virtual techdaysINDIA │ 9-11 February 2011

SQL 2008 Query Tuning Praveen Srivatsa │Principal SME – StudyDesk91│Director, AsthraSoft Consulting │Microsoft Regional Director, Bangalore │MVP, ASP.NET

Query Architecture

Query optimizer and statistics• SQL Server always used a statistic based query optimizer• Statistical information is used to chose the optimal index for a

particular query• The most important information for the optimizer is:

– Selectivity or density: How many different values exist in this column of the table?

– Histogram over the values of a column: How often does one particular value exist?

Analyzing Factors

• Application• Database Design• Microsoft SQL Server• Operating System• Hardware

SQL Coding Issues• Excessive Scanning• Poor use of indexes/missing Indexes• Out of Date Table Statistics• Non-selective Triggers• ‘Where’ clause not limiting record

set enough• Excessive Recompiles• Long blocking durations

(multi-connections)

Where• Limit number of Columns• Avoid <>, OR, and not• Indexed fields• Avoid calculated columns• Avoid functions• Avoid redundant criteria

Search ArgumentsEquality Predicate

SELECT * FROM CUSTOMERWHERE CUSTOMER_NAME = 'CUSTOMER#000000047'

Range PredicateSELECT * FROM CUSTOMERWHERE CUSTOMER_ID BETWEEN 100 AND 200

ConjunctionsSELECT * FROM ORDER_ITEMWHERE SHIP_DATE = '1998-07-02'AND SUPPLIER_ID = 350AND PRODUCT_ID > 80000

Search arguments useful in driving index seeks

Non-Search ArgumentsNot Predicates

SELECT * FROM CUSTOMERWHERE CUSTOMER_ID <> 100

Like pattern on left of columnSELECT * FROM CUSTOMERWHERE CUSTOMER_NAME LIKE ‘%47‘

Function on columnSELECT * FROM ORDER_ITEMWHERE YEAR(SHIP_DATE) = 1998

Non-search arguments may still be evaluated in an indexIn Where clause of Index Scan or Index Seek Operator

Understanding JoinsOuterTable

InnerTable

JoinMethod

Nested Loop Algorithm– Get Row From Outer Table (1st input to Join operator in showplan)

• Get Matching Row From Inner Table (2nd input to Join operatorin showplan)

• Output Composite Result• Loop Through Inner Table• When Inner Table Exhausted ,Loop on Outer Table

OuterTable Inner

Table

Merge Join Algorithm

• Get next row from outer table– Get next row from inner table with same key– If found output and loop on inner table– If not found loop on outer table

Join Sequence

Join Sequence

Match andMerge

OuterTable

InnerTable

Hash Join Algorithm• Scan Smaller (Build) Table

– Hash Build Key Values; Store in Hash Table– 1st input to Join operator in SHOWPLAN

• Scan Larger (Probe) table– Hash Probe Key Value; Look Up in Hash Table– If Found, Output Result– 2nd input to Join operator in SHOWPLAN

Build Table

Hash Table

Probe Table

Lookup in Hash Table

Hash Join Key

Key

Join

Hash

Troubleshooting Join Performance• Is join predicate missing?• Indexes on join column(s)?• Join columns exact data type match?• Examine Statistics Profile for estimates versus actual

– Hash Join: EstimateRows for child operatorproducing Build input

– Nested Loop: EstimateExecute for inner table (2nd input)– Merge Scan: EstimateRows for Sort Operator (if present)

• Examine STATISTCS IO– SET STATISTICS IO ON– If Hash Join, are workfiles present

• Indicate Build tables have spilled to tempdb

• Last Resort: Join Hints

Stored Procedures• Execution Plan Usage• Keep them small• Recompiles

– Causes– Avoiding

• Analysis

Intelligent Indexing• Minimizing result sets to columns

needed (Covering index approach)instead of select *

• Avoid indexing fields that change frequently

• Compound Indexes– Unique >>>>Non-Unique

How do you do Partitioning?• Before a table is partitioned two things have to be created, a

partition Function and Schema• Partition Function

– Each row of an index/table is assigned to a partition (numbered 1, 2, 3, ...) by using a “Partition Function”

• SQL Server 2005 supports Range partitioning only on a single column • User defines the key column, the number of partitions, and the partition

boundary points

• Partition Scheme– Each partition for a partition function is mapped to a physical storage

location (Filegroup) through a “Partition Scheme”