SQLite indexes are a fundamental feature of the SQLite database management system that allows for efficient querying and retrieval of data. Indexes are a mechanism for organizing data in a way that makes it faster to search and retrieve information from a database.
An SQLite index is a data structure that is used to speed up database queries by providing a quick lookup mechanism for specific data within a database. Indexes are used to improve the performance of
DELETE, and other queries that search for data within a database. An index is created by selecting one or more columns from a database table and building a data structure that allows for quick lookups based on the values in those columns.
Indexes are especially useful for large databases that contain a large amount of data. Without an index, queries that search through large amounts of data can be slow and inefficient. With an index, the database engine can quickly locate the specific data that is needed, leading to faster query performance.
In SQLite, an index is created on one or more columns of a table, and it stores a copy of the data in those columns in a separate structure that is optimized for fast searching. When a query is executed on the table, the index is used to quickly locate the rows that match the search criteria, without having to scan the entire table
Creating an index
CREATE INDEX idx_person_name ON person (name);
This creates an index named idx_person_name on the name column of the person table.
Dropping an index
DROP INDEX idx_person_name;
This drops the idx_person_name index from the database.
When creating an index, it is important to carefully select the columns to include in the index. Generally, columns that are frequently searched or sorted on are good candidates for indexing. However, it is also important to balance the benefits of indexing with the overhead of maintaining the index, which can increase the size of the database and slow down data insertion and update operations.
unique index is an index that enforces uniqueness on one or more columns in a table. When a unique index is created, SQLite ensures that no two rows in the table have the same value(s) for the column(s) covered by the index. If an attempt is made to insert a row with a duplicate value for the column(s) covered by the unique index, the insert will fail and SQLite will report an error.
Unique indexes can be created on one or more columns in a table using the
CREATE UNIQUE INDEX statement. For example, to create a unique index on the “email” column of a table named “users”, you could use the following SQL statement:
CREATE UNIQUE INDEX idx_users_email ON users(email);
This statement creates a unique index named “idx_users_email” on the “email” column of the “users” table.
Indexes on Expressions
An index on an expression is an index that is created based on the result of evaluating an expression, rather than on a column value directly. This can be useful when you want to index a computed value or a function result, rather than a raw column value.
To create an index on an expression, you can use the
CREATE INDEX statement and specify the expression as the index key. For example, to create an index on the lowercase version of the “email” column in the “users” table, you could use the following SQL statement:
CREATE INDEX idx_users_email_lower ON users(lower(email));
This statement creates an index named “idx_users_email_lower” on the result of applying the lower() function to the “email” column of the “users” table.
Indexes on Expressions can also be used to improve performance when querying data that involves computed values or function results. For example, if you frequently search for all users with email addresses ending in a certain domain, you could create an index on the result of applying the substr() function to the “email” column to extract the domain name:
CREATE INDEX idx_users_email_domain ON users(substr(email, instr(email, '@')+1));
This statement creates an index named “idx_users_email_domain” on the result of extracting the domain name from the “email” column of the “users” table.
SQLite indexes are a powerful tool for improving database query performance. By carefully selecting the columns to include in an index and choosing the appropriate index type, developers can significantly speed up their database queries and improve the overall performance of their applications.