Index Seek Vs Scan
Nov 4, 2002
I have created a nolock view off a table to prevent locks. I have users coming in through MS Access that have switched their queries to run against the views. Now we are noticing that queries that used to run as a clustered index seek against the table are running as a clustered index scan against the table and performance in the queries has dropped.
Is there any way that the same query that hits the view instead of the table can be made to run faster or at least use the index seek?
Thanks,
Steve
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Oct 4, 2007
Hey,
what is the difference between Table Scan und Index Scan?
I find no difitions in the internet
Finchen
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Sep 21, 2007
Hi,
I want to know wht is a
TABLE SCAN
INDEX SCAN
INDEX SEEKand When they are used, Wht is the difference between all these.????
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Oct 20, 2006
please explain the differences btween this logical & phisicall operations that we can see therir graphical icons in execution plan tab in Management Studio
thank you in advance
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Mar 1, 2004
I have a really strange problem.
I execute this query:
declare @cid int
set @cid = 2003227
select * from sales s, product p where p.product_Id = s.product_Id and customer_id = @cid
select * from sales s, product p where p.product_Id = s.product_Id and customer_id = @cid or @cid = 0
3 Million rows in sales, 120000 in product.
The first does and index seek, the second an index scan.
The execution plan reports that the scan takes 99.87% of the cost, and the seek takes 0.13%
This problem obviously gets worse the bigger the dataset / query /etc.
The reason I query this, is because it never used to take this long to do index scans. Is there something i can change, something i can fix?
Any help would be appreciated.
Josh
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Nov 14, 2006
the query:
SELECT a.AssetGuid, a.Name, a.LocationGuid
FROM Asset a WHERE a.AssociationGuid IN (
SELECT ada.DataAssociationGuid FROM AssociationDataAssociation ada
WHERE ada.AssociationGuid = '568B40AD-5133-4237-9F3C-F8EA9D472662')
takes 30-60 seconds to run on my machine, due to a clustered index scan on our an index on asset [about half a million rows]. For this particular association less than 50 rows are returned.
expanding the inner select into a list of guids the query runs instantly:
SELECT a.AssetGuid, a.Name, a.LocationGuid
FROM Asset a WHERE a.AssociationGuid IN (
'0F9C1654-9FAC-45FC-9997-5EBDAD21A4B4',
'52C616C0-C4C5-45F4-B691-7FA83462CA34',
'C95A6669-D6D1-460A-BC2F-C0F6756A234D')
It runs instantly because of doing a clustered index seek [on the same index as the previous query] instead of a scan. The index in question IX_Asset_AssociationGuid is a nonclustered index on Asset.AssociationGuid.
The tables involved:
Asset, represents an asset. Primary key is AssetGuid, there is an index/FK on Asset.AssociationGuid. The asset table has 28 columns or so...
Association, kind of like a place, associations exist in a tree where one association can contain any number of child associations. Each association has a ParentAssociationGuid pointing to its parent. Only leaf associations contain assets.
AssociationDataAssociation, a table consisting of two columns, AssociationGuid, DataAssociationGuid. This is a table used to quickly find leaf associations [DataAssociationGuid] beneath a particular association [AssociationGuid]. In the above case the inner select () returns 3 rows.
I'd include .sqlplan files or screenshots, but I don't see a way to attach them.
I understand I can specify to use the index manually [and this also runs instantly], but for such a simple query it is peculiar it is necesscary. This is the query with the index specified manually:
SELECT a.AssetGuid, a.Name, a.LocationGuid
FROM Asset a WITH (INDEX (IX_Asset_AssociationGuid)) WHERE
a.AssociationGuid IN (
SELECT ada.DataAssociationGuid FROM AssociationDataAssociation ada
WHERE ada.AssociationGuid = '568B40AD-5133-4237-9F3C-F8EA9D472662')
To repeat/clarify my question, why might this not be doing a clustered index seek with the first query?
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Sep 19, 2015
I've been having some trouble getting a single-column "varchar(5)" field to reliably use a table seek instead of a table scan. The production table in this case contains 25 million rows. As impressive as it is to scan 25 million rows in 35 seconds, the query should run much faster.
Here's a partial table description:
CREATE TABLE [dbo].[Summaries_MO]
(
[SummaryId] [int] IDENTITY(1,1) NOT NULL,
[zipcode] [char](5) COLLATE Latin1_General_100_BIN2 NOT NULL,
[Golf] [bit] NULL,
[Homeowner] [bit] NULL,
[Code] .....
