What is a column index?
A column store index is an index that was designed mainly for improving the query performance for workloads with very large amounts of data (eg. data warehouse fact tables). This type of index stores the index data in a column based format rather than row based as is done with traditional indexes.
How many columns can I have in a SQL table?
1024 columns
For the columns in a table, there is a maximum limit of 1024 columns in a table. SQL Server does have a wide-table feature that allows a table to have up to 30,000 columns instead of 1024.
How does a ColumnStore index improve query performance?
Since a columnstore index scans a table by scanning column segments of individual rowgroups, maximizing the number of rows in each rowgroup enhances query performance. When rowgroups have a high number of rows, data compression improves which means there is less data to read from disk.
Are there columnstore indexes in SQL Server 2012?
SQL Server columnstore indexes are new in the SQL Server 2012 release. They are designed to improve query performance for data warehouses and data marts. This page describes query performance tuning for columnstores.
What’s the maximum number of rows in a ColumnStore index?
For best query performance, the goal is to maximize the number of rows per rowgroup in a columnstore index. A rowgroup can have a maximum of 1,048,576 rows. It’s okay to not have the maximum number of rows per rowgroup. Columnstore indexes achieve good performance when rowgroups have at least 100,000 rows.
Why are columnstore indexes only fetched from disk?
The Query Processor needs to read in an entire row to retrieve specific column values, which increases I/O because extra data is unnecessarily read into memory. For example, if a table has 50 columns and the query only uses 5 of those columns, the columnstore index only fetches the 5 columns from disk.