Ask HN: Good datasets for studying RDBMS in depth

I have a fundamental knowledge on RDMS and have done data modelling in Postgresql (to some extend). I could able to make sense of what is Normalization, Joins, locks (specifically optimistic locking) etc.

I have learnt all these in my side projects mainly, or in production where the data isn't much huge. I haven't got any chance to debug a slow query or even select Indexing strategies etc which I guess is really important to understand RDMBS or postgresql in general.

I wanted to know are there any open source datasets, which will helpful for self studying or even debugging slow queries / seeing their indexing strategies etc.

Any answer would be appreciated.