datetime - Why some dates give worse performance than other in MS SQL Server -

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i have query in ms sql server asking name , date-related information, depending on 2 dates, start- , enddate.

the problem is, i´m not getting same performance. whenever request between dates;

2010-07-01 00:00:00.000 , 2011-07-21 23:59:59.999

the performance excellent. result within mseconds. when request between these dates, example, 

2011-07-01 00:00:00.000 , 2011-07-21 23:59:59.999

the performance is.. less good, taking between 20-28 seconds each query. note how dates giving performance more year between, while latter 20 days.

is there particular reason (maybe related how datetime work) this?

edit: query,

select ename,      sum(case date when 0 1 else 0 end) u2,      sum(case date when 1 1 else 0 end) b_2_4,      sum(case date when 2 1 else 0 end) b_4_8,      sum(case date when 3 1 else 0 end) b_8_16,      sum(case date when 4 1 else 0 end) b_16_24,      sum(case date when 5 1 else 0 end) b_24_48,      sum(case date when 6 1 else 0 end) o_48,      sum(case date when 7 1 else 0 end) status,      avg(avg) avg,      sum(date) total       (select ename,              (case                  when status = 'Öppet' 7                  when date < 48                      (case when date between 0 , 2 0                      when date between 2 , 4 1                      when date between 4 , 8 2                      when date between 8 , 16 3                      when date between 16 , 24 4                      when date between 24 , 48 5                      else - 1 end)              else 6 end) date,              date avg              (select datediff(hour, cases.date, status.date) date,          extern.name ename,          status.status              cases inner join         status on cases.id = status.caseid                  , status.date =                     (select max(date) expr1                     status status_1                     (caseid = cases.id)                     group caseid) inner join                     extern on cases.owner = extern.id                     (cases.org = 'expert')                          , (cases.date between '2009-01-15 09:48:25.633'                          , '2011-07-21 09:48:25.633'))     derivedtbl_1)  derivedtbl_2 group ename order ename

(parts of) tables:

extern     -id (->cases.owner)     -name cases     -owner (->extern.id)     -id (->status.caseid)     -date (case created @ date) status     -caseid (->cases.id)     -status     -date (can multiple, max(status.date) gives date when       status last changed)

i have thought statistics issue.

when selecting recent dates these may unrepresented in statistics yet threshold has not yet been reached trigger auto updating.

see blog post example.


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