In What Order Does A Clustered Index Store Data?
Mar 26, 2007
Hi
I was going through the book by Kalen Delaney where she has mentioned the following paragpraph in Chapter 7 (Index Internals):
Many documents describing SQL Server indexes will tell you that the clustered index physically stores the data in sorted order. This can be misleading if you think of physical storage as the disk itself. If a clustered index had to keep the data on the actual disk in a particular order, it could be prohibitively expensive to make changes. If a page got too full and had to be split in two, all the data on all the succeeding pages would have to be moved down. Sorted order in a clustered index simply means that the data page chain is logically in order.
Then I read the book on SQL Server 2000 (on Perf Tuning) by Ken England. He says the clustered index stores data in physical order and any insert means moving the data physically. Also the same statement is echoed on the net by many articles.
What is the truth? How are really clustered index stored? What does physical order in the above statement really mean?
Regards
SanjaySi
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Mar 5, 2015
I have a clustered index that consists of 3 int columns in this order: DateKey, LocationKey, ItemKey (there are many other columns in this data warehouse table such as quantities, prices, etc.).
Now I want to add a non-clustered index on just one of the other columns, say LocationKey, like this:
CREATE INDEX IX_test on TableName (LocationKey)
I understand that the clustered index keys will also be added as key columns to any NC indexes. So, in this case the NC index will also get the other two columns from the clustered index added as key columns. But, in what order will they be added?
Will the resulting index keys on this new NC index effectively be:
LocationKey, DateKey, ItemKey
OR
LocationKey, ItemKey, DateKey
Do the clustering keys get added to a NC index in the same order as they are defined in the clustered index?
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Feb 29, 2008
I have a table "Client" that has two columns: "ClientID" and "ProductID". I created on clustered index on ClientID and when I opened the table in the management studio, I saw the table was in the order of ClientID.
Then I added another non-clustered index on ProductID. When I open the table again, it is in the order of ProductID. Shouldn't the table always be in the order of clustered index? Non-clustered index should be a structure outside of the table itself? Did I do anything wrong?
Thanks for any hint.
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Jul 20, 2005
Hi allI recently noticed when trying to optimise a major query of a chess websiteI am the webmaster of, that adding an order by for "gamenumber" which is aclustered index field as in for example "order by timeleft desc, gamenumberdesc" actually speeded up the queries and reduced sql server 2000 timeouts.I have an ASP error log and I am fairly sure that a dramatic reduction insql server timeouts is simply attributed to adding an extra seeminglyredundant order by field - which is the clustered index. Is this phenomenaat all possible or is it my imagination?!Other special attributes of the query includes the use of "Top" to obtain amaximum specified number of rows. Perhaps it is just the uniquecharacteristics of the query, but I would have thought that the less orderby fields would imply faster performance. Has anyone else noticed that aseemingly redundant order by column on for example the clustered indexcolumn, can actually help speed up queries?!Best wishesTryfon GavrielWebmasterwww.chessworld.net
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May 29, 2007
Want to check my thinking with you folks...
I have a table with a clustered composite index, consisting of 3 columns, which together form a unique key. For illustration, the columns are C1, C2 & C3.
Counts of distinct values for columns are C1 425, C2 300,000 & C3 4,000,000
C3 is effectively number of seconds since 01/01/1970.
The usage of the table is typically, insert a row, do something else, then update it.
Currently, the index columns are ordered C3,C1,C2. Fill factor of 90%.
My thinking is that this composite index is better ordered C1,C2,C3.
My reasoning is that having C3 as the leading column, biases all the inserts towards one side of the indexes underlying B-tree, causing page splits. Also, there'll be a bunch of "wasted" space across the tree, as the values going into C3 only ever get bigger (like an identity), so the space due to the fill factor in lower values never gets used.
Welcome your thoughts.
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Apr 3, 2015
I've been asked to look at using Clustered Columnstore indexes for one of my tables. The table contains about 5 million records with about 50 columns. The max field size is a NVarchar(MAX) with max field length currently of about 4k characters. It's only about a gigabyte's worth of data. The table is about 50% R/W operations. Currently, we have multiple indexes with no clustered index due to some performance issues that happened in the past. I've been attempting to determine if it's even really worth it to switch over. I feel that the table is still fairly small with minimal columns and don't believe there will be any noticeable improvement over traditional indexing.
