Category: Performance Tuning

Greg Low points out an important property which can help linked server performance:

The on-premises versions of SQL Server have the ability to connect one server to another via a mechanism called Linked Servers.

Azure-based SQL Server databases can communicate with each other by a mechanism called External Tables. I’ll write more about External Tables soon.

With Linked Servers though, I often hear people describing performance problems and yet there’s a configuration setting that commonly causes this. In Object Explorer below, you can see I have a Linked Server called PARTNER.

Read on for more.

Arun Sirpal has a self-Q&A session regarding Azure SQL Database’s automatic tuning options:

What are the options?

1. CREATE INDEX that identifies the indexes that may improve performance of your workload, creates the indexes, and verifies that they improve performance of the queries.

2. DROP INDEX that identifies redundant and duplicate indexes, and indexes that were not used in the long period of time.

3. PLAN REGRESSION CORRECTION that identifies SQL queries that are using execution plan that are slower than previous good plan, and uses the last known good plan instead of the regressed plan.

Very useful information.

Arvind Shyamsundar walks us through a scenario with user-defined functions with optional parameters:

If you notice carefully, the above query is an example of ‘optional parameters’ wherein the same query caters to situations where there are specific values for the parameters as well as other cases where there are none. Due to the implementation of the query (specifically the usage of ISNULL(@paramname, ColName)) what ends up happening is that the query plan thus generated will not leverage any indexes on the table. While this query can be refactored to separate versions for cases where the parameter values are supplied, and where they are not, another viable option is to use OPTION (RECOMPILE) on the statement level. This is an acceptable solution in most cases because the cost of scanning the table is often far higher than the cost of recompiling this query. So here is how we used OPTION RECOMPILE in this case:

Arvind walks us through three separate solutions.  My fourth solution is, don’t use user-defined table-valued functions.

Lukas Eder has a big article on ten query optimizations that don’t involve looking at statistics or query costs:

This optimisation is really silly, but hey, why not. If users write impossible predicates, then why even execute them? Here are some examples:

-- "Obvious"
SELECT * FROM actor WHERE 1 = 0

-- "Subtle"
SELECT * FROM actor WHERE NULL = NULL

The first query should obviously never return any results, but the same is true for the second one, because while NULL IS NULL yields TRUE, always, NULL = NULL evaluates to NULL, which has the same effect as FALSE according to three-valued logic.

This doesn’t need much explanation, so let’s immediately jump to see which databases optimise this:

I was a bit surprised at how well DB2 did in this set.

Daniel Janik shows one of the many pain points around user-defined functions in SQL Server:

Note that Query 1 uses a function around the column and this causes a scan which increases IO and CPU utilization. This is because every value in the column for the whole table must have the hyphen removed to see if it is a match.

In Query 2, we see a seek. This is because the value is modified instead of the column.

Click through for a few examples.

Grant Fritchey is asking for input on a new flowchart he has created:

Lots of people are confused by how to deal with bad parameter sniffing when it occurs. In an effort to help with this, I’m going to try to make a decision flow chart to walk you through the process. This is a rough, quite rough, first draft.

I would love to hear any input. For this draft, I won’t address the things I think I’ve left out. I want to see what you think of the decision flow and what you think might need to be included. Click on it to embiggen.

I think it’s a great first step.  I think a decision to add local variables and use them instead of parameters would be useful, particularly in contrast to using RECOMPILE and OPTIMIZE FOR UNKNOWN.