Month: October 2017

In previous posts, we saw

Part 1 – How to create Temporal Tables

Part 2 – Accessing History data from Temporal Tables

Part 3 – Modifying Temporal Tables, Disabling System versioning and why we need to disable ?

In this post we will see how to manage Temporal tables and History

To get list of Temporal Tables – Snippet 1

SELECT name,
       object_id,
       temporal_type,
       temporal_type_desc,
       history_table_id,
       is_remote_data_archive_enabled,
       is_external,
       history_retention_period,
       history_retention_period_unit,
       history_retention_period_unit_desc
FROM sys.tables
WHERE temporal_type > 0
ORDER BY name;

Output

If we see the output, we can identify the following

1. Type of Temporal Table – System Versioned or History table using “temporal_type” and “temporal_type_desc” columns

2. For System Versioned tables, we can also see the History Table ID

3. History Retention Period can be determined using history_retention_period, history_retention_period_unit and history_retention_period_unit_desc

history_retention_period = -1  = History is kept forever

Positive Integer = Refers to the number of Days/Weeks/Years

history_retention_period_unit will have one of these values

-1 = INFINITE

3 = DAY(S)

4 = WEEK(S)

5 = MONTH(S)

6 = YEAR(S)

By default, when we create temporal tables, if we don’t specify Retention period, it will be created with Infinite period of retention

Modifying Retention Period

Let us see how we can change Retention Period for an existing Temporal Table

Code to alter table to set Retention Period for 2 days – Snippet 2

ALTER TABLE dbo.Demo
	SET (SYSTEM_VERSIONING = ON
			(
				HISTORY_TABLE = dbo.DemoHistory,
				HISTORY_RETENTION_PERIOD = 2 DAYS
			)
		)

If we run query from Snippet 1, we will see below output

To get list of Databases with Retention Enabled – Snippet 3

Once retention period is enabled for any table in the database, SQL Server will set the is_temporal_history_retention_enabled flag = 1 for the database by default and also create background jobs to cleanup the history data

SELECT name,
       database_id,
       is_temporal_history_retention_enabled
FROM sys.databases
WHERE is_temporal_history_retention_enabled = 1
ORDER BY name;

Output

If you want to disable automatic cleanup task, Please use below command

Query to disable cleanup task – Snippet 4

ALTER DATABASE SQL2017_DEMO
     SET TEMPORAL_HISTORY_RETENTION OFF

If we run the query in Snippet 3, we can see the changes

How to cleanup History tables manually ?

If you prefer to cleanup the History tables manually rather than automated, Please follow below steps

1. Make your application offline

2. Run query to disable System versioning on required tables

3. Run query to delete data from History using SysEndTime as Filter criteria

4. Run query to enable System versioning for tables disabled in Step 2

5. Make your application online

If your application has to be on 24/7, better option is to use the in-built automated cleanup task

Hope you find this post helpful !!!

Part 1 – How to create Temporal tables

Part 2 – Accessing History data from Temporal Tables

In this post we will how to modify Temporal Tables, How to disable System Versioning in Temporal tables to update large set data of without affecting history or truncate tables

Modifying Temporal Tables

For adding new columns to Temporal tables, Please use below command

ALTER TABLE dbo.Demo ADD DemoDesc VARCHAR(255) NULL

Screenshot before adding the new column

Screenshot after adding the new column – If we notice, adding the column in the main table automatically modifies the history table as well

Similarly we can modify the tables to drop column or increasing the size of the field, but some of the changes to table are not supported ex. Adding Identify column or changing the data type of the field which is not supported using implicit conversion, in that scenario we need to disable the system versioning to make changes

Disable System Versioning

Screenshot before disabling system versioning

Script to disable system versioning

ALTER TABLE dbo.Demo	SET (SYSTEM_VERSIONING = OFF);

Screenshot after disabling system versioning – If we notice the System versioning is disabled and both Main and history table is displayed as regular tables now

There are other scenarios we need to disable System versioning, if we need to do any of the following operations

1. TRUNCATE TABLE on Main or History table

Msg 13545, Level 16, State 1, Line 1
Truncate failed on table ‘SQL2017_DEMO.dbo.Demo’ because it is not a supported operation on system-versioned tables.

