Introduction to SQL Indexes
An SQL index is a powerful database tool that can greatly increase the speed of retrieval of data from a table in a database. However, it requires more space in the database and also slows down the insertion of new records into the database. In this article, you will learn about the different kinds of indexes in SQL. Explore our SQL course syllabus to get started.
SQL Indexes in Simple Words
Imagine you are in a library that has millions of books. Suppose you need to find a book that has the title “Quantum Physics.” Further, the books are arranged randomly in the library. In that case, you will have to check every book in the library one by one. This is called a Full Table Scan. It is time-consuming and not efficient.
Now imagine further that the books in the library are arranged in a catalog. In the catalog, you can check for the book “Quantum Physics” and find the exact shelf number. This catalog is exactly what an index in SQL is.
What are Indexes in SQL?
Indexes in SQL are data structures that the database search engine can use to speed up data retrieval. In other words, an index in SQL is simply a pointer to the data in the table.
When we create an index on a column of the table, the database creates another data structure, usually a B-Tree, which stores the column values and the pointer to the location of the physical record.
Although indexes make SELECT statements lightning-fast, there are several disadvantages:
- Storage: Indexes require storage.
- DML Performance: Every time we insert, update, or delete data, the database has to update the index, which makes the operation slower.
Begin your learning journey with our SQL tutorial for beginners.
Different Types of Indexes in SQL
It is important to know about the different types of indexes available in SQL. Each type of index is used for a different purpose based on how you access your data.
1. Clustered Index
A clustered index is a type of index in SQL that describes how data is physically stored in a table. Since data can only be sorted one way, you can only have one clustered index per table.
In general, if you have a Primary Key defined for a table, you will already have a clustered index defined for that table.
Example: Phone books are a type of clustered index because they physically store data sorted alphabetically.
2. Non-Clustered Index
A non-clustered index is another type of index available in SQL. Unlike a clustered index, a non-clustered index does not physically sort your data in a table. Instead, it creates a separate object that is sorted and points to your actual data location via a Row ID.
Note: You can have multiple non-clustered indexes per table.
Example: The index at the back of a textbook is a type of non-clustered index. The actual data in the textbook is not physically sorted to correspond to this index.
3. Unique Index
A unique index is used to guarantee that there are no duplicates in the index keys. This is automatically created when defining a constraint on a column as UNIQUE.
Kickstart your DB career with our SQL training in Chennai.
Detailed Breakdown: Types of Indexing in SQL
While talking about different types of indexes used in SQL, there are many times when we divide them based on their structure.
4. Composite Index (Filtered Indexing)
This type of indexing in SQL which is known as a composite index is an index on multiple columns. The position of columns in an index is very important (Left to Right Rule).
Note: This index is useful when you want to search for records based on (State) and (State and City), but not very useful for (City).
How Indexing Works: Under the Hood
Most types of indexing in SQL use a B-Tree (Balanced Tree) data structure.
- Root Node: The starting point of the search.
- Internal Nodes: Directional nodes that point to the next level.
- Leaf Nodes: For a clustered index, this is where actual data is stored. For a non-clustered index, this is where actual data is stored through pointers.
Gain industry expertise with our SQL interview questions and answers.
Different Types of Index in SQL – Advanced Types
In addition to the basic types, specific index types in SQL are used for specific types of data. They are as follows:
5. Full-Text Index
This index is used for complex strings. The standard index has difficulty with the pattern LIKE ‘%value%’. The Full-Text index allows for fuzzy searching.
6. Spatial Index
This index is used for geographical data types, such as Geometry or Geography. This allows for queries such as “Find all stores within 5 miles of this location.”
7. Filtered Index
This index allows for the indexing of only some of the data in the table. This is useful for columns where there are a lot of NULL values or where the values are of a specific status.
Finetune your DB skills witn our SQL project ideas.
Best Practices for SQL Indexing
The type of indexing used in SQL must be chosen strategically:
- Index the “Search” Columns: Columns used in the “WHERE,” “JOIN,” “ORDER BY,” and “GROUP BY” clauses are prime indexing targets.
- Don’t Over-Index: All indexing operations will hinder “INSERT” and “UPDATE” operations. Avoid indexing columns with low “cardinality.” Cardinality refers to the number of different values in a set of data.
- For example, if your table has a “Gender” field, this field should not be indexed unless there are more than two genders.
- Monitor Fragmentation: After a while, as data is constantly added and modified, the data in the indexed table becomes fragmented. This calls for regular “Rebuilding” and “Reorganizing” of the indexed table.
- Use Covering Indexes: If the result set contains all the data requested, then the table does not have to be accessed at all. This is the fastest way data can be retrieved.
Conclusion
Knowing how to use indexes in SQL is the key to moving from an application with functionality to an application with high performance. With the right indexes in SQL, whether it’s the physical arrangement of the data on the disk with the clustered index or the flexible pointers used by the non-clustered index, it’s possible to bring the time taken for queries down from seconds to milliseconds.
However, it’s worth remembering that indexing is all about balance. Make sure to examine your queries, use the EXPLAIN plan to see how the database is looking through your data, and apply the correct indexes in SQL. Learn SQL from basic in our software training institute in Chennai.
