1. Execution Plans Demystified
Ernest Libertucci
Execution Plans Demystified

2. Presenter Info
Senior Database Engineer 17+ Years Using SQL Server Database administration, SSIS, Hadoop, etc Thank you to our sponsers!

3. Execution Plans – What are they?
An execution plan (or query plan) is a description of the operations the SQL Server engine will use to perform a given task, along with the metrics involved in this decision making process.

4. Why should we care? Can answer questions including:
Why is my query so slow?
Is that index I created being used?
Why is TempDB growing?
Where was I last night? (Okay, maybe not that one)

5. Goal – Avoid becoming this:

6. Permissions Required
SHOWPLAN permission is required (database-level) for all databases involved in the query.

7. How do we see them? SSMS Menu. CTRL + L (Show Estimated)
CTRL + M (Include Actual)
SET SHOWPLAN_XML ON

8. Indexes

9. Types of Indexes
Clustered – Logical ordering of data (Page splits can change physical order). Data pages are the leaf nodes, so the table becomes this index. Tables without a clustered index are called a heap.
Non-clustered – One or more columns. Nodes contain pointer to the row data stored in the clustered index (or heap).
Columnstore. Filtered.

10. Statistics

11. Statistics Details
Cardinality: Uniqueness of Data values (Example: Countries would have 193 distinct values).
Cardinality Estimates refer to the engines ‘belief’ about a column’s distribution.
Contributes greatly to the choices of operators used in an execution plan.

12. A day in the life of a Query – Part 1

13. A day in the life of a Query – Part 2

14. A day in the life of a Query – Part 3

15. Query Optimizer
Software that analyzes potential execution plans for a given query and chooses which one to execute. Cost-based, but will choose the best plan it can within a quantum. Parses the query, creating a logical tree of operations. Using this tree, the QO examines some potential ways to satisfy a query and then picks the lowest cost one.

16. Operators Table/Clustered Index Scan Index Seek Sort
Key/Bookmark Lookup
Aggregates
Scalar
Data Modification

17. Join Operators – Nested Loop
In general, better for smaller data inputs.
Cost A * B

18. Join Operators – Merge Join
Join two sorted inputs (Sort operator or Index)
Ideal for large range scans where both sides are indexed.
Cost A + B

19. Join Operators – Hash Match
Build a hash table for both inputs, then look for matching. Typically chosen when at least 1 input is large.
Image: By Jorge Stolfi - Own work, CC BY-SA 3.0,

20. Suspicious Things Seek (go to value) vs Scan (read entire table/index)
SARgable predicates (Search ARGumentable) { f(col1) = value vs col1 = f(value) }
Implicit conversion { ex: column1 = ‘5’ when column1 is integer }
Statistics {Estimated widely different from Actual }

21. Considerations Give the Query Optimizer Information
Constraints, Keys (Primary/Foreign), Nullability
Avoid Implicit Conversion
Database Compatibility Level
Querytraceon hint (9481 or 2312)
9481: Use 2012 QO on a later version of sql server
2312: Use 2014 QO on a 2012 or lower set database (SQL 2014+)

22. Query Store (2016+)
Plan, Runtime stats, wait stores.

23. Automatic Plan Correction (SQL 2017)
Apply previous plan instead of a regressed one. Note: “current” keyword syntax since 2012.

24. Other Things of Note ™
Live statistics
Query Optimizer (2014)
QO Fixes

25. References/Further Information
SQL Server Execution Plans (3rd Ed) – Grant Fritchey
Execution Plan Reference
Sp_WhoIsActive – Adam Machanic
SentryOne Plan Explorer

26. Thank you!