Does ANSI JOIN Syntax Improve T-SQL Query Performance?

ANSI JOIN and T-SQL query performance: an optimization mystery

Many T-SQL users face a dilemma: Will switching to ANSI JOIN syntax improve the performance of their queries? With the introduction of the JOIN keyword in ANSI-92 SQL, it's time to re-evaluate its impact on performance.

Consider the following two queries that have similar functionality but different syntax:

SELECT A.A, B.B, C.C
FROM aaa AS A, bbb AS B, ccc AS C
WHERE
    A.B = B.ID
AND B.C = C.ID
AND C.ID = @param

SELECT A.A, B.B, C.C
FROM aaa AS A
JOIN bbb AS B
   ON A.B = B.ID
JOIN ccc AS C
   ON B.C = C.ID
   AND C.ID = @param

Query performance comparison

The key question is whether ANSI JOIN syntax will bring any performance improvement. Surprisingly, the answer is no. Both queries generate the same internal query plan, resulting in the same execution time.

Advantages of ANSI JOIN syntax

Although there is no performance improvement, ANSI JOIN syntax has several advantages over the old SQL syntax:

• Readability enhancement: Separates relationship logic from filtering logic, enhancing query clarity.
• Reduced ambiguity: It eliminates potentially ambiguous outer joins, ensuring consistent query parsing across different implementations.
• Meet modern standards: ANSI-92 syntax is industry-standard and supported by modern query tools.
• Preventing cross-joins: Using the JOIN clause helps avoid accidental cross-joins, which can severely impact performance.

Conclusion

While ANSI JOIN syntax does not directly improve query performance, its advantages in readability, ambiguity reduction, and compliance make it the first choice for modern SQL programming. The additional clarity and reduced maintenance overhead far outweigh any potential performance tradeoffs.

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