Entity attribute value anti pattern - this has been well known for at least 20 years. It can be tempting when you want to design a "flexible" system but really needs to be used sparingly. I was BI team lead on a product where the architect insisted that it be used on every entity (>300) as you never knew when you might want to add some bit of data. It led to some interesting (multi-page) sqls and the project ultimately failed. This was one of the reasons. Slow performance and often runtime errors when expected data wasn't present and the application layer couldn't cope. It was a good learning experience.
https://mikesmithers.wordpress.com/2013/12/22/the-anti-patte...
We have this as a "meta" field (because MySQL is balls at adding new columns to big tables without multi-hour downtime) with some arcane nonsense format. Totally unqueryable with any efficiency.
EAV pattern has trade-offs you need to compensate for (performance). Production systems that use EAV have flat tables, and heavy caching to have be flexible with acceptable performance.
