This presentation focuses on how transactions affect read operations, how isolations levels work with locking and MVCC.

1. © 2016 by Markus Winand
Transactions
What they are good for and how they work

2. ( )
Transactions are Brackets
Starting a transaction:
Explicit:
START TRANSACTION
Implicit:
INSERT / UPDATE /
DELETE
SELECT
CREATE
Terminating a transaction:
Explicit:
COMMIT
ROLLBACK
Implementation defined:
Due to errors
CREATE
…
Varies
amongst
products

3. Transactions have Characteristics
Access-Mode:
READ_ONLY
READ_WRITE
Constraint Mode:
IMMEDIATE
DEFERRED
Transaction-Isolation-Level:
READ UNCOMMITTED
READ COMMITTED
REPEATABLE READ
SERIALIZABLE
Diagnostic Size:
Archaic

4. Transactions have Characteristics
Access-Mode:
READ_ONLY
READ_WRITE
Constraint Mode:
IMMEDIATE
DEFERRED
Transaction-Isolation-Level:
READ UNCOMMITTED
READ COMMITTED
REPEATABLE READ
SERIALIZABLE
Diagnostic Size:
Archaic
Not strictly a
transaction
characteristic

5. Setting Transaction Characteristics
With explicit transaction begin:
START TRANSACTION ISOLATION LEVEL SERIALIZABLE READ WRITE
For next (implicitly started) transaction:
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE READ WRITE
The standard uses SERIALIZEABLE as default,
but most implementations default to READ COMMITTED.
In practice: Use an API if possible.

6. Use Cases for Transactions

7. Use Cases for Transactions
When writing

8. Use Cases for Transactions
Backing out incomplete changes…
… by the program with rollback
… by the RDBMS in case of crash
When writing

9. Use Cases for Transactions
Backing out incomplete changes…
… by the program with rollback
… by the RDBMS in case of crash
When writing
(Boring)

10. Use Cases for Transactions
Backing out incomplete changes…
… by the program with rollback
… by the RDBMS in case of crash
When writing When reading
(Boring)

11. Use Cases for Transactions
Backing out incomplete changes…
… by the program with rollback
… by the RDBMS in case of crash
When writing When reading
Simplify concurrent programs…
… by utilising the right
transaction isolation level
(Boring)

12. Use Cases for Transactions
Backing out incomplete changes…
… by the program with rollback
… by the RDBMS in case of crash
When writing When reading
Simplify concurrent programs…
… by utilising the right
transaction isolation level
(Boring) Amazing!

13. Standard Transaction Isolation Levels
Phenomena covered by the Standard
Dirty Read
Non-Repeatable
Read
Phantom Read
READ UNCOMMITTED
READ COMMITTED
REPEATABLE READ
SERIALIZABLE
STOP
STOP STOP
STOPSTOPSTOP

14. Standard Transaction Isolation Levels
Phenomena covered by the Standard
Dirty Read
Non-Repeatable
Read
Phantom Read
READ UNCOMMITTED
READ COMMITTED
REPEATABLE READ
SERIALIZABLE
STOP
STOP STOP
STOPSTOPSTOP
Incomplete,
unfortunately

15. How Comes?
SQL is a declarative language:
the standard doesn’t say how to implement it,
it just describes the effects it has.
Unfortunately, SQL-92 was written with locking in mind,
and thus only describes the effects in the granularity
in which they appear whey implemented using locking.
Although a commonly known issue, it was never changed.
https://en.wikipedia.org/wiki/Snapshot_isolation

16. Transaction Isolation Using Locks
Phenomena covered by the Standard
Dirty Read
Non-Repeatable
Read
Phantom Read
READ UNCOMMITTED
READ COMMITTED
REPEATABLE READ
SERIALIZABLE
▶ Row (short) ▶ Row (TX) ▶ Row (TX)
▶ Predicate (TX)
STOP
STOP STOP
STOPSTOPSTOP

17. Transaction Isolation Using Locks
Phenomena covered by the Standard
Dirty Read
Non-Repeatable
Read
Phantom Read
READ UNCOMMITTED
READ COMMITTED
REPEATABLE READ
SERIALIZABLE
▶ Row (short) ▶ Row (TX) ▶ Row (TX)
▶ Predicate (TX)
STOP
STOP STOP
STOPSTOPSTOP
Locks preventing
phenomena:
What (how long)

