Identity increment is jumping in SQL Server database

Question

In one of my tables Fee in column "ReceiptNo" in SQL Server 2012 database identity increment suddenly started jumping to 100s instead of 1 depending on the following two things.

1. if it is 1205446 it is jumps to 1206306, if it is 1206321, it jumps to 1207306 and if it is 1207314, it jumps to 1208306. What I want to make you note is that the last three digits remain constant i.e 306 whenever the jumping occurs as shown in the following picture.

2. this problem occurs when I restart my computer

Answer 1

You are encountering this behaviour due to a performance improvement since SQL Server 2012.

It now by default uses a cache size of 1,000 when allocating IDENTITY values for an int column and restarting the service can "lose" unused values (The cache size is 10,000 for bigint/numeric).

This is mentioned in the documentation

SQL Server might cache identity values for performance reasons and some of the assigned values can be lost during a database failure or server restart. This can result in gaps in the identity value upon insert. If gaps are not acceptable then the application should use its own mechanism to generate key values. Using a sequence generator with the NOCACHE option can limit the gaps to transactions that are never committed.

From the data you have shown it looks like this happened after the data entry for 22 December then when it restarted SQL Server reserved the values 1206306 - 1207305. After data entry for 24 - 25 December was done another restart and SQL Server reserved the next range 1207306 - 1208305 visible in the entries for the 28th.

Unless you are restarting the service with unusual frequency any "lost" values are unlikely to make any significant dent in the range of values allowed by the datatype so the best policy is not to worry about it.

If this is for some reason a real issue for you some possible workarounds are...

1. You can use a SEQUENCE instead of an identity column and define a smaller cache size for example and use NEXT VALUE FOR in a column default.
2. Or apply trace flag 272 which makes the IDENTITY allocation logged as in versions up to 2008 R2. This applies globally to all databases.
3. Or, for recent versions, execute ALTER DATABASE SCOPED CONFIGURATION SET IDENTITY_CACHE = OFF to disable the identity caching for a specific database.

You should be aware none of these workarounds assure no gaps. This has never been guaranteed by IDENTITY as it would only be possible by serializing inserts to the table. If you need a gapless column you will need to use a different solution than either IDENTITY or SEQUENCE