Topic: Index Types in SQL
There are several types of SQL indexes designed to speed up the performance of a database. They function similarly to a textbook index, allowing for swift searches and queries without scanning the entire database.
1. Clustered Index: This type of index reorders the physically stored data in the table to match the logical order (the order of the index). Thus, every table can have only one clustered index. It has a one-to-one relationship with the stored data.
2. Non-Clustered Index: This is similar to having an index at the end of the book. It holds the logical order of the data which does not match the physical stored order of the data on disk. Each table can have multiple non-clustered indexes.
3. Unique Index: This is used not just for performance, but also for data integrity. It does not allow any duplicate values to be inserted into the table, enforcing the uniqueness of all values in a column or combination of columns.
4. Filtered Index: A filtered index is applied only to a certain subset of data. It uses a filter predicate to index a portion of rows in the table. Such Index is beneficial when the table has heterogeneous data values.
5. Full-text Index: This type of index is utilized for full-text queries. It can quickly find the words or phrases you’re looking for, even in a large database.
6. Spatial Index: This Index is used on spatial data types, used for geometric and geographic values.
7. XML Index: These Index is designed for xml data types which shreds xml data into a relational format.
8. Columnstore Index: This Index uses column-based data storage and query processing to achieve high query performance gains.
Interview Questions:
1. Can you explain what an index is in SQL, and what are the different types?
Answer: An index in SQL is used to speed up access to specific records in a table, similar to an index in a book. The different types of indexes in SQL include clustered, non-clustered, unique, full-text, filtered, spatial, XML, and columnstore index.
2. How does a clustered index work in SQL?
Answer: Clustered indexes define the order in which data is physically stored in a table. Table data can be sorted in only way, therefore, only one clustered index can be assigned to a table.
3. What is the difference between clustered and non-clustered indexes?
Answer: A clustered index determines the physical storage order of data in a table. In contrast, a non-clustered index does not alter the physical order of data but creates a logical order that is stored separately from the physical data.
4. What is a unique index, and why might you use it?
Answer: A unique index ensures that all values in the indexed column are unique. Its primary use is to maintain the integrity of data in a column or set of columns, meaning it prevents duplicate entries in the specific column(s).
5. What are filtered and full-text indexes designed to do?
Answer: Filtered indexes are designed to index a portion of rows in a table. This is useful when the table contains heterogeneous values and you often query a well-defined subset of rows. Full-text indexes, on the other hand, are used in full-text search scenarios, where you can quickly find words or phrases in a large database.
