We did it again! Thanks Sanga, Mahela and the team, proud to be a Sri Lankan.
As a best practice, we always add a new key, known as a surrogate key to dimension tables for identifying the records uniquely. However, generally, the unique business key becomes the primary key of relational tables related to business entities but once the surrogate key is added to dimension tables, we make it as the primary key without reusing the business key. Here is an example from AdventureWorksDW data warehouse.
Surrogate key is an integer that is assigned sequentially when records are added. There are many other names for this such as artificial key, synthetic key, meaningless key, integer key or non-natural key. Do we really need this? Does it seem inappropriate using the business keys for primary keys? It may seem sensible for reusing, but here are some valid reasons for not using business keys but using surrogate keys;
Of course, the maintenance cost gets increased and a mechanism has to be introduced for generating surrogate keys and linking them with foreign keys in fact tables. But it is not as difficult as many think because there are many built-in facilities available in DBMS and ETL systems.
Want to know some major features added to SQL Server 2014? Here is an easy way. This book explains how SQL Server 2014 leverages in-memory technology for improving the performance of OLTP and data warehousing solutions, how easily on-premises solutions can be transferred to cloud with added support for hybrid environment, etc. If you are a DBA, DBE or an engineer who wants to know the new features and capabilities, this is the book to be read.
Chapters in this book;
If you are maintaining your profile with world largest professional network (Im sure that you are) and you have done a Microsoft Certification, there is an easy way of adding the certification to your LinkedIn profile rather than manually adding it. The new way has been introduced by teaming up with LinkedIn and it is opened to you via an email which you receive with details once an exam is done. All you have to do is, hit the button populated with all details required (since I have not done a certification recently, I have no idea how this will be appeared in the mail, I believe it is similar to below image).
In order to receive the mail, make sure you have enabled “Promotional email” in MCP profile setting and you are using the same email address for both MCP profile and LinkedIn profile.
For more details, read this post published by Carrie Francey (MS): http://borntolearn.mslearn.net/btl/b/weblog/archive/2014/03/25/announcing-a-new-way-to-add-microsoft-certifications-to-your-linkedin-profile.aspx
Are you planning to do Microsoft SQL Server certifications in next few weeks? If so, here is an announcement on exams updates. All MCSE related SQL Server exams (464 to 467) will be updated on April 24, 2014 adding new features introduced with SQL Server 2014. Numbers of the exams will not be changed but the title will be adjusted with 2014. Official courseware for these exams have not been updated yet, most probably, updated courseware will be published during May 2014.
Here are the exams that will be updated;
Use following links to see the changes done in skills measured;
MCSA related SQL Server exams (461 to 463) will be remain unchanged.
It is a fact that non-clustered indexes improve the performance of data retrieval and we tend to add them mostly based on predicates used. As Data warehouses mainly design for data retrieval, many think that a data warehouse must contain non-clustered indexes as much as possible. In a way, it is true and required, however, we need to thoroughly analyze the requirements on indexing in data warehousing because generally non-clustered indexes do not help much with dimension and fact tables in data warehouses.
Non-clustered indexes generally give better performance on high selective queries. An example for a high selective query would be a query written for getting all customers for a given last name. Mostly attributes in a dimension table are used for pivoting in reports and typically hold few distinct values. Because of that, filters used in such reports are not much selective and will not get any benefits from indexes. However, parameterized reports looking for a particular value from an attribute would benefit as it is very selective. Therefore, when determining an index on an attribute, consider the reporting requirement similar to section example and then add.
It is common practice that adding non-clustered indexes on foreign keys. A typical data warehouse created based on Star-Schema has fact tables and fact tables have foreign keys linked to dimension tables. Do we need to create non-clustered indexes on these keys? Answer is “No”. The reason for that is, “star join query optimizations” feature in SQL Server (Enterprise only). This feature recognizes star join patterns and it uses bitmap filtered hash joins for joining tables in efficient way. Therefore indexes on foreign keys are not required unless a different edition is used. However, if the required dataset is smaller, indexes would be created on foreign keys as merge and nested loops joins give better performance than hash joins. Read more on that here: http://technet.microsoft.com/en-us/library/bb522541(v=sql.105).aspx
Considering above facts, we rarely add non-clustered indexes on both fact and dimension tables. In addition to that, the volume of records involved with the query (IO), the high cost for maintaining indexes, rebuilding every time data is loaded are other factors for not using non-clustered indexes in DWs.