INNER JOIN vs. LEFT JOIN: When is One Significantly Faster Than the Other?

Detailed explanation of the performance difference between INNER JOIN and LEFT JOIN in SQL Server

In SQL Server, performance considerations are often key to query optimization decisions. Choosing between INNER JOIN or LEFT JOIN, especially in queries involving multiple tables, is a common question. Although people often think that LEFT JOIN is inherently faster, this is not always true.

Why is LEFT JOIN usually not faster?

LEFT JOIN will retrieve all rows in the left table, even if there are no matching rows in the right table. In contrast, INNER JOIN only returns rows where a match is found. This means that the LEFT JOIN itself will do more work and generate more result rows, which may result in longer execution times.

Exception: When might a LEFT JOIN be faster?

In rare cases, LEFT JOIN may be faster than INNER JOIN due to certain factors. The main reasons are as follows:

• Very Small Tables: When the table involved is very small (e.g., less than ten rows), the overhead of a hash match (used by INNER JOIN) may exceed that of a nested loop (used by LEFT JOIN ) cost.
• Lack of sufficient indexes: If a table lacks appropriate indexes to support INNER JOIN, SQL Server may choose a less efficient execution plan, resulting in slower queries. In this case, nested loops using LEFT JOIN may be preferable.

Example

Consider the following example:

<code class="language-sql">CREATE TABLE #Test1 (ID int NOT NULL PRIMARY KEY, Name varchar(50) NOT NULL)
INSERT INTO #Test1 (ID, Name) VALUES (1, 'One'), (2, 'Two'), (3, 'Three'), (4, 'Four'), (5, 'Five')

CREATE TABLE #Test2 (ID int NOT NULL PRIMARY KEY, Name varchar(50) NOT NULL)
INSERT INTO #Test2 (ID, Name) VALUES (1, 'One'), (2, 'Two'), (3, 'Three'), (4, 'Four'), (5, 'Five')

SELECT *
FROM #Test1 t1
INNER JOIN #Test2 t2
ON t2.Name = t1.Name

SELECT *
FROM #Test1 t1
LEFT JOIN #Test2 t2
ON t2.Name = t1.Name

DROP TABLE #Test1
DROP TABLE #Test2</code>

When executed on these small tables, LEFT JOIN queries are slightly faster as the overhead of hash matching outweighs the cost of the extra result size. However, on larger tables with more than ten rows, INNER JOIN queries are much faster.

Conclusion

In most cases, INNER JOIN performs better than LEFT JOIN in terms of execution speed. Exceptions are limited to very specific conditions involving extremely small tables and insufficient indexes. Therefore, it is critical to thoroughly examine query designs and indexes to identify potential performance bottlenecks, rather than replacing INNER JOINs with LEFT JOINs just for performance reasons.

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