Our training courses

Book any course as classroom, onsite or online training, or as one-to-one consultancy
Excel Training Basic Excel Advanced Excel VBA Macros Office Scripts Excel Introduction Excel Intermediate Excel Advanced Excel Business Modelling Excel VBA macros Advanced VBA Fast track Excel VBA Basic Office Scripts Advanced Office Scripts Power Platform Training Pure Power BI DAX and fast-track Power Automate Power Apps Introduction to Power BI Adv. Power BI Reports Adv. Power BI Data DAX for Power BI Fast track Power BI Fast track Power BI/DAX Power Automate Desktop Basic Power Automate Advanced Power Automate Fast track Power Automate Power Apps Programming and AI SQL training Visual C# training Python training AI training Introduction to SQL Advanced SQL Fast track SQL Introduction to MySQL Introduction to Visual C# Intermediate C# Fast track C# Introduction to Python Advanced Python Fast track Python Using AI tools Using the OpenAI API SQL Server training Reporting Services Report Builder Integration Services Azure / SSAS Reporting Services Advanced SSRS Fast track SSRS Report Builder Introduction to SSIS Advanced SSIS Fast track SSIS SQL in Azure Data Studio SSAS - Tabular Model
Other training resources

Free resources

Read our blogs, tips and tutorials

Try our exercises or test your skills

Watch our tutorial videos or shorts

Take a self-paced course

Read our recent newsletters

Paid services

License our courseware

Book expert consultancy

Buy our publications

Getting help

Get help in using our site
Our training venues

Where we are

London

Manchester

Where you are

Across the UK

At your site

Online

At your computer
Why we are different

Transparent reviews

551 attributed reviews in the last 3 years

Delivery of courses

Refreshingly small course sizes

Outstandingly good courseware

Whizzy online classrooms

Wise Owl trainers only (no freelancers)

Nicer to work with

Almost no cancellations

We have genuine integrity

We invoice after training

Reliable and established

Review 30+ years of Wise Owl

View our top 100 clients

Search our website

GO
+44 (161) 883 3606 sales@wiseowl.co.uk Web enquiry form
We also send out useful tips in a monthly email newsletter ...

Sign up to newsletter

Our training courses

AI - Articial Intelligence

Power BI and DAX

Other training resources

Tips and tricks

Guides and tutorials

YouTube tutorials

Test your skills

Self-paced courses

Our training venues

Why we are different

Hundreds of published reviews

Small class sizes

Outstanding courseware

Online classroom

(Almost) no cancellations

Wise Owl trainers only

Genuine integrity

Invoices after training

30+ years of Wise Owl

Our top 100 clients

This page has 0 threads | Add post

Blogs

Blogs

Finding our blogs useful? Please consider helping us to share more!

Wise Owl have found one SQL structure which slows down code more than any other
If your stored procedures or SQL queries are running slowly, don't bother looking at correlated subqueries, cursors, functions or any of the other usual culprits. There's one SQL syntax which Wise Owl have found to be an order of magnitude worse than any other, as this blog shows.

Posted by Andy Brown on 13 May 2019

Stored procedure running slowly? Here's the likely culprit

In general our stored procedures aren't that time-critical: we just don't have enough data for them to matter. But one procedure was seizing up our internal Intranet. It used lots of possible culprits:

Possible culprit	Notes
User-defined functions	Everything I read says that scalar functions are slow.
Multiple criteria	My SQL statement had about 15 criteria in.
Nested functions	One scalar function called another.

I tried removing all of these, but the procedure still took over 10 seconds to run. Finally, I found the culprit - some really sloppy programming, because I'm always short of time when I write these things:

-- use main address if gmail, etc

IF EXISTS (

SELECT *

FROM tblUrlHome AS u

WHERE u.UrlHomeName = @DomainMain

)

SET @email = @EmailAddress

So I changed this to code which is almost as sloppy:

-- use main address if gmail, etc

DECLARE @NumberMain AS int = (

SELECT COUNT(*)

FROM tblUrlHome AS u

WHERE u.UrlHomeName = @DomainMain

)

IF @NumberMain > 0

SET @email = @EmailAddress

The difference? The use of IF EXISTS within a function. Lots of websites will tell you the SQL structures you should avoid, but this seems to me to be an order of magnitude slower than correlated subqueries, dynamic SQL, scalar functions, cursors and any of the other usual suspects.

Some other pages relevant to the above blog include:

This blog has 0 threads

Head office

Kingsmoor House

Railway Street

GLOSSOP

SK13 2AA

London

Landmark Offices

99 Bishopsgate

LONDON

EC2M 3XD

Manchester

Holiday Inn

25 Aytoun Street

MANCHESTER

M1 3AE

Visual C# training

Excel training courses

Power Automate training

Power BI / DAX training

Python training courses

SQL training courses

VBA (macros) training

© Wise Owl Business Solutions Ltd 2024. All Rights Reserved.

Terms & Conditions

End of small page here

Please be aware that our website uses cookies!

I'm OK with this Tell me more ...