Recently I had a case where queries against Aurora Reader were 2-3 times slower than on the Writer node. In this blog post, we are going to discuss why.

I am not going to go into the details of how Aurora works, as there are other blog posts discussing that. Here I am only going to focus on one part.

The Problem

My customer reported there is a huge performance difference between the Reader and the Writer node just by running selects. I was a bit surprised, as the select queries should run locally on the reader node, the dataset could fit easily in memory, there were no reads on disk level, and everything looked fine.

I was trying to rule out every option when one of my colleagues mentioned I should have a look at the InnoDB_Adaptive_Hash_Indexes. He was right – it …