statistis, row counts, execution plans and query tuning
TRANSCRIPT
STATISTICS, ROW COUNTS,EXECUTION PLANS AND
QUERY TUNING
Grant Fritchey | Redgate Software
GOALS
• Understand how row counts are determined with SQL Server
• Learn how row counts affect the execution plans generated
by the Query Optimizer
THE PLAN
• Statistics in SQL Server
• Reading Statistics
• How are Rows Estimated
• Row Estimates Affect Execution Plans
• Objects That Are Weird
• More Than Just Rows
• Recommendations
• Recap
STATISTICS
The practice or science of collecting and
analyzing numerical data in large
quantities, especially for the purpose of
inferring proportions in a whole from those
in a representative sample.
STATISTICS
The practice or science of collecting and
analyzing numerical data in large
quantities, especially for the purpose of
inferring proportions in a whole from those
in a representative sample.
SQL SERVER STATISTICS
Statistics for query optimization are
objects that contain statistical
information about the distribution of
values in one or more columns of a
table or indexed view.
SQL SERVER STATISTICS
STATISTICS: HEADER
STATISTICS: DENSITY GRAPH
1.0/ COUNT(DISTINCT Column)
STATISTICS: HISTOGRAM
EXAMPLES
SQL SERVER STATISTICS: OVERVIEW
• Distribution of the data
• Shows actual row counts and estimations
• Uses mathematics to arrive at numbers
• Histogram is the most useful numbers
• Density graph is the second most useful numbers
READING STATISTICS
RANGE HI_KEY RANGE_ROWS
1 0
5 8 1
2
2
2
2
3
3
3
4
5
READING STATISTICS
RANGE HI_KEY RANGE_ROWS EQ_ROWS DISTINCT_RANGE_ROWS
1 0 1 0
5 8 3 3
1 1
5
5
5
5
RANGE HI_KEY RANGE_ROWS EQ_ROWS DISTINCT_RANGE_ROWS
1 0 1 0
5 8 3 3
READING STATISTICS
1
2
2
2
2
3
3
3
4
5
READING STATISTICS
RANGE HI_KEY RANGE_ROWS EQ_ROWS DISTINCT_RANGE_ROWS AVG_RANGE_ROWS
1 0 1 0 1
5 8 3 3 2.67
AVG_RANGE_ROWS = RANGE ROWS
DISTINCT RANGE ROWS=
8
3
EXAMPLES
HOW ROWS ARE ESTIMATED
• Range high key
• Average range rows
• Average values
• Density
• Default values
• Magic estimates
EXAMPLES
ROW ESTIMATES AFFECT EXECUTION PLANS
• Out of Date Statistics
• Missing Statistics
• Parameter Sniffing
• Functions on Columns
• Constant Folding
• Local variables
EXAMPLES
OBJECTS THAT ARE WEIRD
• Table Variables
• Multi-statement, table-valued, user-defined, functions
• XML
• Columnstore
• Constraints
• Foreign Keys
• Joins
EXAMPLES
MORE THAN JUST ROWS
• Memory allocation
ꟷ Overestimates
ꟷ Underestimates
• Tempdb
• I/O
RECOMMENDATIONS
• Enable auto creation and auto update of statistics on the vast majority of systems
• Set database compatibility mode to the latest version
• Manually maintain statistics as necessary
• Enable Traceflag 2371
• Experiment with asynchronous statistics update
• Where necessary adjust statistics sample rate
• Use parameters and constant values
• Avoid operators and functions that limit or prevent statistics use
• Experiment with filtered statistics where applicable
• Use primary keys, foreign keys, constraints
• Ensure WITH CHECK is enabled and enforced
RESOURCES
• SQL Server 2014 Cardinality Estimator, Joe Sack, et al
• 13 Things You Should Know About Statistics, Fabiano Amorim
• Constant Folding and Expression Evaluation During Cardinality
Estimation, Microsoft
• Cardinality Estimation Place in the Optimization Process, Dimitry
Pilugin
• Cardinality Estimation Concepts, Dimitry Pilugin
• Cardinality Estimation Process, Dimitry Pilugin (just read all his stuff)
GOALS
• Understand how row counts are determined with SQL Server
• Learn how row counts affect the execution plans generated
by the Query Optimizer
