SQL Server Performance Tuning

How We Made T-SQL Queries Run 562x Faster

Mark Varnas
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Improvement after tuning

114x

DURATION

171x

CPU

278x

DISK

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

Problem

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

Pre-tuning Metrics

3,402
Duration (ms)
3,114
CPU (ms)
53,116
Disk (number of reads)

Solution

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)
BeforeAfterBeforeAfterBeforeAfter
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
AVG3,420303,41124,653,116191

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

Disk, number of reads(average)

53,116
Before tuning
191
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.

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