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Jun 18, 2015
I have created NONCLUSTERED index on table but my report is taking more time that's why i created columnstore NONCLUSTERED index on the same table but i have one query, if any table have row and column level index(same columns in index) . Which index query will consider.
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Jul 19, 2013
I have created two tables. table one has the following fields,
Id -> unique clustered index.
table two has the following fields,
Tid -> unique clustered index
Id -> foreign key of table one(id).
Now I have created primary key for the table one column 'id'. It's created as "nonclustered, unique, primary key located on PRIMARY". Primary key create clustered index default. since unique clustered index existed in table one, it has created "Nonclustered primary key".
My Question is, What is the difference between "clustered, unique, primary key" and "nonclustered, unique, primary key"? Is there any performance impact between these?
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Sep 17, 2015
How can we get the list of clustered columnstore index in a database in sql server 2014
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Oct 9, 2015
I am trying to use an indexed view to allow for aggregations to be generated more quickly in my test data warehouse. The Fact Table I am creating the indexed view on is a partitioned clustered columnstore index.
I have created a view with the following code:
ALTER view dbo.FactView
with schemabinding
as
select local_date_key, meter_key, unit_key, read_type_key, sum(isnull(read_value,0)) as [s_read_value], sum(isnull(cost,0)) as [s_cost]
, sum(isnull(easy_target_value,0)) as [s_easy_target_value], sum(isnull(hard_target_value,0)) as [s_hard_target_value]
, sum(isnull(read_value,0)) as [a_read_value], sum(isnull(temperature,0)) as [a_temp], sum(isnull(co2,0)) as [s_co2]
, sum(isnull(easy_target_co2,0)) as [s_easy_target_co2]
, sum(isnull(hard_target_co2,0)) as [s_hard_target_co2], sum(isnull(temp1,0)) as [a_temp1], sum(isnull(temp2,0)) as [a_temp2]
, sum(isnull(volume,0)) as [s_volume], count_big(*) as [freq]
from dbo.FactConsumptionPart
group by local_date_key, read_type_key, meter_key, unit_key
I then created an index on the view as follows:
create unique clustered index IDX_FV on factview (local_date_key, read_type_key, meter_key, unit_key)
I then followed this up by running some large calculations that required use of the aggregation functionality on the main fact table, grouping by the clustered index columns and only returning averages and sums that are available in the view, but it still uses the underlying table to perform the aggregations, rather than the view I have created. Running an equivalent query on the view, then it takes 75% less time to query the indexed view directly, to using the fact table. I think the expected behaviour was that in SQL Server Enterprise or Developer edition (I am using developer edition), then the fact table should have used the indexed view. what I might be missing, for the query not to be using the indexed view?
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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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Nov 20, 1998
If I have a table that I want to create a clustered index on. For example sake, say that I have 2 columns in the table. Col1 is char(2), col2 is text (or image). If I create a clustered index on col1, the database needs 1.2% times the size of the table to create the clustered index. Does this include the size of col2, being that text and image data is stored in a separate page chain....?
thanks for your time!
Tim
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Sep 4, 2015
We are going to use SQL Sever change tracking. The problem is that some of our tables, which are to be tracked, have no primary keys. There are only unique clustered indexes. The question is what is the best way to turn on change tracking for these tables in our circumstances.
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Aug 28, 2015
I desire to have a clustered index on a column other than the Primary Key. I have a few junction tables that I may want to alter, create table, or ...
I have practiced with an example table that is not really a junction table. It is just a table I decided to use for practice. When I execute the script, it seems to do everything I expect. For instance, there are not any constraints but there are indexes. The PK is the correct column.
CREATE TABLE [dbo].[tblNotificationMgr](
[NotificationMgrKey] [int] IDENTITY(1,1) NOT NULL,
[ContactKey] [int] NOT NULL,
[EventTypeEnum] [tinyint] NOT NULL,
[code]....
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Jan 4, 2008
I have large table with 10million records. I would like to create clustered or non-clustered index.
