How to Configure SQL Server Temporal Tables Part 2

 

In the initial post, we talked about some very basic items to get you up and running with temporal tables. In part 2, I’ll cover a few more things to think about and implement when working with temporal tables.

In the first post, we looked at altering an existing table to be a temporal table. Now we will look at what is involved in making a table be a temporal table from the very beginning.

Designing a Temporal Table from Scratch

First of all, you can create a table to be a temporal table from the beginning, without naming the history table. But you get an ugly history table name in the following format:

[dbo].[MSSQL_TemporalHistoryFor_tableObjectId].

In my test case that turns out to be this:

[dbo].[MSSQL_TemporalHistoryFor_1913773875].

Also, notice that in this example below, the SysStartTime and SysEndTime required columns are not designated as hidden. This means they can be returned in queries, so be aware of this, particularly when using SELECT *.

This syntax also results in a clustered index being created on the history table
using the SysStartTime and SysEndTime columns.

Observe that I’m taking advantage of the inline index creation syntax that was new in SQL Server 2014. Look at examples S, T, and U to see more information on how to use this feature.

CREATE TABLE dbo.Employee
(
ID INT IDENTITY(1,1) NOT NULL,
LastName VARCHAR(100) NOT NULL,
FirstName VARCHAR(75) NOT NULL,
JobTitle VARCHAR(50) NOT NULL,
BirthDate DATE NOT NULL,
HireDate DATE NOT NULL,
SysStartTime DATETIME2 GENERATED ALWAYS AS ROW START NOT NULL,
SysEndTime DATETIME2 GENERATED ALWAYS AS ROW END NOT NULL,
 PERIOD FOR SYSTEM_TIME (SysStartTime,SysEndTime),
CONSTRAINT PK_Employees PRIMARY KEY CLUSTERED (ID),
INDEX IX_LastName_FirstName (LastName,FirstName),

)
WITH (SYSTEM_VERSIONING = ON);

ALTER TABLE dbo.Employee ADD CONSTRAINT DF_SysStart DEFAULT SYSUTCDATETIME() FOR SysStartTime;
ALTER TABLE dbo.Employee ADD CONSTRAINT DF_SysEndTime DEFAULT SYSUTCDATETIME() FOR SysEndTime;

This allows me to create the index on Lastname, FirstName directly in the CREATE TABLE statement and not have to do that afterward in a lengthier syntax.

INDEX IX_LastName_FirstName (LastName,FirstName)

To clean up this example and try a slightly different approach, run this code:

ALTER TABLE dbo.Employee SET(SYSTEM_VERSIONING = OFF)
DROP TABLE dbo.Employee

 

How To Name the History Table

You can create a temporal table and name the history table so it is more human friendly. This syntax also results in a clustered index being created on the history table using the SysStartTime and SysEndTime columns.

If you want to be able to create a clustered columnstore index, or a different clustered index then there are 2 choices:
1. Use this syntax and then drop the clustered index and make the new clustered index.
2. Create both the temporal table and the history table and when you make the history table, and specify the indexes you want to create.

 

CREATE TABLE dbo.Employee
(
ID INT IDENTITY(1,1) NOT NULL,
LastName VARCHAR(100) NOT NULL,
FirstName VARCHAR(75) NOT NULL,
JobTitle VARCHAR(50) NOT NULL,
BirthDate DATE NOT NULL,
HireDate DATE NOT NULL,
SysStartTime DATETIME2 GENERATED ALWAYS AS ROW START NOT NULL,
SysEndTime DATETIME2 GENERATED ALWAYS AS ROW END NOT NULL,
 PERIOD FOR SYSTEM_TIME (SysStartTime,SysEndTime),
CONSTRAINT PK_Employees PRIMARY KEY CLUSTERED (ID),
INDEX IX_LastName_FirstName (LastName,FirstName),

)
WITH (SYSTEM_VERSIONING = ON(HISTORY_TABLE = dbo.EmployeeHistory));

ALTER TABLE dbo.Employee ADD CONSTRAINT DF_SysStart DEFAULT SYSUTCDATETIME() FOR SysStartTime;
ALTER TABLE dbo.Employee ADD CONSTRAINT DF_SysEndTime DEFAULT SYSUTCDATETIME() FOR SysEndTime;

DROP INDEX IX_EmployeeHistory ON dbo.Employeehistory;

CREATE CLUSTERED COLUMNSTORE INDEX IX_CC ON dbo.EmployeeHistory;

 

Indexing Decisions for Temporal Tables

Per MS Docs, if you are creating this table as a temporal table for the purpose of auditing row changes, then a clustered rowstore index on the  SysStartTime, SysEndtime and the PK columns of the temporal table is a good choice. Otherwise, a clustered columnstore index is a good choice for analytic queries.

How to Handle Data Retention for Temporal Tables

The simplest option for managing data retention on a history table is to use
the HISTORY_RETENTION_PERIOD = syntax. The available time units for history retention are DAYS, WEEKS, MONTHS, YEARS. SQL Server then uses this information to manage the deletion of data from the history table for you. notice below I have added HISTORY_RETENTION_PERIOD = 6 MONTHS to the syntax.

CREATE TABLE dbo.Employee
(
ID INT IDENTITY(1,1) NOT NULL,
LastName VARCHAR(100) NOT NULL,
FirstName VARCHAR(75) NOT NULL,
JobTitle VARCHAR(50) NOT NULL,
BirthDate DATE NOT NULL,
HireDate DATE NOT NULL,
SysStartTime DATETIME2 GENERATED ALWAYS AS ROW START NOT NULL,
SysEndTime DATETIME2 GENERATED ALWAYS AS ROW END NOT NULL,
 PERIOD FOR SYSTEM_TIME (SysStartTime,SysEndTime),
CONSTRAINT PK_Employees PRIMARY KEY CLUSTERED (ID),
INDEX IX_LastName_FirstName (LastName,FirstName),

)
WITH (SYSTEM_VERSIONING = ON
    (HISTORY_TABLE = dbo.EmployeeHistory,
     HISTORY_RETENTION_PERIOD = 6 MONTHS)
    );

ALTER TABLE dbo.Employee ADD CONSTRAINT DF_SysStart DEFAULT SYSUTCDATETIME() FOR SysStartTime;
ALTER TABLE dbo.Employee ADD CONSTRAINT DF_SysEndTime DEFAULT SYSUTCDATETIME() FOR SysEndTime;

Other options for handling data retention include stretch tables, which place the entire history table, or part of it, in Azure. Partitioning the history table an writing your own custom cleanup script are also options.

Next Steps To Take

  1. Look here for a list of other considerations and limitations.
  2. Think about the tables in your environment where this feature could be useful and do some experimenting.
  3. Talk to your business personnel about this feature and ask them if they know of tables where knowing how the data has changed over time would be helpful. In some cases, you might find that the business is sometimes asked questions about how or when their data changed. This feature is perfect for answering that question.
  4. If you would like help with anything in this post, or with something else related to SQL Server, reach out to me here, or on Twitter, and I’ll be glad to offer assistance.

One Reply to “How to Configure SQL Server Temporal Tables Part 2”

Leave a Reply

Your email address will not be published. Required fields are marked *