SQL INDEX

What does the SQL INDEX statement do?

Defines and maintains a data structure that accelerates row retrieval by providing quick lookups on one or more columns.

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Compatible dialects for SQL INDEX: PostgreSQL, MySQL, MariaDB, SQL Server, Oracle, SQLite, Snowflake, BigQuery, Redshift

SQL INDEX Full Explanation

An INDEX is a physical or logical structure that stores the values of specified column(s) in a way that makes searches, joins, and filtering operations faster. Instead of scanning every row, the database engine navigates the index (often a B-tree or hash) to locate matching records quickly. Indexes can enforce uniqueness, support primary and foreign keys, and improve ORDER BY performance. However, they consume storage and slow down INSERT, UPDATE, and DELETE operations because the index itself must be updated. Proper indexing requires balancing read speed with write overhead and choosing the right index type for each workload.

SQL INDEX Syntax

-- Create a standard B-tree index
CREATE INDEX index_name
ON table_name (column1 [ASC|DESC], column2 ...);

-- Create a unique index
CREATE UNIQUE INDEX index_name
ON table_name (column1);

-- Drop an index
DROP INDEX index_name ON table_name;

SQL INDEX Parameters

index_name (identifier) - Name of the index to create or drop
table_name (identifier) - Table on which the index is built
column_list (list) - One or more columns to include in the index, with optional sort order
index_type (keyword) - Optional engine-specific clause (e.g., USING HASH, USING GIN) specifying index method

Example Queries Using SQL INDEX

-- Speed up email lookups
CREATE INDEX idx_users_email
ON users (email);

-- Enforce unique invoice numbers
CREATE UNIQUE INDEX idx_invoice_number
ON invoices (invoice_number);

-- Composite index for common filter and sort pattern
CREATE INDEX idx_orders_customer_date
ON orders (customer_id, order_date DESC);

-- Remove an obsolete index
DROP INDEX idx_old_temp ON temp_table;

Expected Output Using SQL INDEX

Each CREATE INDEX statement builds the index and returns a confirmation such as "CREATE INDEX"
Subsequent SELECT queries that filter or sort on the indexed columns use the index, showing lower execution time
DROP INDEX removes the structure and frees space

Use Cases with SQL INDEX

Accelerate equality and range searches on large tables
Enforce uniqueness without declaring a full PRIMARY KEY
Speed up JOINs by indexing foreign key columns
Optimize frequent ORDER BY or GROUP BY operations
Support full-text search or geospatial queries with specialized index types

Common Mistakes with SQL INDEX

Creating too many indexes, which bloats storage and slows writes
Indexing low-cardinality columns like boolean flags, yielding little benefit
Forgetting to include all columns used in WHERE and ORDER BY, leading to index misses
Assuming indexes improve every query; they only help if the planner chooses them
Neglecting to rebuild or analyze indexes after massive data loads

First Introduced In

SQL-92

Frequently Asked Questions

What is the difference between a clustered and nonclustered index?

A clustered index defines the physical order of rows on disk (only one allowed per table in most engines). A nonclustered index stores a separate structure that points back to the table, allowing multiple per table.

Can I index an expression or function?

Many databases support functional or expression indexes, letting you index computed values like LOWER(email) for case-insensitive searches. Syntax varies by dialect.

How do I check if my query uses an index?

Run an EXPLAIN or EXPLAIN ANALYZE plan before the query. The output shows whether the planner chooses an index scan and which index it uses.

Should I index every foreign key?

Indexing foreign key columns usually speeds up joins and deletes, but small tables may not benefit. Benchmark before adding unnecessary indexes.