Typically, this table is accessed with a query that includes:
SELECT ...
FROM SummaryTable
WHERE ixZIP IN (SELECT ZipCode FROM @ZipCodesForMO)
This query insists on using a table scan. I've tried WITH (FORCESEEK) for example, but that just makes the query fail.
As I've investigated this issue I also tried:
SELECT * FROM Summaries WHERE ZipCode IN ('xxxxx', 'xxxxx', 'xxxxx')
When I run this query with 64 or fewer (actual, valid) ZIP codes, the query uses a table seek.But when I give it 65 or more ZIP codes it uses a table scan.
To summarize, the production query always uses a table scan, and when I specify 65 or more ZIP codes the query also uses a table scan. I'm wondering if the data type of the indexed column (Latin1_General_100_BIN2) is somehow the problem. I'll likely try converting the ZIP codes to an integer to see what happens.
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Jan 29, 2006
How can I improve performance of my search if I am looking in a table with more than ten million rows with a "like query"?
Does putting an index mean only telling the computer to start in a particular order?
if I index all the coloums does my search get faster ?
how can I decide on an indexing strategy?
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May 7, 2008
Hi,
Can I use index seek and still keep 'like' 'or' functionality for the following statement?
--prepare test table
create table entity (Id int identity(1,1), FamName varchar(200), LastName varchar(200))
go
create clustered index entity_id on entity (id)
go
create nonclustered index entity_lastName on entity (lastName)
go
insert into entity select famName,FirstName from table
--test index
declare @LastName varchar(10)
set @LastName = 'a'
select FamName
from entity
where @LastName is null or LastName like @LastName --clustered index scan
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Jun 21, 2004
create table t1(a varchar(50) , b varchar(50))
create index i1 on t1(a)
create index i2 on t1(b)
create view v1
as
select * from t1 where isnull(a,b) = 'test'
select * from v1
The above SQL "select * from v1" is doing a table scan.
What do I do to make it perform an index seek ????
TIA
- ForXLDB
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Dec 10, 2006
If you display the execution plan and run the following:SET STATISTICS IO ONgoSELECT ProductID, SupplierIDFROM ProductsWHERE SupplierID = 1I don't understand how come there is noBookmark Lookup operation happening to get theProductID?I only see an Index Seek happening on SupplierID.There is no composite index SupplierID + ProductIDso what am I not understanding here?Thank you
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Jan 16, 2002
Hi I'm issuing a SELECT on a field with the SUM on SQL Server 7. I have an index on the field in the WHERE clause but upon analysis, the Query Optimizer always uses a Full Table Scan. Can anyone explain why and is there a way to use the index.
HEre's the structure:
SELECT SUM(colA)
FROM TABLE tblB
GROUP BY colC
An index exists on column colC.
Thanks
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Apr 8, 2008
I am running a SELECT on a table. This READ operation ends up going through Clustered Index Scan. I want to know whether Clustered Index Scan , blocks other concurrent transactions trying to INSERT into this table? Does Clustered Index Scan locks the entire clustered index?
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Sep 29, 2006
I have following query to delete the data from fact history table based on fact table. logid, level and post_date uniquely identify the rows on both fact and history table. I want to create indexes on the joined columns.I tried with clustered index (logid, level and post_date) it gives clustered index scan. I also tried with non clustered indexes on each column (logid, level and post_date) but still getting table scan.
Do you have any suggestion on which columns should I create proper indexes to avoid table or index scan? There are about 6 million rows on each table.
DELETE xbar_fact_history
FROM xbar_fact_history AS a
INNER JOIN xbar_fact AS b
ON a.logid = b.logid
AND a.level = b.level
AND a.post_date = b.post_date
AND a.check_CheckSum <> BINARY_CHECKSUM(b.out_mins,b.nor_hrs,b.pdi_call)
______________________________________________________________
Thanks
Sanjeev
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Jul 23, 2005
Hi,Here's my problem: I want to write a stored procedure that returns allrecords from a table that have a certain column starting with giventext. I however find that using LIKE and a variable always causes anindex scan... which is causing performance issues. My table has about3.5M records.Below is a test. In query analyser if I look at the execution plan forthe following it will come up as in index scan. However, if i justhard-code the text it all works fine (index seek).How can I do this with reasonable speed???Thanks GregDECLARE @find varchar(50)SET @find = 'start'SELECT TOP 100*FROM TestWHERECol1 LIKE @find + '%'--Col1 LIKE 'start%'
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Mar 28, 2014
I have a table with clustered index on that. I have only 5 columns in that table. Execution plan is showing that Index scan occurred. What are the cause of the Index scan how can we change that to index seek?
