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General
The Multi-Version Concurrency model is the single most important concept to understand about Oracle and how it works. The underlying mechanisms are the control files, system change numbers (SCN), and how Oracle utilizes ROLLBACK/UNDO segments.
First Available version 4.0
 
Demo

SQL*Plus Demo
CREATE TABLE mvcc_test AS
SELECT * FROM all_objects
WHERE SUBSTR(object_name,1,1) BETWEEN 'A' AND 'W';

SELECT COUNT(*) FROM mvcc_test;

variable x REFCURSOR

BEGIN
  OPEN :x FOR
  SELECT COUNT(*) FROM mvcc_test;
END;
/

DELETE FROM mvcc_test WHERE rownum < 20001;

COMMIT;

SELECT COUNT(*) FROM mvcc_test;

variable y REFCURSOR

BEGIN
  OPEN :y FOR
  SELECT COUNT(*) FROM mvcc_test;
END;
/

DELETE FROM mvcc_test;

COMMIT;

SELECT COUNT(*) FROM mvcc_test;

print x
print y

DROP TABLE mvcc_test PURGE;

CREATE TABLE mvcc_test AS
SELECT * FROM all_objects
WHERE SUBSTR(object_name,1,1) BETWEEN 'A' AND 'W';

SELECT COUNT(*) FROM mvcc_test;

variable z REFCURSOR

set timing on

BEGIN
  OPEN :z FOR
  SELECT * FROM mvcc_test;
END;
/

set timing off

DELETE FROM mvcc_test;

COMMIT;

SELECT COUNT(*) FROM mvcc_test;

print z
Try the same demo substituting TRUNCATE for DELETE and COMMIT
 
Related Topics
DBMS_FLASHBACK
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