Bug #101510 Documentation about when hash join can be used could be more specific

• Submitted: 8 Nov 2020 19:02
• Modified: 11 Nov 2020 0:19
• Reporter: Mark Callaghan
• Status: Closed
• Category: MySQL Server: Documentation
• Severity: S3 (Non-critical)
• Version: 8.0.22
• OS: Any
• Assigned to: Jon Stephens
• CPU Architecture: Any

[8 Nov 2020 19:02] Mark Callaghan

Description:
The docs state:
"Beginning with MySQL 8.0.18, MySQL employs a hash join for any query for which each join has an equi-join condition and uses no indexes, such as this one:"

That statement isn't clear. A hash join is possible when no indexes exist that could be used for a join. If there are indexes that can be used for single-table predicates, then hash join is still possible.

https://dev.mysql.com/doc/refman/8.0/en/hash-joins.html

One of your blog posts makes the same claim as I do:
"In general, hash join will be used if you are joining together tables using one or more equi-join conditions, and there are no indexes available for the join conditions. If an index is available, MySQL tends to favor nested loop with index lookup instead."

https://mysqlserverteam.com/hash-join-in-mysql-8/

How to repeat:
Read the docs and blog post

[9 Nov 2020 3:29] MySQL Verification Team

Hello Mark,

Thank you for the report and feedback!

Thanks,
Umesh

[11 Nov 2020 0:19] Jon Stephens

Fixed in mysqldoc rev 67998, mysql-relnotes rev 21389.

Closed.