Many Reads

Mar 25, 2008

Hi!
I was assigned to solve performance problems for an application. I fired up Sql Server profiler and started a trace. Downloaded Sql Server Trace Analyzer. It's a trial version so it's very limited. What I found is that one stored procedure generates almost 400 000 reads everytime it's used and it's used everytime the user wants to see his orders. I've tried to translate the t-sql to english from swedish, it looks something like this:


select top 100
o.orderid,
o.name,
o.latestdeldate,
os.name as OrderStatus,
os.orderstatusID,
p.placeID,
p.name as place,
p.address,
p.city,
a.name as worktype,
noOfActions=(select count(*) from actions a where a.order_orderid=o.orderid),
noOfServiceObjects = (select count(*) from Serviceobject s, Actions a where s.Place_PlaceID = o.Place_PlaceID and a.order_orderid = o.orderid and a.Serviceobject_serviceobjectid = s.serviceobjectid),
...
...
...


It has 8 select count(*) in the select statement then in the where statement it has 2 more select count(*).

I know it's very difficult for you to come up with a solution but do you know a better way than to use select count(*) everywhere? The count is used for to show different status flags on the website.

/Magnus

Jesus saves. But Gretzky slaps in the rebound.

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May 1, 2008

server: QAT on clustering server ----> 23 seconds
----------------------------------------------------
SS 2000 developer edition SP4
win NT 5.2 (3790) SP4
MeM 7935 MB
processors 4
root directory C:program files...
use a fixed memeroy size 640 MB

reserve physical memory for sql server
minimum query memory 1024 kb

use all available processors
minimum query plan threshold for considering 5

PROFILER READS = 5234




server: MILLER ----> 3 seconds
----------------------------------------------------
SS 2000 developer edition no service pack
win NT 5.2 (3790) SP4
MeM 2047 MB
processors 4
root directory f:MSSQL$INAQAT

dynamically configure sql server memory

use all available processors
minimum query plan threshold for considering 5
PROFILER READS = 598





----------------------------------------------------
Making story short. I got an application that hits only 1 database called RECORDS. I'm getting different duration when running an application. 23 and 3 seconds.
Same database, same objects and same application.
SERVER QAT is our staging server, means lots of databases
SERVER MILLER is just a server i just assembled, means just one database (RECORDS).

Not sure if it's because it's a clustering server that is causing the issue nor the reads. If its the reads, what is causing it? Do you think is the how the memory is configured?. Will the experts pls stand up?

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Jul 18, 2006

So I€™m at a dead-end looking for the reason behind the following behavior. Just to make sure no one misses it, the 'behavior' is the difference in the number of reads between using sp_executesql and not.

The following statements are executed against a SQL 2000 database that contains >1,000,000 records in the act_item table. They are run using Query Analyzer and the Duration and Reads come from SQL Profiler

SQL 1:
exec sp_executesql N'update act_item set Priority = @Priority where activity_code = @activity_code', N'@activity_code nvarchar(40),@Priority int', @activity_code = N'46DF335F-68F7-493F-B55E-5F9BC6CEBC69', @Priority = 0

Reads: ~22000
Duraction: 250-350 ms

SQL 2:
DECLARE @Priority int
DECLARE @Activity_Code char(36)

SET @Priority = 0
SET @Activity_Code = '46DF335F-68F7-493F-B55E-5F9BC6CEBC69'
update act_item set Priority = @Priority where activity_code = @activity_code

Reads: ~160
Duration: 0 ms

Random information:

Activity_code is an indexed field on the table, although it is not the primary key. There are a total of four indexes on the table, none of which include the priority as one of the fields.
There are two triggers on the table, neither of which is executed for this SQL statement (there is an IF UPDATE(fieldname) surrounding the code in the trigger)
There are no foreign relationships
I checked (using perfmon) to see if a compilation/recompilation was happening. No it's not.
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TIA

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I'm trying to insert all the rows from a table to a new table.
(insert A select * from AA)
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First I created a unique clustered index and the reads shows (3258445), then I created a non clustered index expecting to have lower reads. Instead the reads shows (10253548).

I read creating indexes helps reduce reads. But it's not happening.
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=============================
http://www.sqlserverstudy.com

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Hi,

Can any of can explain, what the "Reads" column in Profiler exactly mean ? I'm not comfortable with the explanation given in BOL.


"The number of read operations on the logical disk that are performed by the server on behalf of the event. These read operations include all reads from tables and buffers during the statement's execution"

For the same procedure with same parameters, if the server is not loaded much, the Reads are in a few hundreds, but when there are more than 1000 concurrent users, why it is going to millions ? What other parameters affecting this reads ? And how can I reduce it ?

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Hi,

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The database has isolation set to "READ COMMITTED".

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1.) Start a transaction (for doing a write)

2.) Do a read before the transaction (for doing the write) is committed (e.g. sqlCommand2.ExecuteReader()).

--> the code will appear to lock-up (then time out).

I see the same behavior if I step through the "write" code with the debugger (to a point after the transaction is started, but before it is committed), and run a "SELECT * FROM" type query from Microsoft SqlServer Management Studio.

Following is the code sample demonstates the issue.

