the association to become obsolete elapses only when the database is open. An error is returned if the SCN specified for the argument to SCN_TO_TIMESTAMP is too old.

ORA_ROWSCN

Precision is approximately 3 sec.

CREATE TABLE test (
testcol VARCHAR2(20))
ROWDEPENDENCIES;

SELECT table_name, dependencies
FROM user_tables;

SELECT current_scn
FROM v$database;

INSERT INTO test VALUES ('ABC');

COMMIT;

INSERT INTO test VALUES ('ABC');

COMMIT;

INSERT INTO test VALUES ('ABC');

COMMIT;

SELECT ORA_ROWSCN, rowid, testcol FROM test;

SELECT current_scn
FROM v$database;

UPDATE test
SET testcol = 'DEF'
WHERE rownum = 1;

SELECT ORA_ROWSCN, rowid, testcol FROM test;

COMMIT;

SELECT ORA_ROWSCN, rowid, testcol FROM test;

UPDATE test
SET testcol = 'XYZ';

SELECT ORA_ROWSCN, rowid, testcol FROM test;

COMMIT;

SELECT ORA_ROWSCN, rowid, testcol FROM test;

CREATE TABLE test2 AS
SELECT * FROM test;

COMMIT;

SELECT ORA_ROWSCN, rowid, testcol FROM test2;

INSERT INTO test VALUES ('ABC');
UPDATE test SET testcol = 'DEF' WHERE rownum = 1;
UPDATE test2 SET testcol = 'GHI' WHERE rownum = 1;

COMMIT;

SELECT ORA_ROWSCN, rowid, testcol FROM test;

SELECT ORA_ROWSCN, rowid, testcol FROM test2;

ORA_ROWSCN Granularity

CREATE TABLE t (
testcol NUMBER(10))
ROWDEPENDENCIES;

BEGIN
FOR i IN 1 .. 1000 LOOP
INSERT INTO t VALUES (i);
COMMIT;
user_lock.sleep(0.1);
END LOOP;
END;
/

SELECT ora_rowscn, testcol
FROM t
ORDER BY 2;

ROWID

ROWID is what is referred to a pseudo-column. It is not data in the database or table so much as it is a mapping of the location, in a specific datafile of the physical location of a row of data. Since rows can migrate from location-to-location when they are updated ROWID should never be stored an never be counted on to be the same in any database.

ROWID

CREATE TABLE organization (
organization_name VARCHAR2(20));

INSERT INTO organization
(organization_name)
VALUES
('AAAAA');

INSERT INTO organization
(organization_name)
VALUES
('BBBBB');

INSERT INTO organization
(organization_name)
VALUES
('CCCCC');

COMMIT;

SELECT ROWID, organization_name
FROM organization;

TRUNCATE TABLE organization;

Indexes are segments storing data as are tables. The data stored consists of the data from the columns defining the index and the ROWIDs that correspond with the data. The following creates an index and then dumps the index data

CREATE TABLE ...

CREATE INDEX ...

SELECT

oradebug setmypid
oradebug dump file_hdrs 10

or

ALTER SESSION SET EVENTS 'immediate trace name file_hdrs level 10';

Because ROWID points directly to the physical location of a row of data it is a faster way to affect a record it is faster to update or delete using ROWID than it is to perform a full table scan or look-up the ROWID in an index and then perform the same action.

CREATE TABLE test_table (
seqno        NUMBER(6),
data_element VARCHAR2(10));

ALTER TABLE test_table
ADD CONSTRAINT pk_test_table
PRIMARY KEY (seqno)
USING INDEX;

CREATE SEQUENCE seq;

BEGIN
FOR i IN 1..50000 LOOP
    INSERT INTO test_table
    (seqno, data_element)
    VALUES
    (seq.NEXTVAL, sys.dbms_crypto.randombytes(5));
END LOOP;
END;
/

set timing on

DECLARE
r test_table.seqno%TYPE;
BEGIN
FOR i IN 1 .. 50000 LOOP
    SELECT seqno
    INTO r
    FROM test_table
    WHERE seqno = i;

    UPDATE test_table
    SET data_element = 'AAAAA'
    WHERE seqno = i;
END LOOP;
COMMIT;
END;
/

DECLARE
r UROWID;
BEGIN
FOR i IN 1 .. 50000 LOOP
    SELECT rowid
    INTO r
    FROM test_table
    WHERE seqno = i;

    UPDATE test_table
    SET data_element = 'AAAAA'
    WHERE rowid = r;
END LOOP;
COMMIT;
END;
/

ROWNUM

ROWNUM

ROWNUM is what is referred to as a pseudo-column. It is not data in the database or table and has absolutely no relationship to anything in the database, data file, tablespace, or table or to the order in which a row is inserted into a table. Rather it is the number of a row selected from a table and will change depending on the order in which rows are selected.

CREATE TABLE organization (
organization_name VARCHAR2(20));

INSERT INTO organization
(organization_name)
VALUES
('AAAAA');

INSERT INTO organization
(organization_name)
VALUES
('BBBBB');

INSERT INTO organization
(organization_name)
VALUES
('CCCCC');

COMMIT;

SELECT ROWNUM, organization_name
FROM organization;

SELECT ROWNUM, organization_name
FROM organization
WHERE organization_name <> 'AAAAA';

Because the first row selected is ROWNUM 1, the second ROWNUM 2, trying to view row 2 with a simple use of ROWNUM will not work.

