Is Using 'OR' in INNER JOIN Conditions Always Bad for SQL Performance?

SQL INNER JOIN with OR: Performance Bottleneck

A recent performance optimization effort highlighted a significant issue: using the OR operator within an INNER JOIN condition. The following query exemplifies the problem:

<code class="language-sql">SELECT mt.ID, mt.ParentID, ot.MasterID
FROM dbo.MainTable AS mt
INNER JOIN dbo.OtherTable AS ot ON ot.ParentID = mt.ID OR ot.ID = mt.ParentID</code>

This query proved slow. Rewriting it using LEFT JOINs dramatically improved performance:

<code class="language-sql">SELECT mt.ID, mt.ParentID,
   CASE WHEN ot1.MasterID IS NOT NULL THEN ot1.MasterID ELSE ot2.MasterID END AS MasterID
FROM dbo.MainTable AS mt
LEFT JOIN dbo.OtherTable AS ot1 ON ot1.ParentID = mt.ID
LEFT JOIN dbo.OtherTable AS ot2 ON ot2.ID = mt.ParentID
WHERE ot1.MasterID IS NOT NULL OR ot2.MasterID IS NOT NULL</code>

The revised query executed in seconds, a substantial improvement. This raises concerns about the general use of OR in JOIN conditions.

Why OR in JOINs Can Be Slow

The core issue is that OR in JOIN conditions prevents the SQL Server optimizer from utilizing efficient HASH or MERGE joins. These optimized join methods are typically crucial for fast query execution. The OR condition prevents the server from recognizing the query's equivalence to two separate equijoins:

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

This forces SQL Server to choose a less efficient execution plan, resulting in slower performance.

Best Practices: Avoid OR in JOIN Conditions

While not strictly forbidden, using OR in JOIN conditions often hinders optimization and leads to performance degradation. For multiple join conditions, separate equijoins (as shown above with the LEFT JOIN and UNION or the LEFT JOIN with CASE statement) generally provide superior performance. This allows the query optimizer to leverage its most efficient algorithms.

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