Thoughts on how to resolve the issue (to let me do "read committed" reading of the database table)?

Thanks!

Andy







Module Transaction

Sub Main()

Dim exception1 As Exception

Try

' Create/Open Database Connection

Dim sqlConnection1 As New System.Data.SqlClient.SqlConnection("Server=GRB-AB;Database=Transaction;Trusted_Connection=True;")

sqlConnection1.Open()

' Start transaction

Dim sqlTransaction1 As System.Data.SqlClient.SqlTransaction = sqlConnection1.BeginTransaction()

' Set Parent record

Dim sqlCommand1 As New System.Data.SqlClient.SqlCommand("INSERT INTO Parent (Name) VALUES ('ParentValue');", sqlConnection1)

sqlCommand1.Transaction = sqlTransaction1

sqlCommand1.ExecuteNonQuery()

' Get Id from parent record (note: this code assumes the table was empty when this program starts)

sqlCommand1 = New System.Data.SqlClient.SqlCommand("SELECT Id FROM Parent;", sqlConnection1)

sqlCommand1.Transaction = sqlTransaction1

Dim parentId As Integer = CType(sqlCommand1.ExecuteScalar(), Integer)



'

' Do reading test to test concurrently reading table being written to

'

' Create/Open Database Connection for reading test

Dim sqlConnection2 As New System.Data.SqlClient.SqlConnection("Server=GRB-AB;Database=Transaction;Trusted_Connection=True;")

sqlConnection2.Open()

Dim sqlCommand2 As New System.Data.SqlClient.SqlCommand("SELECT Id FROM Parent;", sqlConnection2)

sqlCommand2.ExecuteReader()

Dim i As Integer

While (sqlCommand2.ExecuteReader.Read = True) ' <===== LOCKS UP HERE **************

i = i + 1

End While

'

' End reading test

'



' Set child record

sqlCommand1 = New System.Data.SqlClient.SqlCommand( _

"INSERT INTO Child (Name, ParentId) VALUES ('ChildValue', " & parentId.ToString & ");", sqlConnection1)

sqlCommand1.Transaction = sqlTransaction1

sqlCommand1.ExecuteScalar()

' Either 1.) commit transaction OR 2.) rollback transaction

Dim test As Boolean = False

If test = False Then

sqlTransaction1.Commit()

Else

sqlTransaction1.Rollback()

End If

sqlConnection1.Close()

sqlConnection2.Close()

Catch ex As Exception

exception1 = ex

End Try

End Sub

End Module

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Thanks again!
mike123



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Table 'WebProxyLog'. Scan count 3, logical reads 873660, physical reads 3493, read-ahead reads 505939, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

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Code Snippet
-- =================== Table and Clustered index ===========================
CREATE TABLE [dbo].[WebProxyLog](
[ClientIP] [bigint] NULL,
[ClientUserName] [nvarchar](514) NULL,
[ClientAgent] [varchar](128) NULL,
[ClientAuthenticate] [smallint] NULL,
[logTime] [datetime] NULL,
[servername] [nvarchar](32) NULL,
[DestHost] [varchar](255) NULL,
[DestHostIP] [bigint] NULL,
[DestHostPort] [int] NULL,
[bytesrecvd] [bigint] NULL,
[bytessent] [bigint] NULL,
[protocol] [varchar](12) NULL,
[transport] [varchar](8) NULL,
[operation] [varchar](24) NULL,
[uri] [varchar](2048) NULL,
[mimetype] [varchar](32) NULL,
[objectsource] [smallint] NULL,
[rule] [nvarchar](128) NULL,
[SrcNetwork] [nvarchar](128) NULL,
[DstNetwork] [nvarchar](128) NULL,
[Action] [smallint] NULL,
[WebProxyLogid] [int] IDENTITY(1,1) NOT NULL,
CONSTRAINT [pk_webproxylog_webproxylogid] PRIMARY KEY CLUSTERED
(
[WebProxyLogid] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

-- =================== Nonclustered Index ===========================

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(
[Action] ASC
)
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[uri],
[mimetype],
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-- =================== Query which is called regularly on the table ===========================

SELECT [User] = CASE
WHEN LEFT(clientusername,3) = domain' THEN RIGHT(clientusername,LEN(clientusername) - 3)
ELSE clientusername
END,
logtime AS [Date],
desthost AS [Site],
uri AS [Actual Site]
FROM webproxylog
WHERE CONVERT(Datetime,CONVERT(VarChar(25),logtime,106),106) BETWEEN '20 apr 2008' AND '14 may 2008'
AND(RIGHT(uri,4) NOT IN('.css','.jpg','.gif','.png','.bmp','.vbs'))
AND (RIGHT(uri,3) NOT IN('.js'))
AND LEFT(mimetype,6) = 'text/h'
AND (uri NOT LIKE '%sometext.local%')
AND (uri NOT LIKE '%sometext.co.uk%')
AND [action] = 9
AND (clientusername IN ('USERNAME'))
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PS There are 60,078,605 rows in the table

Please help!

Many Thanks

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When I'm executing



SELECT * FROM [table] WHERE ClientId = [id] AND ProductId IN (1,2,3,5,7,9,20)

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With this information, which version would you say is fastest and why?

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