Home/ Questions/Q 4382548
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-21T12:47:35+00:00

So really my question is WHY this worked. Anyway, I had this query that

  • 0

So really my question is WHY this worked.

Anyway, I had this query that does a few inner joins, has a where clause and does an order by on a nvarchar column. If I run the query WITHOUT order by, the query takes less than a second. If I run the query WITH order by, it takes 12 seconds.

Now I had a great idea and changed all the INNER JOINs to LEFT JOINs. And also included the ORDER BY clause. That took less than a second. So I remembered the difference between LEFT JOINs and INNER JOINs. INNER JOINs check for NULL and LEFT JOINs don’t. So I went into the table design and unchecked “Allow Nulls”. Now I run the query WITH INNER JOINs and a ORDER BY clause and the query takes less than a second. WHY?

From what I understand, the FROM, JOINS, WHERE, then SELECT clauses should run first and return a result set. Then the ORDER BY clause runs at the very end on the resultant record set. Therefore the query should have taken AT MOST a second, yes, even with the column allowing nulls. So why would the query take less than a second WITHOUT the order by clause, but take 12 seconds WITH order by clause? That doesn’t make sense to me.

Query below:

SELECT PlanInfo.PlanId, PlanName, COALESCE(tResponsible, '') AS tResponsible, Processor, CustName, TaskCategoryId, MapId, tEnd,
CASE MapId WHEN 9 THEN 1 ELSE 2 END AS sor
FROM PlanInfo INNER JOIN [orders].dbo.BaanOrders_Ext ON PlanInfo.PlanName = [orders].dbo.BaanOrders_Ext.OrderNo
INNER JOIN [orders].dbo.BaanOrders ON PlanInfo.PlanName = [orders].dbo.BaanOrders.OrderNo
INNER JOIN Tasks ON PlanInfo.PlanId = Tasks.PlanId
INNER JOIN EngSchedToTimingMap ON Tasks.CatId = EngSchedToTimingMap.TaskCategoryId
WHERE (MapId = 9 OR MapId = 11 or MapId = 13 or MapId = 15)
AND([orders].dbo.BaanOrders_Ext.Processor = 'metest' OR tResponsible = 'metest')
ORDER BY PlanInfo.PlanId
Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    I would have to guess that it is due to having an index on PlanInfo.PlanId, on which you are sorting.

    SQL Server could streamline collection so that it follows the index and build the rest of the columns along that order. When the field is NULLable, the index cannot be used for sorting, because it will not contain the NULL values, which incidentally come first, so it decides to optimize along a different path.

    Showing the Execution Plan always helps. Either paste the images of the plans, or just show the text-mode plans, i.e. add the line above the query, then execute it

    SET SHOWPLAN_TEXT ON;
<the query>
    • 0