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SQL Server 2008
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queries running faster after cached in the...
queries running faster after cached in the query plan
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erics44
erics44
Posted Monday, September 10, 2012 3:53 AM
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Hi
Probly a quick on to answer
Is it the more times you run a query the faster it gets? or is the query plan cached once after the first run and so the query speed should be fairly consitent from the 2nd run onwards?
this probably could have been worded better.
Thanks in advance
Post #1356621
Stewart "Arturius" Campbell
Stewart "Arturius" Campbell
Posted Monday, September 10, 2012 4:18 AM
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When a query is run for the first time, the plan needs to be generated and cached, taking a bit of time.
Thereafter any execution will use the existing cache, provided it has not been flushed. Thus the next execution will be faster than the first, but roughly the same for each execution thereafter
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erics44
erics44
Posted Monday, September 10, 2012 4:29 AM
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thanks for clearing that up :)
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GilaMonster
GilaMonster
Posted Monday, September 10, 2012 4:31 AM
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The latter, assuming that the query continues to process the same number of rows each time.
Gail Shaw
Microsoft Certified Master: SQL Server 2008, MVP
SQL In The Wild
: Discussions on DB performance with occasional diversions into recoverability
We walk in the dark places no others will enter
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Post #1356634
erics44
erics44
Posted Monday, September 10, 2012 4:36 AM
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that is unlikely
rows are added continually through out the day to several of the tables involved in the query in question
Post #1356638
erics44
erics44
Posted Monday, September 10, 2012 4:39 AM
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just to clarify
will the query plan only be cached when the data in the query is consitent? so each time the data changes the query needs to recache?
Post #1356642
GilaMonster
GilaMonster
Posted Monday, September 10, 2012 4:40 AM
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No. Only when the data changes are sufficient to trigger a statistics update, as stats updates invalidate cached plans.
Gail Shaw
Microsoft Certified Master: SQL Server 2008, MVP
SQL In The Wild
: Discussions on DB performance with occasional diversions into recoverability
We walk in the dark places no others will enter
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Post #1356644
HowardW
HowardW
Posted Monday, September 10, 2012 4:40 AM
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It depends. You save on compile time after the first run (which is relatively small for most queries), but the majority of variation in CPU/IO time is more likely to be due to whether the pages to satisfy the query are in the buffer pool or not, which will depend on the amount of memory, the frequency that query is executed, other workloads on the system and the runtime parameters.
Elapsed time can obviously be much more variable based on other load on the server, concurrency,locking etc.
Post #1356645
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