What is the quick way to create? I have tried once and it took more than 10 min.
please help.
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Sep 8, 2006
Hi there, I have a table that has an IDENTITY column and it is the PK of this table. By default SQL Server creates a unique clustered index on the PK, but this isn't what I wanted. I want to make a regular unique index on the column so I can make a clustered index on a different column.
If I try to uncheck the Clustered index option in EM I get a dialog that says "Cannot convert a clustered index to a nonclustered index using the DROP_EXISTING option.". If I simply try to delete the index I get the following "An explicit DROP INDEX is not allowed on index 'index name'. It is being used for PRIMARY KEY constraint enforcement.
So do I have to drop the PK constraint now? How does that affect all the tables that have FK relationships to this table?
Thanks
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Oct 15, 2007
Hi everybody!
I just ran the Database Engine Tuning Advisor on a relative complex query to find out if a new index might help, and in fact it found a combination that should give a performance gain of 94%. Fair enough to try that.
What I wonder about: The index I should create contains 4 columns, the last of them being the Primary Key column of the table, which is also my clustered index for the table. It is an identity integer btw.
I think I remember that ANY index does include the clustered one as lookup into the data, so having it listed to the list of columns will not help. It might at worst add another duplicate 4 bytes to each index entry.
Right? Wrong? Keep the column in the index, or remove it since it is included implicit anyway?
Thanks for suggestions!
Ralf
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Apr 21, 2007
I am trying to create a clustered index on a View of a table that has an xml datatype. This indexing ran for two days and still did not complete. I tried to leave it running while continuing to use the database, but the SELECT statements where executing too slowly and the DML statements where Timing out. I there a way to control the server/cpu resources used by an indexing process. How can I determine the completion percentage or the indexing process. How can I make indexing the view with the xml data type take less time?
The table definition is displayed below.
CREATE TABLE [dbo].[AuditLogDetails](
[ID] [int] IDENTITY(1,1) NOT NULL,
[RecordID] [int] NOT NULL,
[TableName] [varchar](64) NOT NULL,
[Modifications] [xml] NOT NULL,
CONSTRAINT [PK_AuditLogDetails] PRIMARY KEY CLUSTERED
(
[ID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
The view definition is displayed below.
ALTER VIEW [dbo].[vwAuditLogDetails] WITH SCHEMABINDING
AS
SELECT P.ID,D.RecordID, dbo.f_GetModification(D.Modifications,P.ID) AS Modifications
FROM dbo.AuditLogParent P
INNER JOIN dbo.AuditLogDetails AS D ON dbo.f_GetIfModificationExist(D.Modifications,P.ID)=1
The definition for UDF f_GetModification
ALTER function [dbo].[f_GetModification]( @Modifications xml,@PID uniqueidentifier )
returns xml
with schemabinding
as
begin
declare @pidstr varchar(100)
SET @pidstr = LOWER(CONVERT(varchar(100), @PID))
return @Modifications.query('/Modifications/modification[@ID eq sql:variable("@pidstr")]')
end
The definition for UDF f_GetIfModificationExist
ALTER function [dbo].[f_GetIfModificationExist]( @Modifications xml,@PID uniqueidentifier )
returns Bit
with schemabinding
as
begin
declare @pidstr varchar(100)
SET @pidstr = LOWER(CONVERT(varchar(100), @PID))
return @Modifications.exist('/Modifications/modification[@ID eq sql:variable("@pidstr")]')
end
The Statement to create the index is below.
CREATE UNIQUE CLUSTERED INDEX [IX_ID_RecordID] ON [dbo].[vwAuditLogDetails]
(
[ID] ASC,
[RecordID] ASC
)WITH (STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
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Sep 20, 2007
Hi,
I want to ask a basic question, that is
IN WHAT ORDER A CLUSTERED INDEX SORT THE DATA IN THE COLUMN????
Somewhere in the MSDN library I read the following line:
"A clustered index physically sorts the table's contents in the order of the specified index columns"
But Sorting means it will be in ASCENDING ORDER (ASC) or It will be in DESCENDING ORDER (DESC)
So my question is lets suppose a column on which the cluistered index is defined and it contains character data liek abcd so in wht order it will sort the data alphabetically ASC or DESC
or
If the same above case with integer type of values, if column having integer values then in wht order the data in the table will be sorted.
