1. Physical Join Operators
Nesting Merged Hash Loops
Ami Levin
Think Big. Move Fast.

2. Session Goals
SQL Server uses three physical join operators: Nested loops, Merge, and Hash Match.
In this session we will:
See how each of these operators work
Review their advantages and drawbacks
Understand some of the logic behind the optimizer’s decisions on which operator to use
Learn to identify common join-related pitfalls
AD-400|Physical Join Operators

3. Not This Time Outer joins Non equi-joins Logical processing order
NULL issues
Join parallelism
Partitioned joins …
AD-400|Physical Join Operators

4. Equi-Inner-Join
SELECT Foo, Bar, ... FROM T1 INNER JOIN T2 ON T1.C1 = T2.C1 AND T1.C2 = T2.C2 AND ... WHERE ...
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5. Visual Join Simulator
AD-400|Physical Join Operators

6. Nested Loops ? Fetch next row from blue input Start Row exists Quit
False
True
Find matching rows in red input
AD-400|Physical Join Operators

7. Nested Loops I Outer loop determines number of iterations
At least one input should be (relatively) small
Inner operation is performed for every iteration of the outer loop
Index or table scan (naïve)
Index seek + lookup
Covering index seek
Index spool
AD-400|Physical Join Operators

8. Nested Loops II Data pages may be accessed repeatedly
Risky a-sequential page access path
Output of matching row sets is fast
Unordered, but typically grouped
Physical resources
CPU Very low
Physical IO low to very high
Memory low
AD-400|Physical Join Operators

9. Nested Loops with Foreign Key Joins
Foreign keys join parent and child
Most common relationship is one-to-many
Often parent input is significantly smaller
Parent must already be indexed
Either primary key or unique constraint
Therefore, indexing foreign keys often enables efficient use of nested loops
AD-400|Physical Join Operators

10. Nested Loops
DEMO
AD-400|Physical Join Operators

11. Merge ? ? Fetch next row from blue input Start False Row exists
Rows match
Quit ? ?
False
True
True
Fetch next row from red input
AD-400|Physical Join Operators

12. Merge I Inputs must be sorted prior to merge
Sorted by (all?) join expression(s)
Pre-sorted in plan, but not necessarily in DB
Preferred when sorting supports additional plan operations
Merge join types
One to many
Many to many - requires temporary worktable
AD-400|Physical Join Operators

13. Merge II Residual predicates Fast, ordered and grouped output
Physical resources
CPU Very low
Physical IO Very low
Memory Very low
* Excluding sorting costs
AD-400|Physical Join Operators

14. Merge
DEMO
AD-400|Physical Join Operators

15. Hash Match - Phase I (Build)
Start
Fetch next row from blue input ♥ ♦
Row exists
Phase II ?
False ♣
True
Apply hash function ♠
AD-400|Physical Join Operators

16. Hash Match - Phase II (Probe)
Fetch next row from red input ♥
Row exists ♦
Quit ?
False ♣
True
Apply hash function ♠
AD-400|Physical Join Operators

17. Hash Match I Hash function selection Build and probe costs are hidden
Extremely complex
CPU intensive
Build and probe costs are hidden
Do not constitute logical reads
Output of matching row sets is slow
Unordered and typically ungrouped
AD-400|Physical Join Operators

18. Hash Match II In memory hash join Grace hash join Recursive hash join
Hash bailout
Hash warnings event class
Update Statistics
Add more RAM
Role reversal
AD-400|Physical Join Operators

19. Hash Match III May indicate sub-optimal indexing
Best for very large, non covered joins
Physical resources
CPU Very high
Physical IO Low to very high
Memory Very high
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20. Hash Match
DEMO
AD-400|Physical Join Operators

21. Summary Nested Loops Merge Hash Good when CPU Memory Physical IO
Nested Loops
Merge
Hash
Good when
Small outer input
Inner input indexed
Pre-sorted inputs
Sorting needed
Very large inputs
Not well indexed
CPU
Low
* Excluding sorting
High
Memory
Physical IO
Low / High
Logical reads
* Misleading
Output
Fast, unordered, grouped*
Fast, ordered, grouped
Slow, unordered, ungrouped*
AD-400|Physical Join Operators

22. For More Information Books on line White papers
“Inside Microsoft SQL server” books
Craig Freedman’s blog
AD-400|Physical Join Operators

23. Physical Join Operators
Q&A
AD-400|Physical Join Operators

24. Session Code | Session Title

25. Thank you