Subqueries vs. Joins: Why is an Inner Join So Much Faster?

Subqueries vs. Joins: A Performance Comparison

A recent application optimization involved replacing a SQL subquery with an inner join, resulting in a remarkable 100-fold speed increase. This highlights a crucial performance difference between these two SQL techniques.

The key lies in understanding correlated subqueries. These subqueries depend on values from the outer query, necessitating repeated execution—once for every row in the outer query. In contrast, non-correlated subqueries execute only once, independently of the outer query.

The inner join dramatically improves performance by avoiding this iterative execution. The query execution plans illustrate this:

• Subquery: "DEPENDENT SUBQUERY [...] Using where" (repeated execution per outer row)
• Inner Join: "SIMPLE [...] eq_ref [...] Using index" (single execution)

The single execution of the inner join, facilitated by efficient index usage, accounts for the significant speed advantage. While correlated subqueries are sometimes unavoidable, strategies like decomposing them into multiple non-correlated subqueries or optimizing index usage can mitigate performance bottlenecks.

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