??????
Thanks..!!!
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Dec 5, 2007
Dear All.
We had Teradata 4700 SMP. We have moved data from TD to MS_SQL SERVER 2003. records are 19.65 Millions.
table is >> Order_Dtl
Columns are:-
Client_ID varchar 10
Order_ID varchar 50
Order_Sub_ID decimal
.....
...
..
.
Pk is (ClientID+OrderId+OrderSubID)
Web Base application or PDA devices use to initiate the order from all over the country. The issue is this table is not Partioned but good HP with 30 GB RAM is installed. this is main table that receive 18,0000 hits or more. All brokers and users are using this table to see the status of their order.
The always search by OrderID, or ClientID or order_SubNo, or enter any two like (Client_ID+Order_Sub_ID) or any combination.
Query takes to much time when ever server receive more querys. some orther indexes are also created on the same table like (OrderDate, OrdCreate Date and Status)
My Question are:-
Q1. IF Person "A" query to DB on Client_ID, then what Index will use ? (If any one do Query on any two combination like Client_ID+Order_ID, So what index will be uesd.? How does MS-SQL SERVER deal with these kind of issues.?
Q2. If i create 3 more indexes on ClientID, ORderID and OrdersubID. will this improve the performance of query.if person "A" search record on orderNo so what index will be used. (Mind it their would be 3 seprate indexes for Each PK columns) and composite-Clustered index is also available.?
Q3. I want to check what indexes has been used? on what search?
Q4. How can i check what table was populated when, or last date of update (DML)?
My Limitation is i Dont Create a Partioned table. I dont have permission to do it.
In Teradata we had more than 4 tb record of CRM data with no issue. i am not new baby in db line but not expert in sql server 2003.
I am thank u to all who read or reply.
Arshad
Manager Database
Esoulconsultancy.com
(Teradata Master)
10g OCP
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Nov 1, 2007
I have a table<table1> with 804668 records primary on table1(col1,col2,col3,col4)
Have created non-clustered index on <table1>(col2,col3,col4),to solve a performance issue.(which is a join involving another table with 1.2 million records).Seems to be working great.
I want to know whether this will slow down,insert and update on the <table1>?
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Jul 3, 2006
Hi everyone,
When we create a clustered index firstly, and then is it advantageous to create another index which is nonclustered ??
In my opinion, yes it is. Because, since we use clustered index first, our rows are sorted and so while using nonclustered index on this data file, finding adress of the record on this sorted data is really easier than finding adress of the record on unsorted data, is not it ??
Thanks
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Oct 17, 2015
We have a typical issue with Column Store Index, we have a procedure which does 2 activities - Switch & Reverse Switch
Switch:
1. Fetch the Partitions needed to be switched
2. Switch the data from Main Table to Switch table
2. Disable the Column store on Switch table
SSIS Package:
3. Load data to Switch (Insert / Update)
Reverse Switch:
4. Enable the Switch
5. Switch back the data from Switch table to Main table
Issue: Some time the Column store is not getting disabled, and the package fails complaining try disabling the Column store index and try loading data.
If we re-run the procedure, the column store gets disabled.
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Sep 26, 2015
I am trying to create a sample table in the Azure SQL Data warehouse but its giving me a syntax error Incorrect syntax near the keyword 'CLUSTERED'.
CREATE TABLE [dbo].[FactInternetSales]
( [ProductKey] int NOT NULL
, [OrderDateKey] int NOT NULL
, [CustomerKey] int NOT NULL
, [PromotionKey] int NOT NULL
[Code] ....
what's the correct syntax
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Jun 18, 2008
Quick question about the primary purpose of Full Text Index vs. Clustered Index.
The Full Text Index has the purpose of being accessible outside of the database so users can query the tables and columns it needs while being linked to other databases and tables within the SQL Server instance.
Is the Full Text Index similar to the global variable in programming where the scope lies outside of the tables and database itself?
I understand the clustered index is created for each table and most likely accessed within the user schema who have access to the database.
Is this correct?