SELECT ROWNUM, organization_name
FROM organization;

SELECT organization_name
FROM organization
WHERE ROWNUM = 1;

SELECT organization_name
FROM organization
WHERE ROWNUM = 2;

SELECT organization_name
FROM organization
WHERE ROWNUM <= 2;

SELECT organization_name
FROM organization
WHERE ROWNUM < 3;

With an in-line view you can use ROWNUM to obtain records not beginning with the first record.

SELECT rnum, object_name
FROM (
   SELECT rownum rnum, object_name
   FROM all_objects
   WHERE ROWNUM < 100)
WHERE rnum BETWEEN 25 AND 40;

The inner row number is the order of the object names extracted by the query from all_objects. The outer row number is the numbering of the first ten rows selected from the in-line view.

SELECT inner_r num, object_name, rownum outer_rnum
FROM (
SELECT ROWNUM inner_r num, object_name
FROM all_objects
WHERE SUBSTR(object_name,1,1) BETWEEN 'A' AND 'W'
ORDER BY object_name)
WHERE inner_rnum < 10;

Version Query Pseudocolumns

versions_xid

The transaction id that created this version of the row

versions_startscn

The SCN in which this row version first occured

versions_endscn

The SCN in which this row version was changed

versions_starttime

The TIMESTAMP in which this row version first occured

versions_endtime

The TIMESTAMP in which this row version was changed

versions_operation

The action that created this version of the row (one of delete, insert, update)

Versions Demo

CREATE TABLE fb_test (
program_id VARCHAR2(3),
line_number NUMBER(10),
customer_id VARCHAR2(4),
order_date DATE)
ROWDEPENDENCIES;

desc fb_test

BEGIN
INSERT INTO fb_test VALUES ('747',1,'UAL',SYSDATE-10);
COMMIT;
INSERT INTO fb_test VALUES ('757',1,'UAL',SYSDATE-10);
COMMIT;
INSERT INTO fb_test VALUES ('767',1,'UAL',SYSDATE-10);
COMMIT;
INSERT INTO fb_test VALUES ('777',1,'UAL',SYSDATE-10);
COMMIT;
INSERT INTO fb_test VALUES ('787',1,'UAL',SYSDATE-10);
COMMIT;
END;
/

SELECT current_scn
FROM v$database;

SELECT *
FROM fb_test;

SELECT rownum, rowid, ora_rowscn, program_id, line_number
FROM fb_test;

SELECT versions_xid, versions_startscn, versions_endscn, program_id
FROM fb_test
VERSIONS BETWEEN SCN 7742944 AND 7742945;

SELECT versions_xid, versions_startscn, versions_endscn, program_id
FROM fb_test
VERSIONS BETWEEN SCN 7742944 AND MAXVALUE;

SELECT versions_xid, versions_startscn, versions_endscn, program_id
FROM fb_test
VERSIONS BETWEEN SCN MINVALUE AND MAXVALUE;

SELECT current_scn
FROM v$database;

UPDATE fb_test
SET order_date = order_date+20
WHERE program_id = 787;
COMMIT;

UPDATE fb_test
SET order_date = order_date+1
WHERE program_id = 787;
COMMIT;

SELECT *
FROM fb_test
VERSIONS BETWEEN SCN MINVALUE AND MAXVALUE
ORDER BY 1;

SELECT program_id, order_date
FROM fb_test
VERSIONS BETWEEN SCN MINVALUE AND MAXVALUE
WHERE program_id = '787';

set linesize 121

SELECT versions_xid, versions_startscn, versions_endscn,
versions_operation, program_id, line_number, customer_id,
order_date
FROM fb_test
VERSIONS BETWEEN SCN MINVALUE AND MAXVALUE
WHERE program_id = '787';

DROP TABLE fb_test PURGE;

CREATE TABLE fb_test (
program_id VARCHAR2(3),
line_number NUMBER(10),
customer_id VARCHAR2(4),
order_date DATE)
ROWDEPENDENCIES;

desc fb_test

BEGIN
INSERT INTO fb_test VALUES ('747',1,'UAL',SYSDATE-10);
COMMIT;
dbms_lock.sleep(5);
INSERT INTO fb_test VALUES ('757',1,'UAL',SYSDATE-10);
COMMIT;
dbms_lock.sleep(5);
INSERT INTO fb_test VALUES ('767',1,'UAL',SYSDATE-10);
COMMIT;
dbms_lock.sleep(5);
INSERT INTO fb_test VALUES ('777',1,'UAL',SYSDATE-10);
COMMIT;
dbms_lock.sleep(5);
INSERT INTO fb_test VALUES ('787',1,'UAL',SYSDATE-10);
COMMIT;
dbms_lock.sleep(5);
UPDATE fb_test
SET order_date = order_date+20
WHERE program_id = 787;
COMMIT;
dbms_lock.sleep(5);
UPDATE fb_test
SET order_date = order_date+1
WHERE program_id = 787;
COMMIT;
END;
/

SELECT *
FROM fb_test
VERSIONS BETWEEN TIMESTAMP MINVALUE AND MAXVALUE
ORDER BY 1;

col versions_starttime format a22
col versions_endtime format a22

SELECT versions_xid, versions_starttime, versions_endtime,
versions_operation, program_id, line_number, customer_id,
order_date
FROM fb_test
VERSIONS BETWEEN TIMESTAMP MINVALUE AND MAXVALUE
WHERE program_id = '787';

SELECT *
FROM fb_test
VERSIONS BETWEEN TIMESTAMP MINVALUE
AND TO_TIMESTAMP('14-JUN-05 08.35.55 PM')
WHERE program_id = '787';

Related Topics

Connect By

Flashback Query

Index

In-Line View

Miscellaneous Functions

Sequences

XML Tables

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