I am giving that kind of similar query below
SELECT @ProductID= ProductID FROM Product WITH (NOLOCK) WHERE SalesID= '@salesId' and Product = 'Clothes '
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Sep 20, 2006
Hi,
This is on Sybase but I'm guessing that the same situation would happen on SQL Server. (Please confirm if you know).
I'm looking at these new databases and I'm seeing code similar to this all over the place:
if not exists (select 1 from dbo.t1 where f1 = @p1)
begin
select @errno = @errno | 1
end
There's a unique clustered in dex on t1.f1.
The execution plan shows this for this statement:
FROM TABLE
dbo.t1
EXISTS TABLE : nested iteration.
Table Scan.
Forward scan.
Positioning at start of table.
It's not using my index!!!!!
It seems to be the case with EXISTS statements. Can anybody confirm?
I also hinted to use the index but it still didn't use it.
If the existence check really doesn't use the index, what's a good code alternative to this check?
I did this and it's working great but I wonder if there's a better alternative. I don't really like doing the SET ROWCOUNT 1 and then SET ROWCOUNT 0 thing. SELECT TOP 1 won't work on Sybase, :-(.
SET ROWCOUNT 1
SELECT @cnt = (SELECT 1 FROM dbo.t1 (index ix01)
WHERE f1 = @p1
)
SET ROWCOUNT 0
Appreciate your help.
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Jul 30, 2007
Dear All,
I'm working in eVC++3.0 with SQL CE2.0.
I've developed the application to fetch the data from the mdf file by passing the index field and using the command IRowsetIndex()::Seek.
Consider TableExample with Columns A,B,C,D and index Index1 with column A and Index2 with column B,C,D.
Its working fine when index1 is passed for the indexvalue.But when i pass index2 of the same table,its throwing the error as BADINDEX.
I'm passing the correct keyvalues in the Seek command also.
Kindly help me in this regard.
Regards,
Sasi.
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May 18, 2006
Hello I want to seek and find the table named NewCustomer from the database so what would be the query ?
F16 LĂ?GHTĂ?NĂ?NNNG
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Dec 20, 2006
I have an MS Access 2003 database from which I want to seek a specific record in a SQL Server Express 2005 database. I can connect to the table and get a recordcount but the recordset.supports (adseek) and recordset.Supports(adIndex) both return false. Any suggestions? Specific code I'm using is as follows:
Dim cnxn As ADODB.Connection
Dim strCnxn As String
Set cnxn = New ADODB.Connection
cnxn.Provider = "sqloledb"
strCnxn = "Data Source=SERVERSQLEXPRESS2005;Initial Catalog=RAMPSQL;Integrated Security='SSPI';"
cnxn.Open strCnxn
Set rsWSC = New ADODB.Recordset
rsWSC.CursorLocation = adUseServer
strSQL = "DailyData"
rsWSC.Open strSQL, cnxn, adOpenKeyset, adLockReadOnly, adCmdTableDirect
Thank you!
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Nov 30, 2006
Hi
I have a frontend Access and backend SQL
works fine but when i in my customer table seek in name
it takes very very long time . I use ODBC to my connection
is there anayway to this better.?
regards
alvin
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Oct 4, 2007
Hi,
I am reading the book by Steven Roman:
"Access Database : Design and Programming" 3rd edition.
On page 120, Figure 7-2, he showed the the structure of the Jet Database Engine, which is very confusing to me.
According to this picture, I come to such an understanding:
1) VBA is just the hosting language for the Jet Database Engine;
2) Microsoft Visual Basic, Excel, Access, Word are hosting languages for VBA.
Isn't this weird? VB hosts VBA?
Thanks for any input!
