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How to create indexes with included columns in SQL Server?
This topic describes how to add included (or nonkey) columns to extend the functionality of nonclustered indexes in SQL Server by using SQL Server Management Studio or Transact-SQL. By including nonkey columns, you can create nonclustered indexes that cover more queries. This is because the nonkey columns have the following benefits:
When to include columns in non clustered indexes?
By including non-key columns in non-clustered indexes, you can create nonclustered indexes that cover more queries. Note that when an index contains all the columns referenced by a query, the index is typically referred to as covering the query. First, drop the index ix_cust_email from the sales.customers table:
How to include included columns in SQL Server?
Second, specify the name of the table and a list of key column list for the index after the ON clause. Third, list a comma-separated list of included columns in the INCLUDE clause. In this tutorial, you have learned how to use SQL Server indexes with included columns to improve the query performance.
When do you not include columns in a SQL query?
Although this particular query might benefit from an index (#1): This index does not help at all if the query changes slightly, such as: Imagine you had 1,000 employees in Department 5. Using index #1, to find all the Smiths, you’d need to seek through all 1,000 rows in Department 5, as the included columns are not part of the key.
Can a non clustered index be created on a table?
A Non-clustered index contains only the index key columns values in addition to a pointer to the actual data rows stored in the clustered index or the actual table, without specifying the real data order. You can create only one clustered index per each table, with the ability to create up to 999 non-clustered indexes on each table.
Are there different types of indexes in SQL Server?
There are other index types available in SQL Server, such as the Composite index that contains more than one key column, the Unique index that enforces the column values uniqueness and the Covering index that contains all columns needed by the query.
Why do we need an index in SQL Server?
A SQL Server index is considered as one of the most important performance-tuning factors. They are built on a table or view in the shape of the B-Tree structure to provide a fast access to the requested data, based on the index column’s values, speeding up the query processing.
What happens when an index is not included in a query?
If the keys of your index and filters in your query are not selective enough, then the index will be ignored (regardless of what’s in your INCLUDE columns). Every index you create has overhead for INSERT and UPDATE statements; more so for “bigger” indexes.
Why do you need multiple indexes in SQL Server?
Basically it helps your data insert at the end of the index and not cause lots of disk IO and Page splits. Secondly, if you are creating other indexes on your data and they are constructed cleverly they will be reused.