Left Join outperforming Inner Join?

Question

I\'ve been profiling some queries in an application I\'m working on, and I came across a query that was retrieving more rows than necessary, the result set being trimmed dow

Answer 1

If you think that the implementation of LEFT JOIN is INNER JOIN + more work, then this result is confusing. What if the implementation of INNER JOIN is (LEFT JOIN + filtering)? Ah, it is clear now.

In the query plans, the only difference is this: users... extra: using where . This means filtering. There's an extra filtering step in the query with the inner join.

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This is a different kind of filtering than is typically used in a where clause. It is simple to create an index on A to support this filtering action.

SELECT *
FROM A
WHERE A.ID = 3

Consider this query:

SELECT *
FROM A
  LEFT JOIN B
  ON A.ID = B.ID
WHERE B.ID is not null

This query is equivalent to inner join. There is no index on B that will help that filtering action. The reason is that the where clause is stating a condition on the result of the join, instead of a condition on B.