"Query Execution: Expectation - Reality (Level 300)" Денис Резник
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The document provides an overview of SQL Server practices, highlighting key topics such as expectation versus reality, index usage, query performance, and lock types. It includes demonstrations of universal queries, conditional logic, and locking behaviors within SQL transactions. The presentation concludes with a summary of the discussed concepts and includes contact information for the author.
"Query Execution: Expectation - Reality (Level 300)" Денис Резник
- 1. SQL Server. Expectation – Reality Denis Reznik Intapp Email: [email protected] 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
- 17. DEMO: The Most Confusing Behavior
- 18. Summary • Universal Query • Conditional Logic • Implicit Locks • NOLOCK Locks • The Most Confusing Behavior