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Dec 13, 2007
Hi All,
The script below may be use to find out what stored procedure uses a specified column from any of the table. This could be helpful in cases you have change a field name of a table and want to find out what stored procedure uses that column.
create procedure seek_sp_for_columns
@colname_para varchar(500)
as
begin
create table #temp_t
(
textcol varchar(1000)
)
create table #temp_t2
(
procname varchar(500)
)
declare @procname as varchar(500)
declare @found as int
declare @colname as varchar(500)
declare @valid_colname as int
select @valid_colname = count(id)
from syscolumns
where name = @colname_para
if (@valid_colname > 1)
begin
set @colname = '%' + @colname_para + '%'
declare sp_cursor cursor
for select name
from sysobjects
where xtype = 'P'
open sp_cursor
fetch next from sp_cursor
into @procname
while @@fetch_status = 0
begin
insert into #temp_t
exec sp_helptext @procname
set @found = 0
select @found = count(textcol)
from #temp_t
where textcol like @colname
if (@found > 0)
begin
insert #temp_t2 values(@procname)
end
delete #temp_t
fetch next from sp_cursor
into @procname
end
close sp_cursor
deallocate sp_cursor
select *
from #temp_t2
drop table #temp_t
drop table #temp_t2
end
else
begin
select 'Please verify column name'
end
end
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Jul 23, 2005
From what I've read in the docs, ado.net currently supports opening sqlserver ce tables in table-direct mode and performing Seek operations on them(using SqlCeDataReader), but not on the full-blown sql server. Is this(will this be) still true with ado.net 2.0 & sql server 2005?
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May 11, 2006
I am getting error 0x80040E25 when I try to call seek after update on a Recordset opened as (adOpenStatic, adLockOptimistic, adCmdTableDirect)
9 - (13.250) - <2> - *** error in .DbRecordset.cpp, line 908
10 - (13.250) - <2> - ADO_ERRORS FOR pRs = 200a420, seek, err=-2147217883(80040e25)
11 - (13.250) - <2> - ADO_ERROR: E R R O R 1 of 1.
12 - (13.250) - <2> - ADO_ERROR: DESCRIPTION: All HROWs must be released before new ones can be obtained. [,,,,,].
13 - (13.250) - <2> - ADO_ERROR: NUMBER: 80040E25
14 - (13.250) - <2> - ADO_ERROR: NATIVE_ERROR: 0
15 - (13.250) - <2> - ADO_ERROR: SOURCE: Microsoft SQL Server 2005 Mobile Edition OLE DB Provider
I registered SQL Mobile 3.0 dlls using regsvr32.exe so now I can connect to SQLCE3.0 databases on desktop using plain ADO with such connection string _T("Provider=Microsoft.SQLSERVER.MOBILE.OLEDB.3.0; Data Source=") + name of the file
I have not asked this question before as it did not make sense -> there was no official SQLCE3.0 support on desktop. Now, since SQL CE is promiced to be supported on desktop as SQL/E I decided to ask.
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Oct 8, 2007
I write a work about database, and I want to write the diffrence between Table scan and Index scan. When is the index scan effectient? Use the database a Table scan when there is an index on the table or always a index scan when the index is nonselective? Can be a Table Scan effecient than a index scan?
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Aug 14, 2001
I am having problems w/ the indexes on a particular table. Currently the scan density if 25%. I have ran the dbcc dbreindex and the scan density does not improve. I have manually dropped and recreated all of the indexes on the table (there are only two) and still no help.
I have a clustered Primary Key index on my part_number and project_number fields and another index on the project_number field. If I drop the second index and only have my primary key index, it still doesn't help. The fill factor was originally set to 90% and I changed this to 30%, 20%, 10%, 0% and the best scan density I could acheive was 50%.
The data in this table is not updated that often, on average about 15 records a day are updated, and there are only 107 records. This is a small table, but there is a possibility for it to grow rapidly and I want to be prepared now.
Does anyone have any ideas or suggestions? I've tried everything I can think of with no luck.
TIA,
Chris
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May 26, 2000
Can anyone tell me how to get rid of the Table Scan(1 million rows)being performed on the
The last line, option (loop loin) stopped table scanning the B.ss_manifest and started using the index, I'd like both tables to use the index.
This is the argument I get from execution plan under the table scan.
