GitLab Documentation

Storing SHA1 Hashes As Binary

Storing SHA1 hashes as strings is not very space efficient. A SHA1 as a string requires at least 40 bytes, an additional byte to store the encoding, and perhaps more space depending on the internals of PostgreSQL and MySQL.

On the other hand, if one were to store a SHA1 as binary one would only need 20 bytes for the actual SHA1, and 1 or 4 bytes of additional space (again depending on database internals). This means that in the best case scenario we can reduce the space usage by 50%.

To make this easier to work with you can include the concern ShaAttribute into a model and define a SHA attribute using the sha_attribute class method. For example:

class Commit < ActiveRecord::Base
  include ShaAttribute

  sha_attribute :sha

This allows you to use the value of the sha attribute as if it were a string, while storing it as binary. This means that you can do something like this, without having to worry about converting data to the right binary format:

commit = Commit.find_by(sha: '88c60307bd1f215095834f09a1a5cb18701ac8ad')
commit.sha = '971604de4cfa324d91c41650fabc129420c8d1cc'

There is however one requirement: the column used to store the SHA has must be a binary type. For Rails this means you need to use the :binary type instead of :text or :string.