Indexes are a pivotal feature of databases, optimizing the speed of data retrieval operations on database tables. By reducing the amount of data that needs to be examined, indexes enable databases to return results faster for read operations. Essentially, they function as look-up tables that the database search engine can utilize to speed up data retrieval. Here, we’ll delve into the different types of indexes in databases, their unique properties, and how they differ when applied to tables.

Index TypeDescriptionBest Use Case
Single-ColumnIndex on a single column of a tableOperations involve only a single data field
CompositeIndex on multiple columnsOperations often involve multiple fields
UniqueEnsures the indexed data is uniquePrimary keys or unique identifiers
BitmapIndexes using bit arraysColumns with low cardinality
B-TreeAllows data find by branching pathHandling equality and range queries
HashUses a hash function for indexingEfficient for equality searches
PartialIndexes only a portion of a tableLarge tables where only a small subset of rows are frequently accessed
ClusteredRows order in table corresponds to keys orderEfficient for range queries
Non-ClusteredStructure separate from data rowsData pages stored in a heap or a table with a clustered index
Full-textSupports full-text queriesLarge text fields containing full sentences or paragraphs
SpatialIndexes spatial data typesData represents physical location and shape of geometric objects
FilteredIndexes with a WHERE clauseIndexing a subset of rows in a table
Covering (Included)Includes all columns needed to process a queryQuery can be processed using only the index
Reverse KeyReverses the order of bytes in the indexed columnPrevent contention from inserts in monotonically increasing or decreasing indexed columns
Expression-basedCreated on the result of an expression or functionQuerying the database using some computation on a column

Keep in mind that the choice of index type depends on the specific requirements of your database, the nature of the data it holds, and the kind of operations you intend to perform on that data. Each index type has its own strengths and weaknesses, and understanding these can help you optimize your database’s performance.

1. Single-Column Indexes

Single-column indexes, as the name suggests, are created on a single column of a table. They are the most basic type of index and are typically used when operations involve only a single data field. For instance, a unique index might be used on an ID column to quickly find a specific record by its unique identifier.

2. Composite (Concatenated) Indexes

Composite indexes, also known as concatenated indexes, are built on two or more columns of a table. They are useful when queries often involve multiple fields. For instance, a composite index might be created on a ‘first_name’ and ‘last_name’ field to quickly find records based on a person’s full name. The order of columns in the index can significantly impact its effectiveness. Columns used in WHERE clause conditions should be placed earlier in the index for optimal performance.

3. Unique Indexes

Unique indexes ensure the data contained in the indexed column or columns for each row in the table is unique. If the database engine is asked to insert a duplicate entry into a unique indexed column, it will refuse the operation. This type of index is often used for primary keys in a table, such as a ‘user_id’ column in a User table, to ensure no two users have the same ID.

4. Bitmap Indexes

Bitmap indexes utilize bit arrays (bitmaps) and answer queries by performing bitwise operations on these bitmaps. They are most effective in scenarios where the column data is of low cardinality, meaning that the column’s values are very repetitive. For example, a ‘gender’ column, which typically only has a small set of possible values (e.g., ‘male’, ‘female’), would be an ideal candidate for a bitmap index.

5. B-Tree Indexes

B-tree indexes are the most common type of index. They allow the database to find data by following a branching path, leading to the desired value. This makes them excellent for handling equality and range queries. B-tree indexes can be applied to any data type that supports sorting and can be used for indexing both single and multiple columns in a table.

6. Hash Indexes

Hash indexes use a hash function to map index keys to locations in a hash table. They are incredibly efficient for equality searches, but they are unsuitable for range queries or sorting operations. This is because the nature of a hash function is such that it scatters entries across the hash table, and there’s no correlation between the proximity of keys in the index and their corresponding rows in the table.

7. Partial Indexes

Partial indexes index only a portion of a table. They can be particularly useful when there are large tables and only a small subset of rows are frequently accessed. By indexing only these rows, you can save storage and speed up query times.

8. Clustered Indexes

Clustered indexes organize the rows in a table or a view in a specific order to match the index. This order is dictated by the column or columns involved in the index. Therefore, the sequence of data in a clustered index aligns exactly with how the data is stored in the table, making it a highly efficient method for both range and individual record queries.

It’s important to note that there can only be one clustered index per table since the data rows themselves can only be sorted in one order. This does not, however, limit the number of non-clustered indexes that can be applied to the same table.

9. Non-Clustered Indexes

Non-clustered indexes, on the other hand, have a structure separate from the data rows. This type of index contains the non-clustered index key values and each key value entry has a pointer to the data row that contains the key value. The pointer from an index row in a non-clustered index to a data row is called a row locator. The structure of the row locator depends on whether the data pages are stored in a heap or a table with a clustered index.

10. Full-text Indexes

Full-text indexes in SQL Server support full-text queries against character-based data. These queries can include simple words and phrases or multiple forms of a word or phrase. This type of index can be used on large text fields that contain full sentences or paragraphs. For example, if you have a ‘product_description’ field and you want to allow users to search for any word within the description, you would use a full-text index.

11. Spatial Indexes

Spatial indexes are used to index spatial data types. Spatial data represents the physical location and shape of geometric objects. These objects can be point locations or more complex objects such as countries, roads, or lakes. If you’re using a spatial column to store these types of data, a spatial index can help speed up queries that perform operations on this data, such as finding all locations within a certain distance of a point.

12. Filtered Indexes

Filtered indexes are a type of index with a WHERE clause, and they’re only available in Microsoft SQL Server. They’re used to index a subset of rows in a table, which can improve query performance, reduce index maintenance costs, and reduce index storage costs compared with full-table indexes.

13. Covering Index or Included Columns

A covering index includes all the columns needed to process a particular query. For a query to be covered, every column in the query must either be a part of the index or included in it. This means the entire query can be processed using only the index, and the database engine doesn’t need to look up the original data table, which can improve performance.

14. Reverse Key Indexes

Reverse key indexes reverse the order of the bytes in the indexed column before storing the index entry. They are used to prevent contention from inserts that would naturally occur in monotonically increasing or decreasing indexed columns, such as sequence numbers, dates, or timestamps.

15. Expression-based Indexes

Expression-based indexes are created on the result of an expression or function, rather than on a column’s values. These indexes are used when you commonly query the database using some computation on a column. For example, if you frequently query the database for all rows where the length of a string column is above a certain size, you might create an expression-based index using the LEN() function.

Each type of index serves a specific purpose and use case. It’s essential to understand the nature of the data in your database and the kinds of queries you’ll be performing to choose the most suitable index type. Remember that while indexes can significantly speed up data retrieval, they do incur overheads during data insertion, deletion, and updating. As such, careful thought should be given to the number and type of indexes used.


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