In MySQL 8.0.18 there is a new feature called Hash Joins, and I wanted to see how it works and in which situations it can help us. Here you can find a nice detailed explanation about how it works under the hood.

The high-level basics are the following: if there is a join, it will create an in-memory hash table based on one of the tables and will read the other table row by row, calculate a hash, and do a lookup on the in-memory hash table.

Great, but does this give us any performance benefits?

First of all, this only works on fields that are not indexed, so that is an immediate table scan and we usually do not recommend doing joins without indexes because it is slow. Here is …

In my recent SQL work for a large Swiss bank, I have maintained nested database view monsters whose unnested SQL code amounted up to 5k lines of code, joining the same table over and over again in separate subselects combined via UNION operations. This monster performed in way under 50ms, no matter how we queried … Continue reading The Myth About Slow SQL JOIN Operations →

MariaDB 5.3/5.5 has introduced a new join type “Hash Joins” which is an implementation of a Classic Block-based Hash Join Algorithm. In this post we will see what the Hash Join is, how it works and for what types of queries would it be the right choice. I will show the results of executing benchmarks for different queries and explain the results so that you have a better understanding of when using the Hash Join will be best and when not. Although Hash Joins are available since MariaDB 5.3, but I will be running my benchmarks on the newer MariaDB 5.5.

Overview

Hash Join is a new algorithm introduced in MariaDB 5.3/5.5 that can be used for joining tables that have a equijoin conditions of the form tbl1.col1 = tbl2.col1, etc. As I mentioned above that what is actually implemented is the Classic Hash Join. But its known as Block Nested Loop Hash (BNLH) Join in MariaDB.
The Classic Hash Join Algorithm …

This is the third blog post in the series of blog posts leading up to the talk comparing the optimizer enhancements in MySQL 5.6 and MariaDB 5.5. This blog post is targeted at the join related optimizations introduced in the optimizer. These optimizations are available in both MySQL 5.6 and MariaDB 5.5, and MariaDB 5.5 has introduced some additional optimizations which we will also look at, in this post.

Now let me briefly explain these optimizations.

Batched Key Access

Traditionally, MySQL always uses Nested Loop Join to join two or more tables. What this means is that, select rows from first table participating in the joins are read, and then for each of these rows an index lookup is performed on the second table. This means many point queries, say for example if table1 yields 1000 …