Managing temporal table history in sql server 2016 – 【126kr】

SQL Server 2016 introduced a new feature, Temporal Tables, which allow you to keep a historical record of all of the versions of each row in a table. Sql server date As rows get introduced, changed, and deleted over time, you can always see what the table looked like during a certain time period or at a specific point in time.

You may have heard Temporal Tables referred to as system-versioned tables.


Sql server books What happens is that the historical versions of rows, according to the system time of their last modification, are stored in a separate table, while the current version resides in the original table.

For tables that don’t change very often, this works fantastic – queries against the base table know, based on the filter criteria, whether to get the data from the base table or the history table. Sql server tuning For tables with a high volume of insert/delete activity, however, or with rows that get updated frequently, the history table can quickly grow out of control. Latest sql server Imagine a table with 100 rows, and you update those 100 rows 100 times each – you now have 100 rows in the base table, and 9,900 rows in the history table (99 versions of each row).

While there are definitely going to be regulatory/auditing exceptions, in many cases, you won’t want or need to keep every single version of every single row for all of time. Sql server 6 Solution

The way these solutions are explained, though, lead you to make a blanket choice about retaining historical data only based on a specific point in time (say, archive everything from before January 1, 2017) or fixed windows (once a month, switch out the oldest month). Ms sql server versions list This may be perfectly adequate for your requirements, and that’s okay.

When I considered these solutions, I immediately envisioned a scenario that they wouldn’t cover: what if I want to keep only the last three versions of a row, regardless of when those modifications took place? Or all previous versions from the past two weeks or the current calendar year, plus one additional version before that? If I archive or delete based only on a point in time, then I might keep too many versions of some rows, and no historical versions of other rows. Ms sql server 2014 express If I want to keep the three previous versions, I can’t possibly enforce that based on a point in time.

Any criteria can be accomplished, of course, if we put a little more thought into the “custom cleanup script” solution. About sql server 2008 The procedure demonstrated in the documentation accepts a specific datetime value, and deletes all historical data before that point. With en sql server I’d like to demonstrate how to accomplish a selective delete (or archiving into yet another historical location) using a different set of criteria.

Now, even though SYSTEM_VERSIONING is OFF, the ValidFrom and ValidTo values are populated with the time of the insert and the end of the day on 9999-12-31, respectively. Latest version sql server If you update the data in this table right now, the ValidFrom value will update the current time, but no historical version of the row will be stored anywhere.

We can then create a history table. Ms sql server 2005 express The columns and data types must match, but the history table can’t have constraints. What is database in sql server So we’re going to create a clustered index on EmployeeID, ValidFrom, ValidTo: CREATE TABLE dbo.Employees_History

Next, we’ll fictitiously populate it with some historical versions of these rows, just as if I had set this up a couple of years ago (this is *absolutely not* a demonstration of how Temporal Tables should work, nor a recommendation to ever do it this way; we’re just trying to set up some dummy data): — a historical version representing when we updated salary:

What this does, effectively, is moves the existing row from the base table to the history table, updates the ValidTo value to the current time, then creates a new row in the base table with the updated column and a new ValidFrom value. Or in sql server 2008 (That is what happens logically , but not necessarily what happens physically .) Now the two tables look like this – up top, I’ve highlighted the changed value in the base table, and below, the row that now appears in the history table (click to enlarge):

This should demonstrate the purpose of this article: As you update more rows in the base table, the history table can very quickly ramp up and take over your disk, especially if the rows are a lot wider than this simple example. In en sql server Identifying Rows to Clean up

Depending on the rules you want to use to determine which history rows to keep or throw away, it should be easy in this case to visually identify those rows, and then build the proper query.

Again, the rules you want to use to dictate which rows to keep and which rows to delete or archive will have bearing on this. Local sql server database There are a variety of things you may want to do, for example you may want to keep a certain number of history versions of a row regardless of time, or you may always want to keep only a certain window, or you may want to use some combination. Versions of sql server You may even want to have different retention policies applied to different products, departments, teams, or whatever data you’re storing.

Assuming we want to start simple and only keep the previous version of any given row (including the latest version for any key that only exists in the history table), we can first apply shading visually to the above image to highlight the history rows we’re going to delete (click to enlarge):

My suggestion would be to use a common table expression (CTE) to help identify those rows correctly, as you can then easily change the outer query from a SELECT to a DELETE. Sql server into So we can start with this: ;WITH h AS

If we want something more flexible than just keeping the latest history row, like keeping *all* of history for the past month in addition to the latest versioned row before that, it just makes our query a little more complex. Sql server 2012 r2 express In this case it only changes the visual by a single row, marked here as “Now keep!” (click to enlarge):

And the code just uses an additional CTE to first eliminate all of the updates in the past month before determining which rows represent the most recent updates: ;WITH only_history_over_a_month_old AS

The CTEs above use a SELECT as the final query, and have the DELETE commented out. Sql server sql However, you can’t just swap the comments to perform the delete. Sql server download The trick with actually deleting rows from the history table is that system versioning needs to be turned off (true for any DML you want to manually perform). Sql server express upgrade If you try to delete from the table right now, you will get the following error: Msg 13560, Level 16, State 1

You will need to stop versioning the base table, either during a maintenance window, or by implementing the changes within a serializable transaction (or both, to be safe). Sql server versions wiki In either case, you likely want to minimize the amount of time that the base table is not being versioned – by using a serializable transaction, it ensures that any potential writers will be blocked while the changes are taking place.

That CTE + DELETE in the middle would be the only part that needs to change, depending on the retention policy you want to enforce. Download sql server 2005 I should add the disclaimer that you should check @@ROWCOUNT after the DELETE and make sure that not only the right number of rows, but also the right rows, have been deleted (I’ve left out error handling and detailed validation for brevity).

If you are deleting a large number of history rows, you will likely want to add some batching logic to this, rather than delete all the rows at once, in order to minimize the impact on the transaction log and, more importantly, blocking. Sql server manager express (I describe a technique for this in Break large delete operations into chunks . Sql server database version However, since you’re relying on CTEs here, you may need to dump those rows to a #temp table in order to process them x rows at a time, or use a cursor to cycle through a certain number of key values at a time. Sql server 2012 express This is so that you don’t inadvertently delete the “most recent” row for any key more than once.) Another Space-Saving Idea

Since system versioning is all-or-nothing, and you don’t necessarily need to maintain a historical record of every change to every column in a table, another thing you can consider doing is removing the data from columns that don’t need to be audited. Sql server reporting If you had an additional column that stored notes or other unstructured data for each employee, where only the current version really matters, you could use the same process as above to set that column to NULL in the history table. Sql server studio express Summary

Temporal tables provide a rich and powerful way to maintain historical versions of rows, with minimal effort. Sql server express price When history grows to the point that maintenance is required, there are many options for cleaning up, and you aren’t restricted to naïve, hard-coded, time-based windows. Sql server enterprise edition I hope I have shown here how you can accomplish much more flexible retention policies, allowing you to keep exactly the history you want. Sql server web license Next Steps

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