Why can UNION ALL be faster than UNION?

Problem Statement

Explanation

UNION performs a distinct operation across inputs, which costs CPU and memory. UNION ALL concatenates results and is usually faster. Use UNION when you truly need deduplication. Otherwise prefer UNION ALL and dedupe later if needed.

Code Solution

SolutionRead Only

SELECT id FROM a
UNION ALL
SELECT id FROM b;

Practice Sets

This question appears in the following practice sets:

CTEs, Subqueries & Advanced Joins

Next Question

Master Interviews
Anywhere, Anytime

Why can UNION ALL be faster than UNION?

Problem Statement

Explanation

Code Solution

Practice Sets

Related Questions

Which SQL statement retrieves all columns and rows from a table named employees?

Which clause filters rows before aggregation and affects which rows are grouped?

What is the default sort order when ORDER BY is used without ASC or DESC?

Which construct is commonly used to paginate results in SQL engines like Postgres and MySQL?

Which predicate correctly checks for missing values in standard SQL?

More from SQL & Databases

Master Interviews Anywhere, Anytime

Why can UNION ALL be faster than UNION?

Problem Statement

Explanation

Code Solution

Practice Sets

Related Questions

Which SQL statement retrieves all columns and rows from a table named employees?

Which clause filters rows before aggregation and affects which rows are grouped?

What is the default sort order when ORDER BY is used without ASC or DESC?

Which construct is commonly used to paginate results in SQL engines like Postgres and MySQL?

Which predicate correctly checks for missing values in standard SQL?

More from SQL & Databases

Master Interviews
Anywhere, Anytime