According to the document MySQL Explain Output format, MySQL resolves all joins using a nested-loop join method. Is there any way to force MySQL to use a hash join?
- Anybody can ask a question
- Anybody can answer
- The best answers are voted up and rise to the top
Up to the (recently released) 5.6 version, no, you can't. Unfortunately, the only method that the optimizer knows is nested-loop method. As the 5.6 documentation says:
In other words, no hash join algorith or variation has been implemented, not even in version 5.7 - although that is still in development (hope never dies).
There is one alternative. MariaDB, a MySQL fork has implemented various other join methods in the latest (5.3 and 5.5) releases which can be drop-in replacements of MySQL (5.1 and 5.5 respectively) versions.
In their Block-Based Join Algorithms page, they describe the new methods:
There are also some optimizer switches that affect which algorithms will be considered when optimizing a query.
Unfortunately, no. Why ? Two reasons:
REASON #1 : Evaluation Strategy
By design, queries are evaluated in a loop using the following algorithm:
I give more details in my
REASON #2 : Query Optimization with Subqueries
The Query Optimizer has its own secret sauce on top of the Nested Join Approach. This involves removing rows during any transformation phases. This shortens the time for gathering the result set. However, this may not produce correct results when doing
I have posts about this:
UPDATE 2015-10-14 11:09 EDT
Looking over my answer and then looking at @ypercube's answer, I starting to realize that @ypercube actually hinted at changing the optimizer switch setting. Nobody has explicitly mentioned it up to this time, so I will.
Recently, I configured the following for an internal company client in
Then, to skip having to restart mysql, I ran this
The client's troublesome join queries were now running better.
Please read the MySQL Documentation on optimizer_switch.
As for the actually question, only the MySQL Query Optimizer can decide the best join method. Yet, you can help by disabling