How We Made T-SQL Queries Run 34,785 Times Faster

Most SQL Servers bottleneck on Disk, which is number of disk operations required to pull all data necessary to process the the query.

When tuning, the goal is often to get SQL Server to read less data from the disk.

Problem

We identified a long-running query that was throttling CPU usage to 70% and concurrently executing across 12 threads.

Here’s a glance at the pre-tuning metrics:

• Duration (query duration in milliseconds): 3,200,290
• Disk (number of read operations required to get all data needed): 942,222,862

The query execution time was 53 minutes or3,200,290 ms while making almost 1 million disk read operations.

Solution

• After reviewing the query plan, we identified the culprit: a NOT IN operation invoking a VIEW.
• The fix was straightforward: switching the NOT IN operation to a NOT EXISTS operation. See below.

Here is how we modified the original query to fix the problem:

Note: Performance of NOT IN vs. NOT EXISTS – performance vise is often massive. This is one of those examples.

Before vs. After

Small changes, huge difference? Totally!

We slashed a query's run time from 3,200,290 ms down to 92 ms. No joke!

Below is a comparison of the number of disk reads before vs after tuning:

Simple query rewrite reduced the duration of the stored procedure by a 34,785 times!

Final Thoughts

A simple tweak of swapping the NOT IN operation for a NOT EXISTS operation resulted in a 34,785x improvement.

In SQL performance tuning, it's often crucial to understand how the SQL engine retrieves data.