Oracle MultiVersion Concurrency Control (MVCC)
Version 19.2.0.1

General Information
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For how many years have you been working with physical servers that are starving your database of the memory necessary to deploy important new performance features such as the Result Cache, Memoptimize Pool, In-Memory Aggregation, In-Memory Column Store, and Full Database Caching? Too long? Contact me to learn how to improve all queries ... not just some queries.
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
 
SQL*Plus Demo
conn uwclass/uwclass@pdbdev

CREATE TABLE mvcc_test AS
SELECT * FROM all_objects;

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;

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 above demo substituting TRUNCATE for DELETE and COMMIT. What is the difference?

Related Topics
DBMS_FLASHBACK
DBMS_TRANSACTION
Flashback Query
Transactions
What's New In 18cR3
What's New In 19cR2

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