query tuning optimization

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Presented by Harsh Chawla

SQL Community Session – Delhi NCR

DBA: Query Tuning and Optimization - Expert

1. DB Consultant Approaches2. Fetching Execution Plans3. Common execution plan patterns4. Understand the data distribution5. Q&A

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Proactive Approach • Leverage MDW

• Have a Performance Baseline

Understand the load

• Missing Indexes• Duplicate Indexes• Unused Indexes

Understand the Indexing

• Leverage Plan hash and Query Hash to find top resource intensive queries

• Leverage MDW to identify the top queries

Fetch Top Queries for Tuning

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Reactive Approach

• Leverage MDW /SQLNexus to identify the resource contention

Identify the Top Bottlenecks

• Leverage Plan hash and Query Hash to find top resource intensive queries

• Leverage MDW to identify the top queries

Identify the Top Resource intensive queries

• Set Statistics IO ON• Set Statistics Time on• Set Statistics Profile on

Explore the execution plans

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Fetching Execution plans

• Estimated Vs Actual• DMVs show only estimated

plans• Extract the variables from

the execution plan• Execute the query to get the

actual execution plan

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CommonExecution Plan Patterns

• Index Seek• Index Scan• Key/RID Lookups• Implicit Conversions• Missing Statistics

Warning• Sort Warning• Hash Warning

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Let’s Delve Deeper

• Cardinality Estimation• Residual Predicates• Impact of Data

distribution• Sorts

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Cardinality Estimation

• Beauty lies in the data• There is never a

coincidence• Wrong Estimation cause

Hash warnings/sort warning or almost every mess up

• It’s the biggest gotcha while query tuning

• Try to investigate the numbers

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Demo

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Residual Predicates

• What’s a Predicate• If the predicate is not

SARGable, Residual Predicates will be seen

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Demo Impact of data distribution

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Demo Sort

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Q&A