2. Deleting rows from History table

Msg 13560, Level 16, State 1, Line 4
Cannot delete rows from a temporal history table ‘SQL2017_DEMO.dbo.DemoHistory’.

3. Dropping Main or History table

Msg 13552, Level 16, State 1, Line 6
Drop table operation failed on table ‘SQL2017_DEMO.dbo.Demo’ because it is not a supported operation on system-versioned temporal tables.

Hope you find this post helpful !!!

In previous post, we saw how to create Temporal tables or enable System Versioning on an existing table

In this post, we will review how to access History data

Using below code, we will create simple Temporal table and inserted 5 records and querying both the Main and History table

CREATE TABLE dbo.Demo 
 (    
      DemoID int NOT NULL PRIMARY KEY CLUSTERED  IDENTITY 
    , DemoName varchar(50) 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)     
 )    
 WITH (SYSTEM_VERSIONING = ON (HISTORY_TABLE = dbo.DemoHistory)) 
 ;

INSERT INTO dbo.Demo (DemoName) VALUES ('Demo1') 
 INSERT INTO dbo.Demo (DemoName) VALUES ('Demo2') 
 INSERT INTO dbo.Demo (DemoName) VALUES ('Demo3') 
 INSERT INTO dbo.Demo (DemoName) VALUES ('Demo4') 
 INSERT INTO dbo.Demo (DemoName) VALUES ('Demo5')

SELECT DemoID, 
        DemoName, 
        SysStartTime, 
        SysEndTime FROM dbo.Demo

SELECT DemoID, 
        DemoName, 
        SysStartTime, 
        SysEndTime FROM dbo.DemoHistory

Output: Main table has 5 rows and and History has no data, because no changes were made yet to the data, Also if you notice the SysEndTime, all of them are set to 12/31/9999 23:59:59.9999999, which refer to them as current record

Let us modify some data in 2 minute intervals

First Change – Update 1 row and delete 1 row

UPDATE dbo.Demo 
    SET DemoName='Demo11' 
    WHERE DemoID=1

DELETE FROM dbo.Demo WHERE DemoID=4

Output: Now we can see that Main table has 4 rows and History has 2 rows , the green highlighted is the record we updated as you can history table contains the previous version of record and also SysEndTime in History table indicates when it was updated

Similarly Red highlighted record in history is the deleted with Deleted time

Second Update: Update 1 record and Delete 1 record

UPDATE dbo.Demo 
    SET DemoName='Demo12' 
    WHERE DemoID=2

DELETE FROM dbo.Demo WHERE DemoID=3

Output:

Let us now review how we can access the History data

Method 1: AS OF

We can use FOR SYSTEM_TIME AS OF to get records in Demo as of the specific time, This will be helpful for scenarios when you want to see How much Inventory was in hand as of a particular date and time or What products were in hand as of particular date and time

In below example we are using 3 different time with 2 minute intervals

SELECT DemoID, 
       DemoName, 
       SysStartTime, 
        SysEndTime FROM dbo.Demo 
    FOR SYSTEM_TIME AS OF '2017-10-20 14:42:34' 
    ORDER BY DemoID

SELECT DemoID, 
       DemoName, 
       SysStartTime, 
       SysEndTime FROM dbo.Demo 
    FOR SYSTEM_TIME AS OF '2017-10-20 14:44:34' 
    ORDER BY DemoID

SELECT DemoID, 
       DemoName, 
        SysStartTime, 
       SysEndTime FROM dbo.Demo 
    FOR SYSTEM_TIME AS OF '2017-10-20 14:46:34' 
    ORDER BY DemoID