18. Transaction Isolation Using Locks
Phenomena covered by the Standard
Dirty Read
Non-Repeatable
Read
Phantom Read
READ UNCOMMITTED
READ COMMITTED
REPEATABLE READ
SERIALIZABLE
▶ Row (short) ▶ Row (TX) ▶ Row (TX)
▶ Predicate (TX)
STOP
STOP STOP
STOPSTOPSTOP
Locks preventing
phenomena:
What (how long)

19. Transaction Isolation Using Locks
Phenomena covered by the Standard
Dirty Read
Non-Repeatable
Read
Phantom Read
READ UNCOMMITTED
READ COMMITTED
REPEATABLE READ
SERIALIZABLE
▶ Row (short) ▶ Row (TX) ▶ Row (TX)
▶ Predicate (TX)
STOP
STOP STOP
STOPSTOPSTOP
Locks preventing
phenomena:
What (how long)

20. Transaction Isolation Using Locks
Phenomena covered by the Standard
Dirty Read
Non-Repeatable
Read
Phantom Read
READ UNCOMMITTED
READ COMMITTED
REPEATABLE READ
SERIALIZABLE
▶ Row (short) ▶ Row (TX) ▶ Row (TX)
▶ Predicate (TX)
STOP
STOP STOP
STOPSTOPSTOP
Locks preventing
phenomena:
What (how long)

21. Example: Write Skew
Make sure the sum of two rows is positive
(e.g., two bank accounts belonging to the same person)
Code to withdraw from one account
(naive schema and not considering concurrency)
UPDATE accounts
SET balance = balance - 200
WHERE account = 1
SELECT SUM(balance)
FROM accounts
WHERE account IN (1,2)
>= 0 commit
<0 rollback

22. Examples: Disclaimer
The following examples just demonstrate
one possible way two transactions could interact.
The examples are by no means exhaustive.

23. Write Skew in Lock-based READ UNCOMMITTED
Account Balance
1 100
2 100
Transaction 1 Transaction 2

24. Write Skew in Lock-based READ UNCOMMITTED
Account Balance
1 100
2 100
Transaction 1 Transaction 2
▶ 1 -100
2 100
UPDATE accounts
SET balance=balance-200
WHERE account=1

25. Write Skew in Lock-based READ UNCOMMITTED
Account Balance
1 100
2 100
Transaction 1 Transaction 2
▶ 1 -100
2 100
UPDATE accounts
SET balance=balance-200
WHERE account=1
▶ 1 -100
2 -100 ◀
UPDATE accounts
SET balance=balance-200
WHERE account=2

26. Write Skew in Lock-based READ UNCOMMITTED
Account Balance
1 100
2 100
Transaction 1 Transaction 2
▶ 1 -100
2 100
UPDATE accounts
SET balance=balance-200
WHERE account=1
▶ 1 -100
2 -100 ◀
UPDATE accounts
SET balance=balance-200
WHERE account=2
▶ 1 -100
2 -100 ◀
SELECT sum(balance)
FROM accounts
WHERE account IN (1,2)

27. Write Skew in Lock-based READ UNCOMMITTED
Account Balance
1 100
2 100
Transaction 1 Transaction 2
▶ 1 -100
2 100
UPDATE accounts
SET balance=balance-200
WHERE account=1
▶ 1 -100
2 -100 ◀
UPDATE accounts
SET balance=balance-200
WHERE account=2
▶ 1 -100
2 -100 ◀
SELECT sum(balance)
FROM accounts
WHERE account IN (1,2)
-200

28. Write Skew in Lock-based READ UNCOMMITTED
Account Balance
1 100
2 100
Transaction 1 Transaction 2
▶ 1 -100
2 100
UPDATE accounts
SET balance=balance-200
WHERE account=1
▶ 1 -100
2 -100 ◀
UPDATE accounts
SET balance=balance-200
WHERE account=2
▶ 1 -100
2 -100 ◀
SELECT sum(balance)
FROM accounts
WHERE account IN (1,2)
▶ 1 -100
2 -100 ◀
SELECT sum(balance)
FROM accounts
WHERE account IN (1,2)
-200

