This document discusses SQL query performance analysis and optimization. It covers key topics like indexes, different types of scans (table scan, index scan, index seek), clustered vs non-clustered indexes, covering indexes to optimize queries, and using statistics to help the query optimizer generate efficient execution plans. The goal is to help analyze and tune queries to achieve fast index seeks and optimize performance.

1. SQL Query Performance Analysis

2. What is an index ?
• Index is a way to organize data to make
searching, sorting and grouping fasters
• we need indexing when :
1. WHERE, ON, HAVING clause (Searching)
2. ORDER BY clause (Sorting)
3. GROUP BY clause (Grouping) etc.

3. Table scan:
SELECT * FROM Student WHERE RollNo = 111
Time complexity of table scan is : O(n)
RollNo Name Country Age
101 Greg UK 23
102 Sachin India 21
103 Akaram Pakistan 22
107 Miyabi China 18
108 Marry Russia 27
109 Scott USA 31
110 Benazir Banglades 17
111 Miyabi Japan 24
112 Rahul India 27
113 Nicolus France 19

4. Types of Index
• Table without any index is called Heap
• There are two type of index:
1. Clustered index
2. Non-Clustered index

5. Clustered index
• When we create a clustered index on any
table physical organization of table is changed.
• Now data of table is stored as binary search
tree(B tree).
CREATE UNIQUE [CLUSTERED] INDEX <Name>
ON <ObjectName>(
<ColumnName> [ASC | DESC ] [ ,...n ]
)

7. Types of scanning
• Table scan: It is very slow can and it is used only if table has
not any clustered index.
• Index scan: It is also slow scan. It is used when table has
clustered index and either in WHERE clause non-key columns
are present or query has not been covered (will discuss later)
or both.
• Index Seek: It is very fast. Our goal is to achieve this.

8. Clustered index
• If we create table with primary key, sql server
automatically creates clustered index on that
table
• A table can have only one clustered index .
• Physical order of rows of table is same as
logical order of key columns of clustered
index.

9. Terms of execution plan
• Predicate: It is condition in WHERE clause which is either non-
key column or column which has not been covered.
• Object: It is name of source from where it getting the data. It
can be name of table, Clustered index or non-clustered index
• Output list: It is name of the columns which is getting from
object.
• Seek Predicate: It is condition in WHERE clause which is either
key column or fully covered.

10. Non-clustered index
• It is logical organization of data of table. A non-clustered index
can be of two types.
1. Heap
2. Based on clustered index.
• If table has clustered index then leaf node of non-clustered
index keeps the key columns of clustered index.
• If the table has not any clustered index then leaf node of non-
clustered index keeps RID which unique of each row of table.

11. Based on clustered Index

12. Based on heap

13. Covering of queries
• We can specify maximum 16 column names.
• Sum of size of the columns cannot be more than 900 bytes.
• All columns must belong to same table.
• Data type of columns cannot be ntext, text,
varchar (max), nvarchar (max), varbinary (max), xml, or image
• It cannot be non-deterministic computed column.

14. Statistics Analysis
• The query optimizer uses statistics to create query plans that
improve query performance
• A correct statistics will lead to high-quality query plan.
• Auto create and updates applies strictly to single-column
statistics.
• The query optimizer determines when statistics might be out-
of-date by counting the number of data modifications since
the last statistics update and comparing the number of
modifications to a threshold.

15. Goal
• Should we use sub query or inner join?
• Should we use temp table or table variable?
Other tools:
• Sql query profiler
• Database Tuning Advisor
• Resource Governor

16. THANK YOU