This document discusses different techniques for performing joins in data warehousing, including nested loop joins, sort-merge joins, and hash joins. It provides code examples and diagrams to illustrate how each type of join works. Specifically, it explains that nested loop joins examine each row in one table to find matching rows in the other table, while sort-merge joins first sort both tables on the join key before merging to find matches. Hash joins hash one table and probe it with the other to identify matches. The document also discusses factors that affect the performance of each join technique, such as table order and skew.

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Need for Speed: Join TechniquesNeed for Speed: Join Techniques
Virtual University of PakistanVirtual University of Pakistan
Ahsan Abdullah
Assoc. Prof. & Head
Center for Agro-Informatics Research
www.nu.edu.pk/cairindex.asp
National University of Computers & Emerging Sciences, Islamabad
Email: ahsan101@yahoo.com

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Need for Speed: Join TechniquesNeed for Speed: Join Techniques

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BackgroundBackground

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About Nested-Loop JoinAbout Nested-Loop Join

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FOR i = 1 to N DO BEGINFOR i = 1 to N DO BEGIN /*/* N rows in T1N rows in T1 */*/
IF iIF ithth
row of T1 qualifies THEN BEGINrow of T1 qualifies THEN BEGIN
For j = 1 to M DO BEGINFor j = 1 to M DO BEGIN /* M rows in T2/* M rows in T2 */*/
IF the iIF the ithth
row of T1 matches to jrow of T1 matches to jthth
row of T2 on join keyrow of T2 on join key
THEN BEGINTHEN BEGIN
IF the jIF the jthth
row of T2 qualifies THEN BEGINrow of T2 qualifies THEN BEGIN
produce output rowproduce output row
ENDEND
ENDEND
ENDEND
ENDEND
ENDEND
Nested-Loop Join: CodeNested-Loop Join: Code
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““What is the average GPA ofWhat is the average GPA of
undergraduate male students?”undergraduate male students?”
For each qualifying row of Personal table,
Academic table is examined for matching rows.
Student Personal Table Student Academic Table
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62------------------
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Nested-Loop Join: Working ExampleNested-Loop Join: Working Example
Results
Search
Results
Search
Results
Search
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Nested-Loop Join: Order of TablesNested-Loop Join: Order of Tables

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Nested-Loop Join: Cost FormulaNested-Loop Join: Cost Formula
Join cost =Join cost = Cost of accessing Table_A +
# of qualifying rows in Table_A × Blocks of
Table_B to be scanned for each qualifying row
OR
Join cost =Join cost = Blocks accessed for Table_A +
Blocks accessed for Table_A × Blocks
accessed for Table_B
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Nested-Loop Join: Cost of reorderNested-Loop Join: Cost of reorder
Table_A = 500 blocks and
Table_B = 700 blocks.
Qualifying blocks for Table_A QB(A) = 50
Qualifying blocks for Table_B QB(B) = 100
Join cost A&B = 500 + 50×700 = 35,500 I/Os
Join cost B&A = 700 + 100×500 = 50,700 I/Os
i.e. an increase in I/O of about 43%.
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Nested-Loop Join: VariantsNested-Loop Join: Variants

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Sort-Merge JoinSort-Merge Join

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Sort-Merge Join: ProcessSort-Merge Join: Process

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Table_A Table_B
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Table_A Table_B
Sort-Merge Join Example

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Sort-Merge Join: NoteSort-Merge Join: Note

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Hash-Based joinHash-Based join

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Hash-Based Join: WorkingHash-Based Join: Working

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Hash-Based Join: ExampleHash-Based Join: Example
Table_B on disk
DiskDisk
Original
Relation
Table_A
hash
function
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Join Result
. . .
Table_B
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Table_A in main memory
MAIN MEMORY
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Hash-Based Join: Large “small” TableHash-Based Join: Large “small” Table

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Hash-Based Join: Partition SkewHash-Based Join: Partition Skew

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Hash-Based Join: Intrinsic SkewHash-Based Join: Intrinsic Skew