Comments on: A TOP Query

By: Rob Farley

Rob Farley — Wed, 14 May 2014 04:06:57 +0000

Hi Nick,
Yes, that method works very well for finding gaps too.
Rob

By: Nick

Nick — Tue, 13 May 2014 15:34:31 +0000

Hi Rob,
yes, you’re right of course.
Though one thing is to identify the extents (upper and lower limit) of gaps, another is to identify the actual missing keys within the gaps, which is what mine does.
I gave it a try with ROW_NUMBER, but before I got it working I suddenly also remembered the new LAG and LEAD functions. This is what I got working:
;with cte as
(
select
lag(key) over(order by key) as previous_key,
key as current_key
from tbl
)
select
previous_key as gap_start,
current_key as gap_end
from cte
where current_key-previous_key > 1
order by current_key

By: Rob Farley

Rob Farley — Tue, 13 May 2014 03:56:45 +0000

Hi Nick,
Using ROW_NUMBER() is a better option for generating a table of numbers, as it realises it can stop if you only need a few dozen rows. Your version doesn’t get to leverage that. But actually, to find gaps, you don’t even need a table of numbers. Just use ROW_NUMBER(), and look for the times when the difference between the values with gaps and the rownum increases (indicating that things have been missed).
Hope this helps,
Rob

By: Nick

Nick — Mon, 12 May 2014 18:45:48 +0000

This is my absolute favourite sequence generator. Employed here to find gaps in a primary key.
No idea about the plan though…
;with
digits (i) as(
select 1 as i
union all select 2
union all select 3
union all select 4
union all select 5
union all select 6
union all select 7
union all select 8
union all select 9
union all select 0
),
sequence (i) as (
select D1.i
+ (10*D2.i)
+ (100*D3.i)
+ (1000*D4.i)
+ (10000*D5.i)
+ (100000*D6.i)
from digits as D1,
digits as D2,
digits as D3,
digits as D4,
digits as D5,
digits as D6
)
select
(select top 1 admcdt from adm where admkey < i order by admkey desc) as before,
i as ‘missing adm’,
(select top 1 admcdt from adm where admkey > i order by admkey asc) as after
from sequence
where i between 1 and (select max(admkey) from adm)
and not exists (select 1 from adm where admkey=i)
order by i
All credit to — http://drsql.spaces.live.com/blog/cns!80677FB08B3162E4!1232.entry

By: Rob Farley

Rob Farley — Tue, 12 Mar 2013 02:05:18 +0000

All the scripts here are from SQL 2012. I would suggest that 4199 should be applied by default on new servers, yes. By all means do some testing though. http://support.microsoft.com/kb/974006 says: "Starting with Microsoft SQL Server 2000 Service Pack 3 (SP3), the SQL Server query processor team adopted a policy that any hotfix that could potentially affect the execution plan of a query must be controlled by a trace flag. Except for fixes to bugs that can cause incorrect results or corruption, these hotfixes are turned off by default, and a trace flag is required to enable the fix. This policy change helps avoid unexpected changes to the execution plan that may occur when a hotfix or a security update is installed." So if something in the QO has been deemed worthy of a hotfix, you still need to turn on a traceflag to get the benefit of that fix. 4199 is how you turn these things on. I'd be surprised if you found a situation where 4199 was detrimental.

By: tobi

tobi — Tue, 12 Mar 2013 01:57:25 +0000

So what’s the recommendation regarding that trace flag? Apply it by default on new servers?
Is it meaningful to 2012? the article does not list anything regarding 2012.