Why is the SQL Server Bad Execution Plan View Crucial for Dev?

Dear Dev, if you’re working with SQL Server, you know that optimizing query performance is key. One of the tools at your disposal is the execution plan view. But what happens when the execution plan is bad? That’s where this article comes in. We’ll explore what causes bad execution plans, how to identify them, and most importantly, how to fix them. Let’s dive in!

What is an Execution Plan in SQL Server?

Before we get into what makes a plan bad, let’s make sure we’re on the same page about what an execution plan is in the first place. Simply put, an execution plan is a set of steps that SQL Server takes to execute a query. The plan is generated by the query optimizer, which analyzes various factors like table size, indexes, and statistics to determine the most efficient way to execute the query.

The execution plan view in SQL Server Management Studio (SSMS) allows developers to see exactly how the query optimizer has decided to execute a given query. This can be incredibly useful in understanding why a query is performing slowly, and where optimizations can be made.

What Causes a Bad Execution Plan?

So, what makes an execution plan bad? In general, a bad plan is one that takes longer to execute or uses more resources than it should. There are several factors that can contribute to a bad plan:

How to Identify a Bad Execution Plan?

Now that we know what can cause a bad execution plan, the next step is to identify when we have one on our hands. There are several ways to do this:

Check the Execution Plan View

The most obvious way to identify a bad execution plan is by looking at the execution plan view in SSMS. If the plan shows a lot of expensive operations (like table scans or sorts), that’s a good indication that the plan could be optimized.

Use Dynamic Management Views (DMVs)

SQL Server provides a set of DMVs that can be used to get information about query execution. The most useful DMV for identifying bad plans is sys.dm_exec_query_stats, which provides performance statistics for each query that has been executed. By sorting by metrics like CPU time or logical reads, you can quickly spot queries that are consuming more resources than they should.

Use Profiler or Extended Events

Finally, you can use tools like SQL Server Profiler or Extended Events to capture query performance data in real-time. This can be useful in identifying when a query is suddenly performing poorly, which can be a sign of a bad plan.

How to Fix a Bad Execution Plan?

Once you’ve identified that you have a bad execution plan, the next step is to fix it. Here are some approaches you can take:

Update Statistics

As we mentioned earlier, out-of-date statistics can be a cause of bad execution plans. By updating your statistics, you give the optimizer more accurate information to work with, which can lead to better plans. You can update statistics using the sp_updatestats stored procedure or by setting up a regular maintenance plan.

Use Query Hints

In some cases, you might know that a specific query plan is the optimal one for your situation. In these cases, you can use query hints to force SQL Server to use that plan. For example, you might use the OPTION (HASH JOIN) hint to force a hash join instead of a nested loops join.

Add or Modify Indexes

If you’ve identified that missing indexes are contributing to a bad plan, you can add or modify indexes to better support your queries. Keep in mind that adding too many indexes can also have a negative impact on performance, so it’s important to strike the right balance.

Recompile the Query

If you suspect that parameter sniffing is causing a bad plan, you can try recompiling the query using the WITH RECOMPILE option. This tells SQL Server to generate a new plan for each execution, which can be more optimal for the current parameters.

Use Plan Guides

If you have a query that is generating a bad plan and you can’t modify the query itself (for example, if it’s part of a third-party application), you can use plan guides to force SQL Server to use a specific plan for that query. Plan guides can be created using the sp_create_plan_guide stored procedure.

FAQ

What happens when SQL Server generates a bad execution plan?

When a bad execution plan is generated, the query might take longer to execute, use more resources, or return incorrect results. This can lead to slow application performance and user frustration.

Can a bad execution plan cause blocking?

Yes, a bad execution plan can contribute to blocking. If a query is taking longer to execute than it should, it might hold locks on resources for longer, which can block other queries from accessing those resources.

How often should I update my statistics?

There’s no one-size-fits-all answer to this question, as it depends on the size and volatility of your data. In general, it’s a good idea to set up a regular maintenance plan that includes updating statistics, so that your optimizer has up-to-date information to work with.

Can query hints be harmful?

Yes, query hints can be harmful if used improperly. For example, if you force a hash join when the optimizer would have chosen a nested loops join, you might end up with a plan that is slower than the original. It’s important to thoroughly test any query hints before deploying them to production.

What should I do if none of these approaches work?

If you’ve tried all of these approaches and you’re still experiencing bad execution plans, it might be time to call in a SQL Server expert. They can help you identify and resolve more complex issues, such as poorly-designed database schema or hardware limitations.