However, I wish there's something in reverse: Redis as a MySQL storage engine. The most obvious useful things is that you can save a `tags` table without m2m tables, juse use Redis sets.
Why have an m2m table? Just have a table with two columns: post_id (or whatever is being tagged) and tag (varchar). If you want to present a list of possible tags just query for uniques. I may be missing something, so please correct me if there is a better way or I am misunderstanding your point.
One might choose to use the array datatype, combined with GiST, GIN, and/or expression based indexes, depending on the workload. That would be somewhat more lucid in most situations than a string packed into an attribute.
However, I wish there's something in reverse: Redis as a MySQL storage engine. The most obvious useful things is that you can save a `tags` table without m2m tables, juse use Redis sets.