Object ((D4000).(dbo).(shipstop)as (A))
update drivers set dr_miles_run = case when D1.miles > 0 then d1.miles else 0 end
from (select mf_dr_nbr, sum( case when A.ss_end_dt < '05/17/00' then
( cast((datediff(day, '05/17/00' , B.ss_end_dt ) + 1) as float) /
cast( (datediff(day, A.ss_end_dt , B.ss_end_dt ) + 1) as float) * mf_ld_miles)
else mf_ld_miles end) as miles
from manifest, shipstop A, shipstop B
where mf_manifest_nbr = A.ss_manifest_nbr
and mf_manifest_nbr = B.ss_manifest_nbr
and A.ss_stop_type in ('OR','SA')
and B.ss_stop_type in ('DT','RD')
and ((A.ss_end_dt >= '05/17/00 00:00' and A.ss_end_dt < '05/24/00 00:00')
OR ((B.ss_end_dt >= '05/17/00 00:00' and B.ss_end_dt < '05/24/00 00:00'))) and mf_status > 3 group by mf_dr_nbr ) as D1
where Drivers.dr_driver_nbr = D1.mf_dr_nbr
option (loop join)
Thanks for any ideas!
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Aug 21, 2006
Hi,
I have a simple temp table which looks like this:
CREATE TABLE #t7e07c01fa80143ff84cb14a2307809f7
(
[AUTOID_TEMP] [int] IDENTITY (1, 1) NOT NULL,
...
)
ON [PRIMARY]
ALTER TABLE #t7e07c01fa80143ff84cb14a2307809f7 ADD
CONSTRAINT [PK_t7e07c01fa80143ff84cb14a2307809f7] PRIMARY KEY NONCLUSTERED
(
[AUTOID_TEMP]
)
ON [PRIMARY]
Insert Into #t7e07c01fa80143ff84cb14a2307809f7
Selet columns... from t....
-- get total records
Declare @TotalCount as int
select @TotalCount = count(AUTOID_TEMP) from #t7e07c01fa80143ff84cb14a2307809f7
Now the above last statement does a index scan. I am new to indexing/tuning and was wondering if its normal - if so why and can I somehow enhance this?
Thanks
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Nov 1, 2007
Hey guys,
I have a procedure that runs everyday which takes the IIS log file from the previous day and imports it and calcs values. I would like to change this from taking the previous day to scanning all the file names in the folder and comparing them to a database to see if they have already been scanned before. I already have a table called DatesScanned which lists the filename and scandate.
IE:
D:Logsfile1.log
D:Logsfile2.log
D:Logsfile3.log
D:Logsfile4.log
The 4 file names will be scanned and compared to DatesScanned to see which has been imported already.
Can anyone point me to a resource or information on how I would get the directory listing of a folder so I could compare it to the DatesScanned table.
Thanks a bunch!
Jeff
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Jul 23, 2005
Hi,Can someone please explain the following1. Meaning of scan count as reported when "statistics io" is turned onprior to running a query.2. In which situations could you have an identical database runningon two diferent servers , with identical database serverconfigurations, running identical queries, with identical query plan,report large discrepency in the scan count . This is one of thepossibilities we are looking at in terms of the reason why one serverruns the job in 12 hours and the other in 24 hours.Server 1--------Table 'TRANS_HISTORY'. Scan count 216, logical reads 897093, physicalreads 44, read-ahead reads 900599.SQL Server Execution Times:CPU time = 27766 ms, elapsed time = 46850 ms.UNIT_NUMBERACCOUNT_TYPETRANSACTION_TYPEServer 2--------Table 'TRANS_HISTORY'. Scan count 491, logical reads 952759, physicalreads 51, read-ahead reads 954414.SQL Server Execution Times:CPU time = 31563 ms, elapsed time = 145595 ms.UNIT_NUMBERACCOUNT_TYPETRANSACTION_TYPEI thank you in advance for your assistance.Puvendran
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Jul 9, 2007
Does SQL Server allow a table scan to be used when querying a table that has a clustered-index? If yes, could someone please show me the syntax? I have tried with (index(0)) but this appararently means a clustered index scan when there is a clustered index on the table to be queried.
Or does clustered index scan mean the same thing as a table scan when the table has a clustered index? Confused.
Thanks
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Nov 28, 2007
Friends,
I am using this query in a table with 29,000 records having non-clustered index on emp_id
select * from employee
where emp_id between 100 and 1000
and gender = 'M'
order by emp_id
execution plan shows a table scan in table employee. Any reason?
select * from employee with (index = emp_id)
where emp_id between 100 and 1000
order by emp_id
ofcourse this uses index seek
regards,
Priw
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