Yes, this is very wide problem: you expect something, but reality is a bit different. You expect that query will run 1 sec, but it runs (Oh, my God!) 1 hour. You expect that your query will perform index seek, but it performs index scan instead. You expect your query doesn't use locks, but it uses them. So this session will be focused on understanding the internals of such situations and making our expectations more close to reality.

1. SQL Server. Expectation – Reality
Denis Reznik
Intapp
Email: denisreznik@gmail.com
Skype: denisreznik
Blog: http://reznik.uneta.com.ua
Twitter: @denisreznik

2. Agenda
• Some Useful Stuff to Remind
• Expectation - Reality 1
• Expectation - Reality 2
• Expectation - Reality 3
• Expectation - Reality 4
• Expectation - Reality 5

3. Index (B-Tree) - Seek
…
…
1 .. 1M
1 .. 2K 2K+1 .. 4K
1M-2K .. 1M
1 .. 300 301..800 801..1,5K 1,5K+1..2K
SELECT * FROM Users
WHERE Id = 523

4. Index (B-Tree) - Scan
…
…
1 .. 1M
1 .. 2K 2K+1 .. 4K
1M-2K .. 1M
1 .. 300 301..800 801..1,5K 1,5K+1..2K
SELECT * FROM Users

5. Statistics
500
1000
10
1200
800
1 800 2000 2800 4500 5400
SELECT * FROM Users
WHERE Id BETWEEN 2100 AND 2500
SELECT * FROM Users
WHERE Id BETWEEN 1 AND 5300

6. Exec SomeProc @p
Optimizer
Buffer Pool
Query Plan
cache
@p
Plan is cached for the
first value of Query Plan
cache
Parameter Sniffing – Intro

7. Lock Types - Shared
S S
X

8. Lock Types - Exclusive
X
X
S

9. Lock Types - Update
U
U
S
SX

10. READ COMMITTED
ID City
1 Kiev
2 London
3 London
4 London
5 London
6 New York
7 New York
ID City
1 London
2 London
3 London
4 London
5 London
6 New York
7 New York
SELECT * FROM Users
WHERE City = 'Kiev'
BEGIN TRAN
UPDATE Users
SET City = 'London'
WHERE City = 'Kiev' SELECT * FROM Users
WHERE City = 'Kiev'
Wait for
Shared lock on
the row
X S

11. READ COMMITTED SNAPSHOT
ID City
1 Kiev
2 London
3 London
4 London
5 London
6 New York
7 New York
ID City
1 London
2 London
3 London
4 London
5 London
6 New York
7 New York
BEGIN TRAN
UPDATE Users
SET City = 'London'
WHERE City = 'Kiev'
SELECT * FROM Users
WHERE City = 'Kiev'
SELECT * FROM Users
WHERE City = 'Kiev'
X
tempdb
ID City
1 Kiev
Version Store

12. READ UNCOMMITTED
ID City
1 Kiev
2 London
3 London
4 London
5 London
6 New York
7 New York
ID City
1 London
2 London
3 London
4 London
5 London
6 New York
7 New York
SELECT * FROM Users
WHERE City = 'Kiev'
BEGIN TRAN
UPDATE Users
SET City = 'London'
WHERE City = 'Kiev' SELECT * FROM Users
WHERE City = 'Kiev'
0 RecordsROLLBACK
SELECT * FROM Users
WHERE City = 'Kiev'
X

13. DEMO: Universal Query

14. DEMO: Conditional Logic

15. DEMO: Implicit Locks

16. DEMO: NOLOCK Locks

17. DEMO: The Most Confusing
Behavior

18. Summary
• Universal Query
• Conditional Logic
• Implicit Locks
• NOLOCK Locks
• The Most Confusing Behavior

19. Thank You!
@denisreznik
denisreznik@gmail.com
http://reznik.uneta.com.ua/
https://www.facebook.com/denis.reznik.5
https://www.linkedin.com/pub/denis-reznik/3/502/234