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How to Reduce the Logical Reads, to imporve the Performance of the Query Expand / Collapse
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Posted Wednesday, September 17, 2008 10:29 AM
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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
Posted Wednesday, September 17, 2008 10:47 AM


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From BOL

Logical reads - number of pages read from the data cache
Physical 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 filtered by getting a smaller set prior to pulling that data then you can always cut down on reads that way. With improved indexing, specifically with covering indexes, you can reduce 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 the queries that are running in your production environment. A query like the following (not mine but got this from somewhere else so, no credit here) should help in getting those queries.

SELECT TOP 20
SUBSTRING(qt.text, (qs.statement_start_offset/2)+1, ((
CASE qs.statement_end_offset
WHEN -1 THEN DATALENGTH(qt.text)
ELSE qs.statement_end_offset
END - 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_plan
FROM
sys.dm_exec_query_stats qs CROSS APPLY sys.dm_exec_sql_text(qs.sql_handle) qt
CROSS APPLY sys.dm_exec_query_plan(qs.plan_handle) qp
WHERE
qt.encrypted=0
ORDER BY
qs.last_logical_reads DESC


David

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Post #571183
Posted Wednesday, September 17, 2008 10:16 PM
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Hi,

Thank you for your valuable information.

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


Regards,
Santosh
Post #571459
Posted Wednesday, September 17, 2008 10:53 PM
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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, proper indexes, etc.

You need to open a good book about relational DB design and follw its recommendations.
Post #571473
Posted Tuesday, January 5, 2010 3:04 PM
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Yes, indexing helps but reducing logical reads does not help performance. maybe a tiny bit

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

Dr. Inner Join
Post #842453
Posted Tuesday, January 5, 2010 3:11 PM


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


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Posted Tuesday, January 5, 2010 3:21 PM
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Posted Tuesday, January 5, 2010 3:37 PM
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papapumpy (1/5/2010)
Yes, indexing helps but reducing logical reads does not help performance. maybe a tiny bit

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

Dr. Inner Join



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

Using SET STATISTICS IO ON when optimizing queries can help you quickly, by looking 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 sections of your query.

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




John Rowan

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Post #842470
Posted Wednesday, January 6, 2010 6:21 AM
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Dear All,

Thank you very much for your information.
Post #842715
Posted Wednesday, February 8, 2012 2:15 AM
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Summary Info:

Logical Reads : Reading Data pages from Cache
Physical Reads : Reading Data pages from Hard Disk
Buffer 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 data cache to process query. It is very possible that logical read will access same data pages many times, so count of logical read value may be higher than actual number of pages in a table. Usually the best way to reduce logical read is to apply correct index or to rewrite the query.

Physical Reads
Physical read indicates total number of data pages that are read from disk. In case no data in data cache, the physical read will be equal to number of logical read. And usually it happens for first query request. And for subsequent same query request the number will be substantially decreased because the data pages have been in data cache.

Buffer Cash Hit Ratio
Buffer hit ratio will be calculated based on these two kinds of read as the following formula: (logical reads – physical reads)/logical read * 100%. The high buffer hit ratio (if possible to near 100%) indicates good database performance on SQL Server level. So use information from physical read and buffer hit ratio to measure performance in server level and logical read to measure individual query level

Thanks,
Dinesh Babu Verma
Post #1248790
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