What is Fill Factor in SQL Server

Fill Factor works in the performance tuning area, for the index the most important property is Fill Factor. Fill Factor responsible to determine the percentage of space on each leaf-level page to be filled with data. As we know the page is the smallest unit of SQL server which size is 8k. Every page cans one or more than one row which is depending on the size of the row.

The Fill Factor specifies the % of the fullness of the leaf level pages of an index. When an index is created or rebuild then filled up pages with data depend on Fill Factor. For example, if we create an index and put the value of Fill Factor is 70 then pages will be filled up with data 70% other 30% space will be remain.

For Example, I am creating a Temp named table for testing of Index with Fill Factor.

CREATE TABLE Temp (        id INT IDENTITY(1,1),        Name VARCHAR(100) ) DECLARE @count INT=100000; WHILE (@count>0) BEGIN        INSERT INTO Temp        VALUES('SQL Server tutorial by codefari.com Type'+CONVERT(VARCHAR(100),@count))        SET @count=@count-1 END SELECT COUNT(*) FROM Temp

Now run following script

EXEC sp_spaceused 'dbo.Temp'

ResultSet

High Fill Factor value

You can see index size 8kb and unused 8kb both are same, because we did not create any index on this table.

Now see the following script I am creating a non-clustered Index with Fill Factor 100%

USE [Test] GO /****** Object:  Index [Name]    Script Date: 11/27/2015 6:34:53 PM ******/ CREATE NONCLUSTERED INDEX [Name] ON [dbo].[Temp] (        [Name] ASC )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF,  DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON,  FILLFACTOR = 100) ON [PRIMARY] GO

Again run following script

EXEC sp_spaceused 'dbo.Temp'

ResultSet

Here Index size becomes 6404 kb and unused 176kb. Here we have taken Fill Factor 100%. 

Note: You may choose high Fill Factor value if it is very little or no changes in the underlying table's data. This means if you have an index that is constantly changing you would want to have a lower value to keep some free space available for new index entries.  Otherwise, SQL Server would have to constantly do page splits to fit the new values into the index pages.

Low Fill Factor value

Now if we put the value of Fill Factor as 50% then what will happen you may see in the following example.

Drop the created index first and run the following script again.

USE [Test] GO /****** Object:  Index [Name]    Script Date: 11/27/2015 6:34:53 PM ******/ CREATE NONCLUSTERED INDEX [Name] ON [dbo].[Temp] (        [Name] ASC )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF,  DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON,  FILLFACTOR = 50) ON [PRIMARY] GO

Again run the following script

EXEC sp_spaceused 'dbo.Temp'

ResultSet

Now here you can see index_size is 12544kb and unused 248kb.

Note: With new data records added, the index pages need to have sufficient space to take the new entries. When there is not enough space a page split needs to occur which could impact performance depending on how frequently page splits need to occur.