29. Write Skew in Lock-based READ UNCOMMITTED
Account Balance
1 100
2 100
Transaction 1 Transaction 2
▶ 1 -100
2 100
UPDATE accounts
SET balance=balance-200
WHERE account=1
▶ 1 -100
2 -100 ◀
UPDATE accounts
SET balance=balance-200
WHERE account=2
▶ 1 -100
2 -100 ◀
SELECT sum(balance)
FROM accounts
WHERE account IN (1,2)
▶ 1 -100
2 -100 ◀
SELECT sum(balance)
FROM accounts
WHERE account IN (1,2)
-200
-200

30. Write Skew in Lock-based READ COMMITTED

31. Write Skew in Lock-based READ COMMITTED
Account Balance
1 100
2 100
Transaction 1 Transaction 2

32. Write Skew in Lock-based READ COMMITTED
Account Balance
1 100
2 100
Transaction 1 Transaction 2
▶ 1 -100
2 100
UPDATE accounts
SET balance=balance-200
WHERE account=1

33. Write Skew in Lock-based READ COMMITTED
Account Balance
1 100
2 100
Transaction 1 Transaction 2
▶ 1 -100
2 100
UPDATE accounts
SET balance=balance-200
WHERE account=1
▶ 1 -100
2 -100 ◀
UPDATE accounts
SET balance=balance-200
WHERE account=2

34. Write Skew in Lock-based READ COMMITTED
Account Balance
1 100
2 100
Transaction 1 Transaction 2
▶ 1 -100
2 100
UPDATE accounts
SET balance=balance-200
WHERE account=1
▶ 1 -100
2 -100 ◀
UPDATE accounts
SET balance=balance-200
WHERE account=2
▶ 1 -100
2 -100 ◀
SELECT sum(balance)
FROM accounts
WHERE account IN (1,2)
Blocked
▶

35. Write Skew in Lock-based READ COMMITTED
Account Balance
1 100
2 100
Transaction 1 Transaction 2
▶ 1 -100
2 100
UPDATE accounts
SET balance=balance-200
WHERE account=1
▶ 1 -100
2 -100 ◀
UPDATE accounts
SET balance=balance-200
WHERE account=2
▶ 1 -100
2 -100 ◀
SELECT sum(balance)
FROM accounts
WHERE account IN (1,2)
Blocked
Dead lock
▶
◀▶ 1 -100
2 -100 ◀
SELECT sum(balance)
FROM accounts
WHERE account IN (1,2)▶

36. First Write, then Read: Conclusion
For this code, READ COMMITTED is enough
to make one of the transactions succeed
(assuming a proper deadlock detection).

37. First Write, then Read: Conclusion
For this code, READ COMMITTED is enough
to make one of the transactions succeed
(assuming a proper deadlock detection).
Exercise: Does this hold true in these cases too?

38. First Write, then Read: Conclusion
For this code, READ COMMITTED is enough
to make one of the transactions succeed
(assuming a proper deadlock detection).
Exercise: Does this hold true in these cases too?
SELECT sum(balance)
FROM accounts
WHERE account IN (1,2)
(iff >= 200)
UPDATE accounts
SET balance = balance - 200
WHERE account = :a

39. First Write, then Read: Conclusion
For this code, READ COMMITTED is enough
to make one of the transactions succeed
(assuming a proper deadlock detection).
Exercise: Does this hold true in these cases too?
SELECT sum(balance)
FROM accounts
WHERE account IN (1,2)
(iff >= 200)
UPDATE accounts
SET balance = balance - 200
WHERE account = :a
SELECT balance FROM accounts
WHERE account = :a
UPDATE accounts SET balance = :b
WHERE account = :a
SELECT sum(balance)
FROM accounts
WHERE account IN (1,2)

40. Write-Skew: General Case
When using lock-based isolation
REPEATABLE READ
covers with write-skew too.

41. What’s Bad About Locking?

42. What’s Bad About Locking?
Writers will always need to block writers anyway

43. What’s Bad About Locking?
Writers will always need to block writers anyway
Readers never block readers

44. What’s Bad About Locking?
Writers will always need to block writers anyway
Readers never block readers
Writers block readers,
readers block writers
(except in READ UNCOMMITTED)

45. What’s Bad About Locking?
Writers will always need to block writers anyway
Readers never block readers
Writers block readers,
readers block writers
(except in READ UNCOMMITTED)
Great!

46. What’s Bad About Locking?
Writers will always need to block writers anyway
Readers never block readers
Writers block readers,
readers block writers
(except in READ UNCOMMITTED)
Not so
good
Great!

