What is a columnstore index and how is different from clustered and non-clustered indexes?

A columnstore index is a type of index in a relational database management system that organizes data by columns instead of by rows. In a traditional row-based storage system, the data is stored in rows, and each row is treated as a single unit. With a columnstore index, the data is instead stored in columns, and each column contains the values for a single attribute or field.

The main advantage of using a columnstore index is improved query performance. Because the data is stored in columns, the database management system can access and process only the columns that are required for a particular query, instead of reading the entire row. This can result in much faster query times, especially for large data sets.

A clustered index is an index that physically reorders the rows in a table to match the order of the index. In other words, the data is stored in the same order as the index, and the index contains a pointer to the actual data row. There can only be one clustered index per table.

A non-clustered index, on the other hand, is a separate structure that contains a subset of the data in the table, along with a pointer to the location of the actual data row. Unlike clustered indexes, non-clustered indexes do not physically reorder the data in the table. Multiple non-clustered indexes can be created on a single table.

In summary, columnstore indexes, clustered indexes, and non-clustered indexes are different ways of organizing data in a relational database, and each type has its own advantages and use cases. Columnstore indexes are designed to improve query performance for large data sets, while clustered indexes physically reorder the data to match the order of the index, and non-clustered indexes provide an additional layer of organization that can help speed up queries.