I once learned, in the good old mainframe times, that there are 3 sizes of databases: Small size that fit into RAM, medium size that fit on one computer, and bi

I once learned, in the good old mainframe times, that there are 3 sizes of databases: Small size that fit into RAM, medium size that fit on one computer, and big databases, that require a cluster of computers. The relational model, and SQL databases play their strong roles in medium size databases, but are to much overhead for a fast small database, and do not scale well for big databases. It was hoped at that time, that Moors law will beat Wirths law (who claimed this law much later), that big databases will soon be medium sized, nobody would care about performance of small databases that much, and we could happy use SQL for all problems. This was true for surprisingly long time, and still is, if your problem fits into a medium size database. Unfortunate, computer history turns in cycles, and tends to forget lessons from the past. Coding access to a bunch of different databases was at least standard under COBOL. Coding for half a dozen NoSQL databases now, is a complete mess.