Just Check ALL the Boxes

Today I ran into something on a client server I unfortunately see too often.  The DBA goes through the trouble of configuring and setting up alerts\operators but doesn’t really understand what the options in the configurations mean. So unfortunately, that means they take the CYA (cover your ass) approach and they check all of them. Now, not only have I seen this with alerts but also with things like security configurations as well. My advice is to always in to take a second and research what each option is before you check the little boxes, especially when it comes to security. Always follow the rule of less is more.

In the example below the administrator enabled alerts for an operator using the CYA approach. They checked email, pager, and netsend.

So, what’s the big deal? This server experienced an insufficient resources (space) alert that fired every minute and by having PAGER notifications enabled it caused the error log to bloat, consumed unnecessary space, and created noise in the logs.

The administrator of this environment really only needed to configure the email notification, as the company did not use netsend nor have pagers duties configured. To be honest, I have yet to see an environment use more than that, and per Microsoft both Pager and Net Send will be removed in future versions.

So, the morale of the story is, please take the time to research what the little checkboxes are before you enable them. The example above is a pretty benign one, but you can imagine what kind of messes you can get yourself in for other more critical things like security.

A Side Note:

If you want to learn how to setup your alerts and operators I’ve already written a blog on that with scripts you can find it here.

You can also visit github.com/dc-ac for a full install script that includes the Alert and Operator setups https://github.com/DC-AC/SQL2016_Scripted_Install

Hmmm… What’s This?

OK So, I am doing some digging and peaking around again in SQL Server and came across a database option called Date Correlation Optimization Enabled = False. Honestly, I had no clue what it did, so I took it as a learning opportunity to look into it and do a little research. Who knows, it may actual help me solve one of the many problems I run into day to day for clients.

Syntax

So, What Does It Do?

According to MSDN – The DATE_CORRELATION_OPTIMIZATION database SET option improves the performance of queries that perform an equi-join between two tables whose date or datetime columns are correlated, and which specify a date restriction in the query predicate.

How many of you read what MSDN says and thinks “wuuuuuttt, English please”? I do.

In English

Basically, it uses a foreign key relationship key between tables in SQL Server to enhance performance of date and date time queries when the dates fall within a certain defined range of each other (correlates). Ok that’s cool, but what’s the big deal? The power really comes in for things like reporting, validation, and data warehouses. With this option turned on, SQL Server maintains statistics between correlated columns and creates improved execution plans that reads less data.

Let’s See It in Action

Consider this, all internet orders that are received have a must send out by date (due date) of 10 days after order is received. Therefore, the OrderDate and DueDate are correlated, related to each other.

Here is a query you would normally run.

Without DATE_CORRELATION_OPTIMIZATION turn on the optimizer would create a plan just like anything else, however with it set to ON the optimizer can make more granular execution plans.

Here’s how

With each INSERT, UPDATE and DELETE between these two tables SQL Server is gathering statistics which helps the optimizer infer the query to be more like the one below. This is where the power comes in. The optimizer can better narrow down the records it needs to read and therefor returning faster results.

Here is the way SQL interprets the dates now that correlation is turned on and statistics are being gathered. It based on those statistics it can now infer that each DueDate is exactly 10 days after the OrderDate.

Depending on the number of records in the two tables this can be a VERY significant decrease to execution times.

The Caution

You should not enable DATE_CORRELATION_OPTIMIZATION in update-intensive database environments. SQL Server keeps all the correlation information in statistics form, this means with every INSERT, UPDATE and DELETE you gain additional overhead.

As always, be sure to test it before you use it in production.

What does this little check box do?

Ever wander around SQL Server properties and wonder what these little check boxes turn on? I do, and I get very tempted to check them. Here is one of those tempting little boxes that seems pretty handy, Use query governor to prevent long running queries.

Syntax

How Does it Work?

It’s simple. This option, available in SQL Server 2008 standard and forward, will prevent long running queries based on run time measured in seconds. If I specify a value of 180 the query governor will not allow any execution of a query that it estimates will exceed that value. Notice it says ESTIMATES which means it will be based on optimizer estimates and not ACTUAL run times. It does NOT KILL an actively running query after designated amount of time.  There is no worries for rollback scenarios or partial data.

CAUTION

This is an advanced option, keep in mind this is a server instance wide option. This will also effect your maintenance queries, so please use with caution, this is not “a let me check this box for fun” option.

But Wait There’s More

Now there is a query “transaction” based option available to us that will limit a specific query. This option will estimate a transaction and prevent it from running if it will go over the boundary we have set. Notice we set the limit before the query and then back to 0 after.

Again, playing with any old check box is not a recommended practice. Make sure you research it first and understand the full impact before checking that tempting little box.

Please Don’t Do This!

Please, please, please Admins do not leave your default index fill factor at 0. This means you are telling SQL Server to fill the page 100% full when creating indexes. This also means you are forcing it to a new page when additional inserts are done. These are called PAGE SPLITS which can take time to perform and is a resource intensive operation. Having a high fill factor will cause more index fragmentation, decrease performance and increase IO.

If you find that this is how your system is configured, all is not lost. You can correct this by changing the default value so that new indexes will be created with proper factor and rebuilding your existing indexes with another fill factor value. I like to use 80 across the board for most, of course there is always the “it depends” scenario that arises but 80 is a pretty safe bet. One of those “it depends” would be on logging table that has the correct clustering key and never gets updates in between values (make sense?), I don’t want a fill factor of 80.  I’d want 0/100 to maximize page density as page splits wouldn’t occur if the clustered key is monotonically increasing.

Note, having the additional 20% free on a page will increase your storage requirements but the benefit outweighs the cost.

Example syntax for changing the default

Example script for rebuilding in index with new fill factor