Why Are OR Conditions in INNER JOINs a Performance Bottleneck?

SQL Query Optimization: Avoiding OR in INNER JOINs

Optimizing a slow SQL query often reveals performance bottlenecks. One common culprit is the use of OR conditions within INNER JOIN statements. Replacing such OR conditions with LEFT JOINs and a WHERE clause can dramatically improve query speed. But is using OR in joins always a problem?

The Performance Penalty of OR in JOINs

JOINs with OR conditions frequently impede optimization. This prevents SQL Server (or other database systems) from utilizing highly efficient join algorithms like HASH JOIN and MERGE JOIN.

Understanding the Performance Issue

The OR condition forces the database to treat the join as a series of UNION operations. Consider this example:

A query originally written with an OR condition in the join:

(This is a simplified representation, the actual execution plan can be more complex)

The database might effectively execute it as:

<code class="language-sql">SELECT *
FROM maintable m
JOIN othertable o
ON o.parentId = m.id
UNION ALL
SELECT *
FROM maintable m
JOIN othertable o
ON o.id = m.parentId</code>

Each UNION ALL operation represents an equijoin, but the database may fail to recognize this efficient structure in the original OR-based query.

Best Practices: Replacing OR with LEFT JOINs and WHERE

While OR in join conditions is syntactically correct, it often severely impacts performance by hindering query optimization. Refactoring to use multiple LEFT JOINs combined with a WHERE clause is generally recommended for optimal performance. This allows the database to apply its most efficient join strategies.

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