Monday, January 6, 2014

ColumnStore Indexes...

With the increasing amounts of data, there is lot of expectations from data warehousing solutions in terms of performance and accuracy. There is a constant demand for increased performance of queries from a data warehouse. With huge amount of rows sometimes performance of queries get affected due to the time spent in pre-aggregating or aggregating data. With this backdrop there has been the emergence of columnstore indexes. What is a columstore index in SQL Server? Quoting from Microsoft Technet: Columnstore indexes in the SQL Server Database Engine can be used to significantly speed-up the processing time of common data warehousing queries. Typical data warehousing workloads involve summarizing large amounts of data. The techniques typically used in data warehousing and decision support systems to improve performance are pre-computed summary tables, indexed views, OLAP cubes, and so on. Based on this definition what is the advantage over traditional row based storage. In Databases with ColumnStore indexes SQL Server stores data in a columnar format based on the column with columnstore Index. Since data is stored one column At a time, this allow sql server to perform aggregations on the single column very quickly instead of traversing through each row. Also Since only the columns needed must be read this greatly reduces the Disk I/O activity. Most queries do not encompass all columns in a table which means that lesser data would be bought into memory thereby enhancing the speed of the queries.
Some important points to keep in mind with respect to ColumnStore index
Clustered index key – If a base table is a clustered index, all columns in the clustering key must be present in the nonclustered columnstore index. If a column in the clustering key is not listed in the create index statement, it will be added to the columnstore index automatically.
Partitioning – Columnstore indexes works with table partitioning. No change to the table partitioning syntax is required. A columnstore index on a partitioned table must be partition-aligned with the base table. Therefore a nonclustered columnstore index can only be created on a partitioned table if the partitioning column is one of the columns in the columnstore index.

In SQL Server 2012 Columnstore indexes are not updatetable, i guess in SQL Server 2014 there are plans to make them updatetable.

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