47. Multi Version Concurrency Control (MVCC)

48. Multi Version Concurrency Control (MVCC)
Instead of waiting for writers,
just use the previous committed version of the data
(pretend the read happened before the write)

49. Multi Version Concurrency Control (MVCC)
Instead of waiting for writers,
just use the previous committed version of the data
(pretend the read happened before the write)
The reader effectively works on a snapshot

50. Multi Version Concurrency Control (MVCC)
Instead of waiting for writers,
just use the previous committed version of the data
(pretend the read happened before the write)
The reader effectively works on a snapshot
This prevents all three phenomena mentioned in the standard:
dirty read
non-repeatable read
phantom read

51. Write Skew When Using a Snapshot
Transaction 1 Transaction 2Account Balance
1 100
2 100

52. Write Skew When Using a Snapshot
Transaction 1 Transaction 2
1 -100
2 100
UPDATE accounts
SET balance=balance-200
WHERE account=1
Account Balance
1 100
2 100

53. Write Skew When Using a Snapshot
Transaction 1 Transaction 2
1 100
2 -100
UPDATE accounts
SET balance=balance-200
WHERE account=2
1 -100
2 100
UPDATE accounts
SET balance=balance-200
WHERE account=1
Account Balance
1 100
2 100

54. Write Skew When Using a Snapshot
Transaction 1 Transaction 2
1 100
2 -100
UPDATE accounts
SET balance=balance-200
WHERE account=2
1 -100
2 100
UPDATE accounts
SET balance=balance-200
WHERE account=1
Account Balance
1 100
2 100

55. Write Skew When Using a Snapshot
Transaction 1 Transaction 2
1 100
2 -100
UPDATE accounts
SET balance=balance-200
WHERE account=2
1 -100
2 100
UPDATE accounts
SET balance=balance-200
WHERE account=1
Account Balance
1 100
2 100

56. Write Skew When Using a Snapshot
Transaction 1 Transaction 2
1 100
2 -100
UPDATE accounts
SET balance=balance-200
WHERE account=2
1 -100
2 100
UPDATE accounts
SET balance=balance-200
WHERE account=1
Account Balance
1 100
2 100
SELECT sum(balance)
FROM accounts
WHERE account IN (1,2)
1 -100
2 100

57. Write Skew When Using a Snapshot
Transaction 1 Transaction 2
1 100
2 -100
UPDATE accounts
SET balance=balance-200
WHERE account=2
1 -100
2 100
UPDATE accounts
SET balance=balance-200
WHERE account=1
Account Balance
1 100
2 100
SELECT sum(balance)
FROM accounts
WHERE account IN (1,2)
1 -100
2 100
1 100
2 -100
SELECT sum(balance)
FROM accounts
WHERE account IN (1,2)

58. Write Skew When Using a Snapshot
Transaction 1 Transaction 2
1 100
2 -100
UPDATE accounts
SET balance=balance-200
WHERE account=2
Account Balance
1 -100
2 -100
1 -100
2 100
UPDATE accounts
SET balance=balance-200
WHERE account=1
Account Balance
1 100
2 100
SELECT sum(balance)
FROM accounts
WHERE account IN (1,2)
1 -100
2 100
1 100
2 -100
SELECT sum(balance)
FROM accounts
WHERE account IN (1,2)

59. Write Skew When Using a Snapshot
Transaction 1 Transaction 2
1 100
2 -100
UPDATE accounts
SET balance=balance-200
WHERE account=2
Account Balance
1 -100
2 -100
1 -100
2 100
UPDATE accounts
SET balance=balance-200
WHERE account=1
Account Balance
1 100
2 100
SELECT sum(balance)
FROM accounts
WHERE account IN (1,2)
1 -100
2 100
1 100
2 -100
SELECT sum(balance)
FROM accounts
WHERE account IN (1,2)

60. Can Snapshots and Serialisability Coexist?

61. Can Snapshots and Serialisability Coexist?
Simple snapshots cannot prevent write skew abnormalities.

62. Can Snapshots and Serialisability Coexist?
Simple snapshots cannot prevent write skew abnormalities.
But most transactions are
not vulnerable to write skew anyway.
(e.g., TPC-C is not vulnerable at all)

