Month: August 2019

SQL Server: Don’t Make the Query Optimiser’s Job More Difficult

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Part of my job is tuning complex queries: I’ve seen some recently with eye watering complexity. A post from Erik Darling explains how abstraction can be the cause of poor performance in SQL Server:

Sometimes it’s views, CTEs, or derived tables. Sometimes it’s functions. obviously functions can have a weirder set of effects, but the general idea is the same.

If you start chaining things, or nesting them together, you’re making the optimizer’s job harder and likely introducing a lot of overhead.

There’s no “caching” of steps in a query. If you nest a view however-many-levels-deep, each step isn’t magically materialized.

Same goes for CTEs. If you string a bunch together and reference them multiple times, you’ll start to see some very repetitive branches in your query plans.

Now, there are tricks you can play to get what happens inside of one of these steps “fenced off”, but not to get the result set fully materialized.

In addition, as your query becomes complex, the query optimiser eventually gives up and produces a less than efficient query plan because there are too many potential query plans to choose from.

Erik references Grant Fritchey’s post from 2012, The Seven Sins against TSQL Performance, which is still as relevant today.

Hardening SQL Server Security

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Three part article on hardening SQL Server Security:

Below are some Microsoft recommended best practices for network settings:

  • Enable Windows Firewall and limit the network protocols supported.
  • Do not enable network protocols unless they are needed.
  • Disable NETBIOS and SMB protocol unless specifically needed.
  • Do not expose a server that is running SQL Server to the public Internet.
  • Configure named instances of SQL Server to use specific port assignments for TCP/IP rather than dynamic ports.
  • Use extended protection in SQL Server 2012 if the client and operating system support it.
  • Grant CONNECT permission only on endpoints to logins that need to use them. Explicitly deny CONNECT permission to endpoints that are not needed by users or groups.

SQL Server: Poison Waits

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SQL Server performance tuning often starts by examining your top wait statistics. There are certain wait types where even a small number of occurrences can indicate performance problems. These are termed Poison Waits.

RESOURCE_SEMAPHORE_QUERY_COMPILE
A query was sent to SQL Server, and there wasn’t an execution plan for it in the query plan cache. In order to create an execution plan, SQL Server requests a small amount of memory, but due to memory pressure the requested memory wasn’t available. So SQL Server had to wait for memory to become available before it could even build an execution plan, let alone execute the query. In this situation, cached query plans and small un-cached plans may be able to run depending on how much pressure the server is under, but complex queries will experience memory request waits and feel sluggish.

RESOURCE_SEMAPHORE
SQL Server compiled an execution plan (or retrieved the query plan from cache), but now it needs memory to actually execute the query (a memory grant request). If other queries are already using a lot of memory, then our query won’t be able to start executing because there is insufficient memory available. Similar to the RESOURCE_SEMAPHORE_QUERY_COMPILE wait, smaller queries may be able to execute, but complex ones will be blocked from executing and wait for memory to become available.

THREADPOOL
At startup, SQL Server creates a predefined number of worker threads based on how many logical processors the server has (each worker thread uses 2MB of memory). As queries arrive, they get assigned to worker threads. If enough queries queue up, such as when queries get blocked, you can run out of available worker threads (worker thread starvation). You might be tempted to increase max worker threads (and Microsoft support sometimes makes this suggestion), but then you might simply escalate the problem to a RESOURCE_SEMAPHORE or RESOURCE_SEMAPHORE_QUERY_COMPILE issue. Blocking is the most common culprit of THREADPOOL waits, but it can also be due to a large amount of connections trying to run queries at the same time. If you are unable to connect to SQL Server to troubleshoot because of worker thread starvation, try connecting using the Dedicated Admin Connection.

Whenever any of these poison waits occur, you have to get to the root cause of the problem. For a list and explanation of the various waits: https://docs.microsoft.com/en-us/sql/relational-databases/system-dynamic-management-views/sys-dm-os-wait-stats-transact-sql