With an extra field sic.key_ordinal and order by clause more readible, because the column fields are then in the exact order of the index:

SELECT OBJECT_NAME(sis.OBJECT_ID) TableName, si.name AS IndexName, sic.key_ordinal, sc.Name AS ColumnName,
sic.Index_ID, sis.user_seeks, sis.user_scans, sis.user_lookups, sis.user_updates, sis.last_user_seek
FROM sys.dm_db_index_usage_stats sis
INNER JOIN sys.indexes si ON sis.OBJECT_ID = si.OBJECT_ID AND sis.Index_ID = si.Index_ID
INNER JOIN sys.index_columns sic ON sis.OBJECT_ID = sic.OBJECT_ID AND sic.Index_ID = si.Index_ID
INNER JOIN sys.columns sc ON sis.OBJECT_ID = sc.OBJECT_ID AND sic.Column_ID = sc.Column_ID
WHERE sis.Database_ID = DB_ID(‘prdomgcod’) AND sis.OBJECT_ID in
( OBJECT_ID(‘oas_docline’), OBJECT_ID(‘oas_balance’))
order by
OBJECT_NAME(sis.OBJECT_ID), si.name, sic.key_ordinal, sc.Name,
sic.Index_ID, sis.user_seeks, sis.user_scans, sis.user_lookups, sis.user_updates, sis.last_user_seek
GO

Please write an article about the Selectivity, describing the situations where we shall create index in a column.

Also it will be great to have an article about the Fill Factor and its practical usage.

Thanks again for the valuable articles.

Very gud article.

Could you please tell me what is the cons of using index ?
Thanks.

Han.

Really very good explanation of index scan & index seek. And by your article every one will familier with how can make faster seeking objects by index. And also you have cleared in your article for seeking the objects when it will fetching by leaf level and when it will fetching by table.

Also I have a question? Thats the general question off all interviewer..

If Yesterday on the Server T-SQL statement was running fine but today it’s take too much time. What are the reason?

Thanks & Regards,
Rajiv Singh

So, is those column are eligible for index?
Or, can you tell, how much minimum rows require for eligibility for creating index?

Thanks

select name , city from tables where id = 2
select * from tables where id = 2

Two indexes are created on table as following :

create index IC_Col1 on table(id) include (name, city)
create index IC_Col2 on table(id)

1. which of above index is used for
select name , city from tables where id = 2

2. create index IC_Col2 on table(id) can be used for both query
select name , city from tables where id = 2
select * from tables where id = 2

So Selectivity ratio is more or around 80-90 percentage is the best option to create for index, Right?

Thanks all for you help.

Thanks.

Often an index needs to be very selective before it will be used, not 50% as you state but often 10% or lower.

Thanks
Ian

An index scan is a complete scan of all the pages in a non-clustered index.
A clustered index scan is a complete scan of all pages in a clustered index (ie, the table itself)
Neither scan uses the b-tree structure of the index, but just reads the leaf pages in order, using each page’s reference to the next in the chain.

An index seek is a seek through the b-tree structure of a non-clustered index, from the root down to the leaf.
A clustered index seek is a seek through the b-tree structure of a clustered index, from the root down to the leaf.

Example with good Selectivity

A table having 100’000 records and one of its indexed column has 88000 distinct values, then the selectivity of this index is 88’000 / 10’0000 = 0.88.

The selectivity of an index is the percentage of rows in a table having the same value for the indexed column. An index’s selectivity is good if few rows have the same value.

Example with bad Selectivity

lf an index on a table of 100’000 records had only 500 distinct values, then the index’s selectivity is 500 / 100’000 = 0.005 and in this case a query which uses the limitation of such an index will retum 100’000 / 500 = 200 records for each distinct value. It is evident that a full table scan is more efficient as using such an index where much more I/O is needed to scan repeatedly the index and the table.

select count (distinct job) “Distinct Values” from emp;

Distinct Values
—————
5

select count(*) “Total Number Rows” from emp;

Total Number Rows
—————–
14

Selectivity = Distinct Values / Total Number Rows
= 5 / 14
= 0.35

This post is a dangerous mix of poorly worded sentences and incorrect facts.

The fact that you are with Solid Quality Learning and an MVP is astonishing and is a testament to the quality of those organizations.

I feel sorry for the people that look to you for information and guidance.

Best of luck, Jason

Can you give me description?

How much rows (minimum require for selectivity to create index?

Suppose one table have 200 rows, in this 175 rows are same ans 25 are distinct ? then will it be fine to create an index on that column?

or Can you give me exact scenario when column is liable for selective for index?

An index scan means that SQL Server reads all rows in a table, and then returns only those rows that satisfy the search criteria.

or

An index scan means that SQL Server reads all rows in a INDEX, and then returns only those rows that satisfy the search criteria.

?

Not to much information to over load first thing on a Monday but just enough to get you started!