Output: As we can see in first output we are able to see all records as how it was inserted and in 2nd put, we can see 1st record was updated and Demo ID 4 record is missing and in output 3 23 can seem Demo ID 1 and 2 are updated and 3 and 4 are deleted

Method 2: FROM…TO and  BETWEEN…AND option

We can use FROM…TO and BETWEEN…AND option to get records changed between 2 specified date times

SELECT DemoID, 
       DemoName, 
       SysStartTime, 
       SysEndTime 
    FROM dbo.Demo 
    FOR SYSTEM_TIME FROM '2017-10-20 14:42:33' TO '2017-10-20 14:46:55' 
    ORDER BY DemoID, SysStartTime

SELECT DemoID, 
        DemoName, 
       SysStartTime, 
       SysEndTime 
    FROM dbo.Demo 
    FOR SYSTEM_TIME BETWEEN '2017-10-20 14:42:33' AND '2017-10-20 14:46:55' 
    ORDER BY DemoID, SysStartTime

Output: In below output you can see all changes made between the specified time intervals, but if you notice it, its providing us both version of the records, so based on your application need, you will need to use this or else, it might provide more records for display than you need

Method 3: CONTAINED IN

When we use CONTAINED IN option, it provides data from history table only for the specified datetime(s)

SELECT DemoID, 
        DemoName, 
        SysStartTime, 
       SysEndTime 
    FROM dbo.Demo 
    FOR SYSTEM_TIME CONTAINED IN ('2017-10-20 14:42:33', '2017-10-20 14:45:56') 
    ORDER BY DemoID, SysStartTime

Output: As we can see the below output doesn’t contain the current version of record or other records from Main table

Method 4: ALL

When we use ALL option, it provides data from both Main and History table for all the changes made so far

SELECT DemoID, 
        DemoName, 
        SysStartTime, 
       SysEndTime FROM dbo.Demo 
    FOR SYSTEM_TIME ALL 
    ORDER BY DemoID, SysStartTime

Output

We can see all records from main and history table combined, Please be cautious using this option when running without Filter criteria for larger tables

We can create Views using Temporal Tables and those views supports these clauses as well

Hope you find this post helpful !!!

Temporal Tables is a new feature introduced in SQL Server 2016, which makes database/application developers life easy to implement History feature in their application without having to write Triggers or any extra logic in stored procedures

Method 1: Creating Temporal Tables for New Table without explicitly specifying History table name

Please add these 3 columns to your table definition and use WITH (SYSTEM_VERSIONING = ON)

CREATE TABLE dbo.Demo
(
DemoID int NOT NULL PRIMARY KEY CLUSTERED
, DemoName varchar(50) 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)
)
WITH (SYSTEM_VERSIONING = ON);

In below screenshot, you can see that Demo table is created with History table, but History table name is generated by SQL Server itself, the default naming conversion is dbo.MSSQL_TemporalHistoryFor<OBJECTID of Table>

Method 2: Creating Temporal Tables for New Table with explicit History table name

CREATE TABLE dbo.Demo2
(
DemoID int NOT NULL PRIMARY KEY CLUSTERED
, DemoName varchar(50) 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)
)
WITH (SYSTEM_VERSIONING = ON (HISTORY_TABLE = dbo.Demo2History));

In the below screenshot, you can see the dbo.Demo2 table is created with History table name

Method 3: Altering an existing table to implement Temporal table

The below table dbo.Demo3 already exists

ALTER TABLE dbo.Demo3 ADD SysStartTime datetime2 GENERATED ALWAYS AS ROW START NOT NULL
, SysEndTime datetime2 GENERATED ALWAYS AS ROW END NOT NULL
, PERIOD FOR SYSTEM_TIME (SysStartTime,SysEndTime)

ALTER TABLE dbo.Demo3
SET (SYSTEM_VERSIONING = ON (HISTORY_TABLE = dbo.Demo3History))

In below screenshot you can see Demo3 table is modified to include

In next post, we will see how INSERT/UPDATE/DELETE operation ran on main table generates History data and how we can access it

Hope you find this post helpful !!!