63. Can Snapshots and Serialisability Coexist?
Simple snapshots cannot prevent write skew abnormalities.
But most transactions are
not vulnerable to write skew anyway.
(e.g., TPC-C is not vulnerable at all)
The only two ways to prevent write skew
abnormalities at the database are
(1) putting locks when reading and
(2) proofing the serialization graph is acyclic

64. Can Snapshots and Serialisability Coexist?
Simple snapshots cannot prevent write skew abnormalities.
But most transactions are
not vulnerable to write skew anyway.
(e.g., TPC-C is not vulnerable at all)
The only two ways to prevent write skew
abnormalities at the database are
(1) putting locks when reading and
(2) proofing the serialization graph is acyclic
READ ONLY
transactions
are an important
special case

65. Can Snapshots and Serialisability Coexist?
Simple snapshots cannot prevent write skew abnormalities.
But most transactions are
not vulnerable to write skew anyway.
(e.g., TPC-C is not vulnerable at all)
The only two ways to prevent write skew
abnormalities at the database are
(1) putting locks when reading and
(2) proofing the serialization graph is acyclic
READ ONLY
transactions
are an important
special case
InnoDB
SQL Server
DB2

66. Can Snapshots and Serialisability Coexist?
Simple snapshots cannot prevent write skew abnormalities.
But most transactions are
not vulnerable to write skew anyway.
(e.g., TPC-C is not vulnerable at all)
The only two ways to prevent write skew
abnormalities at the database are
(1) putting locks when reading and
(2) proofing the serialization graph is acyclic
READ ONLY
transactions
are an important
special case
InnoDB
SQL Server
DB2 PostgreSQL

67. Can Snapshots and Serialisability Coexist?
Simple snapshots cannot prevent write skew abnormalities.
But most transactions are
not vulnerable to write skew anyway.
(e.g., TPC-C is not vulnerable at all)
The only two ways to prevent write skew
abnormalities at the database are
(1) putting locks when reading and
(2) proofing the serialization graph is acyclic
READ ONLY
transactions
are an important
special case
InnoDB
SQL Server
DB2 PostgreSQL
Oracle?

68. Can Snapshots and Serialisability Coexist?
Simple snapshots cannot prevent write skew abnormalities.
But most transactions are
not vulnerable to write skew anyway.
(e.g., TPC-C is not vulnerable at all)
The only two ways to prevent write skew
abnormalities at the database are
(1) putting locks when reading and
(2) proofing the serialization graph is acyclic
READ ONLY
transactions
are an important
special case
InnoDB
SQL Server
DB2 PostgreSQL

69. Can Snapshots and Serialisability Coexist?
PostgreSQL 9.1+ is the only(?) database that
uses MVCC and detects serialization graph cycles.
➜ Use SERIALIZABLE and you are done
In InnoDB (MySQL, MariaDB) and SQL Server, MVCC and Lock-Based SERIALIZABLE
isolation can be

70. How to use Snapshots and Prevent Write Skew
Transaction type
READ ONLY READ/WRITE
DB2 LUW
REPEATABLE READ
(=SERIALIZABLE)
REPEATABLE READ
(=SERIALIZALBE)
InnoDB
(MySQL, MariaDB)
REPEATABLE READ1 SERIALIZABLE
PostgreSQL 9.1+ SERIALIZABLE SERIALIZABLE
Oracle SERIALIZABLE SERIALIZABLE
SQL Server 2005+ SNAPSHOT SERIALIZABLE
1: MySQL’s REPEATABLE READ also protects against phantom reads.
KEY
No shared locks
No write skew issues
Shared locks
No write skew issues
No shared locks
Write skew issues

71. SQL Server is Special
Except SQL Server, all MVCC capable databases use it per default.
In SQL Server, it must be enabled:
ALTER DATABASE … SET allow_snapshot_isolation on;
This will make write operations keep old versions
(needs more space in permanent DB and tempdb)
Then you can use SNAPSHOT isolation (e.g., in read-only transactions).
Staying in SERIALISABLE for read/write transactions prevents write skew issues.
Note: Hint remain effective: NOLOCK will still do dirty-read in SNAPSHOT isolation.

72. Keep Transactions Focused
Don’t select data that is irrelevant for the transaction.
(an innocent looking function-call might query something you are not aware of)
Do unrelated stuff in distinct transactions.
Work fast.
(Never, ever keep a transaction open waiting for someone)
(In the code, but also not in your ad-hoc SQL session!)
Tune your statements.
(That makes your transactions shorter too)