Cross Join with WHERE vs. Inner Join: Which SQL Join Offers Better Performance?

Performance Comparison: Cross Join with WHERE Clause vs. Inner Join

The efficacy of using a cross join followed by a WHERE clause is often debated against the performance of an inner join. While both approaches produce similar results, there may be certain situations where one performs better than the other.

Cross Join with WHERE Clause

A cross join combines all rows from two or more tables, regardless of any relationships between them. When combined with a WHERE clause, a cross join effectively filters out the desired rows based on the specified predicates.

Inner Join

An inner join matches rows from two tables based on equality comparisons defined by the ON or USING clauses. The resulting rows include only the columns of the matching rows.

Performance Considerations

Generally, inner joins are more efficient than cross joins with WHERE clauses.

In cross joins, the DBMS must first generate the entire cartesian product of the two tables, which can be computationally expensive, especially for large tables. Applying the WHERE clause then eliminates unwanted rows, requiring additional processing.

Inner joins, on the other hand, only combine rows that meet the join conditions, eliminating the need for an initial cartesian product. This results in faster execution times.

Example

Consider the example queries provided:

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

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

The second query using an inner join will typically execute faster than the first query with a cross join and WHERE clause. The DBMS can directly select the desired rows from the User table based on the equality comparison, without first generating a large cartesian product.

Conclusion

While cross joins with WHERE clauses can achieve similar results to inner joins, inner joins generally provide better performance. However, it's important to note that specific vendor performance guidelines may impact the optimal choice of join in different situations.

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