SQL Server Performance Tuning

How We Made T-SQL Queries Run 562x Faster

Mark Varnas
No comments

Improvement after tuning







Reading from disk is SQL Server’s slowest operation, making the reduction of disk reads a primary target for tuning.


A stored procedure that ran hundreds of times per hour was taking too long.

Pre-tuning Metrics

Duration (ms)
CPU (ms)
Disk (number of reads)


The fix involved multiple code changes in the stored procedure to reduce the number of disk reads.

Before vs. After

The transformation drastically cut execution time from 3,420 ms to just 30 ms!

Here’s the before-and-after snapshot that showcases the efficiency gains:

We took 6 separate runs to analyze the query performance before and after the fix:

RunDuration (ms)CPU (ms)Disk (8k page reads)
Run 13,833323,8443153,658231
Run 2193261883125,799162
Run 34,654244,6561565,655167
Run 43,287313,2503145,807225
Run 54,187434,15661,039151
Run 64,367284,3751566,738214

We significantly lowered the total number of disk reads which also led to a decrease in CPU and Duration:

Disk, number of reads(average)

Before tuning
After tuning

BeforeAfterImprovement (%)Improvement (x)
Duration, ms*3,4203011,400114
CPU, ms*3,41124,613,865139
Disk, number of reads*53,11619127,809278

*The numbers are an average of multiple T-SQL runs

After our tuning, the stored procedure became significantly faster, showing an overall improvement of 56,264%!

That’s 562x faster!

Final thoughts

Most SQL Servers bottleneck is on Disk access (or disk “reads”).

It’s neither the CPU nor the RAM, which are usually the first suspects for most customers.

And that makes much sense. Here is why:

  • Inefficient queries scan (or read) much data.
  • Data read in is stored in RAM. As more data is read in, “older” data is pushed out from RAM.
  • If there isn’t enough RAM to keep ALL data in memory (which is often not possible), SQL Server has to read from disk – and that is the slowest operation SQL Server can do.
  • When the query can be tuned to read 10 rows vs. 10 million – less CPU and RAM automatically are necessary.

Disk resources are the cause of 95% of SQL Server bottlenecks, so reducing the disk reads is usually the primary objective.

For the end-user, nothing is more crucial than the speed (or duration) of the query, in every case.

Tuning to reduce CPU/RAM resources is helpful too.

When queries are tuned to need less CPU & RAM, it means that the same server now has more capacity.

Article by
Mark Varnas
Founder | CEO | SQL Veteran
Hey, I'm Mark, one of the guys behind Red9. I make a living performance tuning SQL Servers and making them more stable. I channel my SQL into our SQL Managed Services, SQL Consulting and our internal database products.

Managed SQL Server services, consulting, and emergency support from expert DBAs to improve performance, predictability, and cost.

Get started with Red9 today.

Contact us

Discover More

Discover what clients are saying about Red9

Red9 has incredible expertise both in SQL migration and performance tuning.

The biggest benefit has been performance gains and tuning associated with migrating to AWS and a newer version of SQL Server with Always On clustering. Red9 was integral to this process. The deep knowledge of MSSQL and combined experience of Red9 have been a huge asset during a difficult migration. Red9 found inefficient indexes and performance bottlenecks that improved latency by over 400%.

Rich Staats 5 stars
Rich Staats
Cloud Engineer

Always willing to go an extra mile

Working with Red9 DBAs has been a pleasure. They are great team players and have an expert knowledge of SQL Server database administration. And are always willing to go the extra mile to get the project done.
5 stars
Evelyn A.
Sr. Database Administrator

Boosts server health and efficiency for enhanced customer satisfaction

Since adding Red9 to the reporting and DataWarehousing team, Red9 has done a good job coming up to speed on our environments and helping ensure we continue to meet our customer's needs. Red9 has taken ownership of our servers ensuring they remain healthy by monitoring and tuning inefficient queries.
5 stars
Andrew F.
Datawarehousing Manager
See more testimonials