Topic: Index in SQL
An Index in SQL is a database object that improves the speed of data retrieval operations on a database table. It is essentially a lookup table that the database can use to retrieve data more swiftly. Indexes provide faster access to data for operations that return a small portion of a table’s rows. They can be created on one or more columns of a database table, providing the basis for both rapid random lookups and efficient ordering of access to records.
Indexes are crucial for achieving high levels of database performance. Having the right indexes in place can dramatically speed up your data retrieval queries.
However, they also come with some disadvantages. Indexes take up disk space. Indexes also slow down the speed of writing operations such as INSERT, UPDATE, DELETE. Therefore, the decision of what indexes to create is often a trade-off between the amount you’re willing to spend on disk storage and query speed.
Interview Questions:
1. What is the purpose of an index in SQL?
Answer: The aim of an index is to enhance the speed of data retrieval operations on a database. It acts as a lookup table that the database can use to fetch data more rapidly, saving time and computing resources.
2. What are the different types of Indexes in SQL?
Answer: Some of the different types of indexes in SQL include Clustered Index, Non-Clustered Index, Unique Index, and Full-text Index. The Clustered Index sorts and stores the data rows in the table based on their key values. A Non-Clustered Index stores a second set of data in a different order. A Unique Index does not allow any duplicate values to be inserted into the table. A Full-text Index aids in performing full-text searches.
3. How do you create an index in SQL?
Answer: Indexes in SQL can be created using the CREATE INDEX statement. Below is the syntax example:
`CREATE INDEX index_name ON table_name (column1, column2, … );`
4. When should you avoid using indexes in SQL?
Answer: Avoid using indexes in situations where the table data is frequently updated, as the indexes will need to be updated and this can lead to slower performance. Also, do not use indexes on small tables, as the performance gain will be negligible.
5. What is the difference between Clustered and Non-Clustered Index in SQL?
Answer: A clustered index determines the order of the physical data in a table. There can only be one clustered index on a table. Non-clustering indexes, on the other hand, do not change the physical order of data, but create a logical order that is stored separately from the data, much like an index in a book. We can have multiple non-clustered indexes on a single table.
