...
At first the gaps in functionality and reliability between MySQL and PostgreSQL were perceived as annoying nuisances, but as the scale of Open edX and the sites running it have grown, so has the scale of the pain in dealing with MySQL’s foibles. Some of the more notable ones:
No transactional Transactional DDL. If a database schema migration fails in MySQL, it has a nontrivial risk of leaving the database in a broken state that triggers a CAT-1 incident to restore site functionality. In PostgreSQL, such a failure simply results in the transaction being rolled back so the database is left in the state it was in before the migration attempt.
2nd1st-class Django support. Django has many useful features that only work with PostgreSQL, and many more cases where “this works on MySQL, but not as well as on PostgreSQL”. And this is a logical prioritization choice for the project’s maintainers - according to the 2022 JetBrains survey of Django developers, 79% of them work with PostgreSQL while only 28% use MySQL in any capacity.
Better full text search. MySQL comes with a rudimentary full text search engine, but the one in PostgreSQL is good enough to replace dedicated tools like Elasticsearch for many use cases (including many Open edX installations).
Better JSON support. MySQL can store JSON data has text, but its availability to work with it as structured data is very limited. PostgreSQL does this well enough to outperform dedicated document databases like MongoDB in many use cases.
Proper Unicode behavior. MySQL made the odd choice to make its default “utf8” encoding only support 3 of the 4 available bytes in the UTF-8 standard. The full 4-byte encoding was only added later as “utf8mb4”, and the default character set was still the much more restrictive latin1 until MySQL 8.0.
Developer Experience/Hiring/Retention. For many reasons like the ones noted above, PostgreSQL has become strongly preferred over MySQL by a solid majority of developers. StackOverflow’s 2023 developer survey notes that about 43% of developers would like to be working with PostgreSQL vs. only 24% for MySQL (and 71% of PostgreSQL users are happy with it, vs. only 51% for MySQL). Consequently, despite MySQL’s early lead in popularity for historical reasons, PostgreSQL is now more widely used. (The MariaDB fork of MySQL fares even worse on most of these metrics.). This may not be a determining factor in software engineers deciding where to work, but it lands database selection solidly in the “con” column for many such decisions and could help tip the scales one way or the other.
...