8/26/2014 How to Reduce the Logical Reads, to imporve the Performance of the Query

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Home » SQL Server 7,2000 » T-SQL » How to Reduce the Logical Reads, to imporve...

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santoooo Posted Wednesday, September 17, 2008 10:29 AM

SSC Journeyman

Group: General ForumMembers Last Login: Wednesday,July 10, 2013 10:13 PM Points: 83, Visits: 253

Hi,

I heard abt the Logical/ Physical Reads In Sql Server.

Could any one please explain me

What are Logical Reads?What are Physical Reads?

and also heared, Less no of Logical Reads improves the Query performance.

please explain me how can we reduce the logical reads..?

Thanks,Santosh

Post #571160

David Benoit Posted Wednesday, September 17, 2008 10:47 AM

SSCrazy

From BOL

Logical reads - number of pages read from the data cachePhysical reads - number of pages read from disk

To reduce reads you need to look at a couple of things, first being query design,secondly being indexing. If your query is pulling a large number of records that could be

8/26/2014 How to Reduce the Logical Reads, to imporve the Performance of the Query

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Group: General ForumMembers Last Login: Friday,August 22, 2014 12:19PM Points: 2,107, Visits:3,582

filtered by getting a smaller set prior to pulling that data then you can always cut downon reads that way. With improved indexing, specifically with covering indexes, you canreduce the number of pages that are being read as well.

All of this is just a basic guideline but should be applied to the analysis of all thequeries that are running in your production environment. A query like the following (notmine but got this from somewhere else so, no credit here) should help in getting thosequeries.

SELECT TOP 20SUBSTRING(qt.text, (qs.statement_start_offset/2)+1, ((CASE qs.statement_end_offsetWHEN -1 THEN DATALENGTH(qt.text)ELSE qs.statement_end_offsetEND - qs.statement_start_offset)/2)+1) AS SQLText, qs.execution_count, qs.total_logical_reads, qs.last_logical_reads, qs.min_logical_reads, qs.max_logical_reads, qs.total_elapsed_time, qs.last_elapsed_time, qs.min_elapsed_time, qs.max_elapsed_time, qs.last_execution_time, qp.query_planFROMsys.dm_exec_query_stats qs CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) qtCROSS APPLY sys.dm_exec_query_plan(qs.plan_handle) qpWHEREqt.encrypted=0ORDER BYqs.last_logical_reads DESC

David

@SQLTentmaker

SQL Tentmaker

“He is no fool who gives what he cannot keep to gain that which he cannot lose” - JimElliot

Post #571183

santoooo Posted Wednesday, September 17, 2008 10:16 PM

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Group: General ForumMembers Last Login: Wednesday,July 10, 2013 10:13 PM Points: 83, Visits: 253

Hi,

Thank you for your valuable information.

So, to reduce the Logical Reads we need proper indexing for the tables.

Regards,Santosh

Post #571459

Sergiy Posted Wednesday, September 17, 2008 10:53 PM

SSCarpal Tunnel

Group: General ForumMembers Last Login: Monday,August 18, 2014 9:16PM Points: 4,576, Visits:8,347

santoooo (9/17/2008)

So, to reduce the Logical Reads we need proper indexing for the tables.

Regards,Santosh

No, you need proper database.Proper schema, proper data normalisation, proper data types, proper keys, properindexes, etc.

8/26/2014 How to Reduce the Logical Reads, to imporve the Performance of the Query

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You need to open a good book about relational DB design and follw itsrecommendations.

Post #571473

papapumpy Posted Tuesday, January 5, 2010 3:04 PM

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Group: General ForumMembers Last Login: Tuesday,January 5, 2010 3:20 PMPoints: 2, Visits: 1

Yes, indexing helps but reducing logical reads does not help performance. maybe a tinybit

read some article about buffer hit ratio and data cache. physical reads is the one takessignificantly longer time. i actually wrote my final term paper about that when i was inCS in my college long time ago.

Dr. Inner Join

Post #842453

GilaMonster Posted Tuesday, January 5, 2010 3:11 PM

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You did notice that this thread is over a year old?

Gail ShawMicrosoft Certified Master: SQL Server 2008, MVPSQL In The Wild: Discussions on DB performance with occasional diversions intorecoverability

We walk in the dark places no others will enterWe stand on the bridge and no one may pass

Post #842458

papapumpy Posted Tuesday, January 5, 2010 3:21 PM

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LOL!

Post #842463

John Rowan Posted Tuesday, January 5, 2010 3:37 PM

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Group: General ForumMembers Last Login: Tuesday,August 19, 2014 10:01AM Points: 3,844, Visits:3,841

papapumpy (1/5/2010)

Yes, indexing helps but reducing logical reads does not help performance. maybe atiny bit

read some article about buffer hit ratio and data cache. physical reads is the onetakes significantly longer time. i actually wrote my final term paper about that when iwas in CS in my college long time ago.

Dr. Inner Join

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The thread may be over a year old, but I have to add a correction as to not confuse newreaders. Reducing logical reads [b]does[/do] improve query performance! In fact, logicalreads is one of the best metric to look at when optimizing a query. Logical readsrepresents the data that must be read from cache in order to process a query. The lessamount of data needed to process a query, the better it performs.

Using SET STATISTICS IO ON when optimizing queries can help you quickly, bylooking at the logical reads counter, determine where to focus your tuning efforts.Seeing the logical reads by object can quickly point you to poorly performing sectionsof your query.

So to recap, I think the statement that "reducing logical reads does not helpperformance. maybe a tiny bit" is not true.

John Rowan

============================================================================================================Forum Etiquette: How to post data/code on a forum to get the best help - byJeff Moden

Post #842470

santoooo Posted Wednesday, January 6, 2010 6:21 AM

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Group: General ForumMembers Last Login: Wednesday,July 10, 2013 10:13 PM Points: 83, Visits: 253

Dear All,

Thank you very much for your information.

Post #842715

Dinesh Babu

Verma Posted Wednesday, February 8, 2012 2:15 AM

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Group: General ForumMembers Last Login: Tuesday,April 2, 2013 1:37 AM Points: 6, Visits: 83

Summary Info:

Logical Reads : Reading Data pages from CachePhysical Reads : Reading Data pages from Hard DiskBuffer Cach Hit Ratio logical reads – physical reads)/logical read * 100%

Details Info:

Logical Reads:Logical read indicates total number of data pages needed to be accessed from datacache to process query. It is very possible that logical read will access same datapages many times, so count of logical read value may be higher than actual number ofpages in a table. Usually the best way to reduce logical read is to apply correct index orto rewrite the query.

Physical Reads Physical read indicates total number of data pages that are read from disk. In case nodata in data cache, the physical read will be equal to number of logical read. Andusually it happens for first query request. And for subsequent same query request thenumber will be substantially decreased because the data pages have been in datacache.

Buffer Cash Hit RatioBuffer hit ratio will be calculated based on these two kinds of read as the followingformula: (logical reads – physical reads)/logical read * 100%. The high buffer hit ratio (ifpossible to near 100%) indicates good database performance on SQL Server level. So

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use information from physical read and buffer hit ratio to measure performance in serverlevel and logical read to measure individual query level

Thanks,Dinesh Babu Verma

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