Indexes
SQL indexes are specialized lookup tables that help the database search engine get data faster.
In simple words, a SQL index is a tool for quickly identifying rows with certain column values. If there were no indexes, the SQL server would have to start with the first row and work its way through the entire table to find the relevant entries. This technique, known as a full-table scan, can be extremely inefficient for large tables.
Why indexes in SQL are used ?
1. Improved Query Performance
The major reason for indexing is to speed up query processing. Indexes can dramatically minimize the amount of data the server must evaluate.
2. Efficient Data Access
Indexes allow you to quickly retrieve row data for SELECT statements. This is very useful for tables with a high number of rows.
3. Sorting and Grouping Speed
Indexes speed up data retrieval processes by presenting a sorted version of the data that is easier to handle for ORDER BY and GROUP BY operations.
4. Unique Constraints
Indexes can be used to enforce column uniqueness, ensuring that no two rows of a table have duplicate values in a single column or a combination of columns.
Types of Indexes
1. Primary key Indexes
A primary key is a field or set of fields in a database table that uniquely identifies each record (row) in the table. A primary key index is an automatically created index that is linked to the primary key column(s) to improve data retrieval and ensure data uniqueness.
Importance of Primary Key Indexes
i). Data Uniqueness
The main key index applies the uniqueness constraint to the specified column(s).
Specifically, no two records in the table can have the same values in the primary key column(s). It prevents duplicate records and ensures data accuracy.
ii). Data Retrieval Efficiency
By constructing a primary key index, the database management system (DBMS) creates a data structure that enables for quick data retrieval. Rather than scanning the entire database, the DBMS can use the main key index to determine the specific place of a given record, considerably enhancing query performance.
iii). Join Operation
Primary keys are commonly used in join procedures, which integrate data from various tables. The main key index guarantees that records are matched quickly and efficiently during these processes, hence saving processing time.
2. Unique Key Indexes
A unique index in a relational database is a data structure that enforces the uniqueness requirement on one or more columns in a table. Its main function is to verify that the values stored in the indexed column(s) are unique across all records in the database.
Role in maintaining unique values
- A unique index prevents duplicate data entries. It prevents the insertion of rows with duplicate values in the indexed column(s), hence maintaining a table's data integrity.
- When a unique index is applied to a column, the database management system (DBMS) checks for duplicate values whenever a new record is inserted or an existing record is modified in the table.
- If an insertion or update operation will result in a duplicate value in the indexed column(s), the DBMS will raise an error and reject the operation, preventing the introduction of duplicate data.
3. Clustered Indexes
A clustered index arranges and saves a table's data according to the values in one or more defined columns. Each table can only have one clustered index, and the clustering column(s) used has a major impact on data storage and retrieval.
Importance of Clustered Indexes
i). Physical Data Organization
The fundamental function of a clustered index is to physically organize the data rows in the table according to the values in the indexed column(s). This structure enables efficient data retrieval when queries request data in the same order as the clustered index.
ii). Optimized Data Retrieved
Clustered indexes are especially effective for boosting query efficiency when selecting sorting, or filtering data using the clustered index's columns. They remove the requirement for a separate data lookup operation because the data rows are already stored in the desired order.
iii). Sequential Access
A clustered index is extremely efficient for queries involving range scans or fetching a range of data values. It enables sequential access, which reduces disk I/O operations and increases query speed.
4. Non-Clustered Indexes
A non-clustered index is a type of index used in relational databases to increase data retrieval performance. Unlike clustered indexes, which change the actual order of data rows in a table, non-clustered indexes establish independent data structures to provide quick access to certain data subsets. This means that non-clustered indexes do not change the physical organization of data, but instead build a different structure to allow for faster access to the data.
Importance of Non-Clustered Indexes
i). Faster Data Retrieval
Non-clustered indexes considerably increase query performance because they enable the database management system (DBMS) to easily discover and obtain specified data rows depending on the indexed column(s).
ii). Reduced Disk I/O
When querying data, non-clustered indexes eliminate the need for full-table scans. This results in less disk input/output (I/O) operations, which speeds up query processing.