I am kind of confused on why you would use full text index as opposed to clustered index.
Thank you
Goldmember
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Apr 15, 2015
We are running SQL Server 2014 Enterprise Edition (64-Bit) on Windows 2012 R2 Standard (64-Bit).
1. When to create indexes, before or after data is added? Please address Clustered and Non-Clustered Indexes.
2. To move indexes to it's own filegroup, is it best to create the NON-Clustered Indexes on the separate filegroup with code similar to the example below?
CREATE NONCLUSTERED INDEX IX_Employee_OrganizationLevel_OrganizationNode
ON HumanResources.Employee (OrganizationLevel, OrganizationNode)
WITH (DROP_EXISTING = ON)
ON TransactionsFG1;
GO
I have read the following links that states that if you create the Clustered Index on a separate filegroup, it would also move the base table to that particular filegroup. (So I take it that you ONLY can move NON-CLustered Indexes to a separate filegroup.)
Placing Indexes on Filegroups:
[URL]
By default, indexes are stored in the same filegroup as the base table on which the index is created. A nonpartitioned clustered index and the base table always reside in the same filegroup. However, you can do the following:
• Create nonclustered indexes on a filegroup other than the filegroup of the base table.
Move an Existing Index to a Different Filegroup:
[URL]
Limitations and Restrictions
• If a table has a clustered index, moving the clustered index to a new filegroup moves the table to that filegroup.
• You cannot move indexes created using a UNIQUE or PRIMARY KEY constraint using Management Studio. To move these indexes use the CREATE INDEX statement with the (DROP_EXISTING=ON) option in Transact-SQL.
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Apr 1, 2006
Hello I want to learn disparity clustered index or nonclustered index and in queries which one run better.
example
select * from orders where orderID=5
to this query clustered or nonclustered
thanks
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Dec 6, 2005
What does an index add to the performance?
Why do we use Clustered Index and Non-clustered Index?
thanks
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Sep 30, 2015
I have a really super slow stored proc that does something simple. it updates a table if certain values are received.
In looking at this the matching is done on the Primary Key, which is set as a Clustered index, looking further I have another constraint, that sets the same column to a Unique, Non-Clustered.
I am not sure why this was done, but it seems to be counter productive. I have read only references to Which one is better on a primary key, but not can their be both and if it is "Smart".
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May 19, 2014
I am building three partitioned, clustered column store tables.I was researching whether it was faster to populate a staging table and swap it into the partitioned table or to directly insert into the partitioned table.The first partition for the three tables will have:
Table F: 50M rows, 6 columns wide, partitioned on a date column (1 date, 2 bigint keys, and two varchar columns)
Table D1: 50M rows, 150 columns wide, partitioned on a bigint
Table D2: 19M rows, 300 columns wide, partitioned on a bigint
If build the data that would go into partition 1 in a non partitioned column store, I get these table sizes:
Table F: 476 MB
Table D1: 6,800 MB
Table D2: 5,496 MB
If build the same data directly in the partitioned column store, my table sizes end up being:
Table F: 579 MB
Table D1: 6,800 MB
Table D2: 5,364 MB
That's a 20% difference on Table F, the narrow table.Looking at the row groups, I see 47 identical row groups in partition 1 and the unpartitioned table, but the average "size_in_bytes" is consistently 20% smaller in the unpartitioned table.
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May 22, 2000
Hi,
I have a small table (around 10,000 rows) that is constantly selected from, deleted from, and inserted into. Basically we fill it with content, our web application selects the content, and when we run out, we regenerate (about 50 rows at a time). We currently have a nonclustered PK on the first two columns, both INTs. How can I determine if a clustered index would be better? I am concerned about bottlenecks due to a hotspot with the nonclustered index. When our site really starts to get users, this could become a big issue. I am thinking that I could use a clustered index, and set up a job to reindex the table once every hour or so....any help is appreciated greatly.
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Dec 9, 2000
Does anybody know if a key defined on Uniqueidentifier datatype is a good candidate for the clustered index or otherwise.
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Jun 29, 2007
Hi,
Is it advisable to create a Non Clustered Index in "ALLow NULL" column?
Thanks,
Rahul Jha
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