SQL Query Optimisation

Posted on 2012-04-03
Last Modified: 2012-04-04
I have a query that is selecting the last known Latitude and Longitude along with the Name from a table. At the moment the query is taking over 10 seconds to execute with only 18 Units to sort through, I can see this being a rather large issue when there are 300+ Units.

So I need a way to optimise this Query to execute much faster.

                          (SELECT     TOP (1) Latitude
                            FROM          tGPSData AS T3
                            WHERE      (VesselID = T1.ID)
                            ORDER BY ID DESC) AS LastLat,
                          (SELECT     TOP (1) Longitude
                            FROM          tGPSData AS T3
                            WHERE      (VesselID = T1.ID)
                            ORDER BY ID DESC) AS LastLon
FROM         tUser INNER JOIN
                      tUserVesselMap ON tUser.ID = tUserVesselMap.iUserID INNER JOIN
                      tVesselMaster AS T1 ON tUserVesselMap.iVesselID = T1.ID
WHERE     (T1.bEnabled = 1) AND (tUser.sUsername = @Username)
Question by:mgordon-spi
  • 4
  • 2
  • 2
LVL 69

Accepted Solution

Scott Pletcher earned 500 total points
ID: 37803956
For *this* query -- not taking into account any other workload on the tables -- you need to have these indexes:

tUser (sUsername) --preferably clustered
tUserVesselMap (iUserID)
tVesselMaster (ID)
tGPSData (VesselID, bEnabled ) --if bEnabled is bit, make it a tinyint instead

Author Comment

ID: 37804014
I'm sorry - I dont quite understand your response?

I was more referring to the fact that I was using two nested select statements to retrieve the last record in the GPS table for each vessel. I was wondering if there was a faster way to do that?

The GPSData table holds hundreds of thousands of records for many vessels.

Untimately I am trying to retrieve a list of vessels and their last known location. This query achieves that but takes about 20 seconds to execute.
LVL 75

Expert Comment

by:Anthony Perkins
ID: 37804200
I was wondering if there was a faster way to do that?
Yes, as Scott has pointed out it is called indexes.  Until you have confirmed the existence of those indexes, there is not a lot we can add...
Back Up Your Microsoft Windows Server®

Back up all your Microsoft Windows Server – on-premises, in remote locations, in private and hybrid clouds. Your entire Windows Server will be backed up in one easy step with patented, block-level disk imaging. We achieve RTOs (recovery time objectives) as low as 15 seconds.


Author Comment

ID: 37804406
I'm sorry but this might be stretching my SQL abilities. Jsut to confirm we are not talking about primary keys?

I have not created indexes before. Will the creation of them help with query execute speed?
LVL 75

Expert Comment

by:Anthony Perkins
ID: 37804525
Jsut to confirm we are not talking about primary keys?
Maybe, but not necessarily.

Will the creation of them help with query execute speed?
Yes.  Although you should be aware that too many indexes can slow down INSERTs and UPDATEs, But I suspect you are a long way from that.

Author Comment

ID: 37804535
So it is as simple as creating indexes (the ones mentioned above) and I should see a difference?

Author Closing Comment

ID: 37804592
Your help has directed me to learn a whole new aspect of SQL. Indexing!

LVL 69

Expert Comment

by:Scott Pletcher
ID: 37806452
>> creating indexes ... and I should see a difference? <<

Yes, definitely!

Featured Post

Best Practices: Disaster Recovery Testing

Besides backup, any IT division should have a disaster recovery plan. You will find a few tips below relating to the development of such a plan and to what issues one should pay special attention in the course of backup planning.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

Title # Comments Views Activity
SQL - MAX of multiple date columns in a SINGLE Row 4 29
backup and restore 21 27
SQL query and VBA 5 41
SQL Server / Update DB? 22 36
How to leverage one TLS certificate to encrypt Microsoft SQL traffic and Remote Desktop Services, versus creating multiple tickets for the same server.
This article explains how to reset the password of the sa account on a Microsoft SQL Server.  The steps in this article work in SQL 2005, 2008, 2008 R2, 2012, 2014 and 2016.
This video shows, step by step, how to configure Oracle Heterogeneous Services via the Generic Gateway Agent in order to make a connection from an Oracle session and access a remote SQL Server database table.
Viewers will learn how to use the INSERT statement to insert data into their tables. It will also introduce the NULL statement, to show them what happens when no value is giving for any given column.

792 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question