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Introduction
In SQL, indexes are used to retrieve data from the database more quickly. They work similarly to an index in a book and are an essential part of any database structure. There are two main types of indexes: clustered and non-clustered. While both are powerful tools, they are used differently and have distinct advantages and drawbacks.
Clustered Indexes
A clustered index determines the physical order of data in a table. Think of it like a telephone directory, which is sorted by last name. Because of this sorting, a table can only have one clustered index. Here’s an example:
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CREATE CLUSTERED INDEX ix_table_column ON schemaName.tableName(columnName) |
This code snippet creates a clustered index on ‘columnName’ of ‘tableName’, which is efficient for range queries.
Non-Clustered Indexes
A non-clustered index doesn’t sort the physical data inside the table. Instead, it creates a separate object within the table that contains the column(s) included in the index, also contains a pointer to the location of the row data. Due to this separate object, a table can have multiple non-clustered indexes. For example:
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CREATE NONCLUSTERED INDEX ix_table_column ON schemaName.tableName(columnName) |
This sample code creates a non-clustered index on a ‘columnName’ of ‘tableName’. Non-clustered indexes are best for precise queries, as they return data faster than their clustered counterparts.
Differences Between Clustered and Non-Clustered Indexes
The main differences between clustered and non-clustered indexes are:
- A table can only have one clustered index, but multiple non-clustered indexes.
- Clustered indexes sort and store data rows in the table or view based on their key values. Non-clustered indexes, however, have a structure separate from the data rows, enabling faster query performance.
Conclusion
Both clustered and non-clustered indexes can dramatically improve SQL query performance. However, they should be used judiciously, as creating too many indexes can actually degrade performance. The best approach is to understand the differences between these indexes and choose the most suitable for each scenario.