Cross Join vs. Inner Join with WHERE Clause: Which Performs Better?

Cross Join vs Inner Join with WHERE Clause: Performance Comparison

In contrast to inner joins, cross joins return all possible row combinations across multiple tables without establishing any relationship. This can result in a significant number of rows, especially for large tables.

Consider the following concrete example:

SELECT User.*
FROM User, Address
WHERE User.addressId = Address.id;

This cross join, equivalent to an inner join with a WHERE clause, produces all combinations of rows from the User and Address tables.

SELECT User.*
FROM User
INNER JOIN Address ON (User.addressId = Address.id);

In contrast, an inner join filters the results based on the join condition, resulting in a smaller set of rows where values match.

Contrary to popular belief, optimizations do not automatically convert cross joins into inner joins. While performance may not be noticeably improved after switching to inner joins, this is not due to optimizations but rather the specific query characteristics and DBMS behavior.

Cross joins often produce more rows than inner joins, consuming more memory and processing resources. Therefore, inner joins are generally preferred for performance considerations.

Factors to consider when selecting between cross joins and inner joins include query complexity, table size, and desired output. If data filtering is required, inner joins offer a more efficient and accurate means to retrieve the desired results.

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