> I also don't see how this is slow; it's all in-memory.

Are you not writing to disk on every single transaction, then? My bad. And good luck keeping that power cord plugged in.

> Why bring a big honking DBMS into the picture when all you wanted was a hash table?

I didn't bring that DBMS into the picture. You did, in the second half of your first sentence:

"Start simple: store the data in-memory in whatever way makes sense, and just write a log file that contains pickled transactions that can be played back."

Indeed, an in-memory cache is really fast. But it's unfair to compare it with an RDBMS, which is handicapped by the need to write every transaction to disk before it can be committed. The hash table is great only up to the point where a stray cosmic ray crashes the server and makes the whole thing disappear, after which you realize that you need a log file.

In the general case, writing a transaction log file is a hard problem. If you write a really robust tool for managing that log file - a tool which is efficient at reading and writing even when the number of transactions grows large; one that lets you specify when the log gets written, and how often, and whether the system can be queried during the write, and how long those queries will block, and what they will return; one which allows multiple threads and multiple machines to read and write the data without concurrency problems; one which prevents the in-memory cache from getting out of sync with the filesystem - you will have implemented a substantial portion of MySQL, and probably memcached as well.

It is easy to start out designing a fast, simple, non-transactional DB and end up reinventing MySQL. If you don't believe me, ask the folks who invented MySQL!

In certain special cases (e.g. Google), rolling your own persistent storage system is a big, big win. You may know, in advance, that your website is one of those cases. If you are correct, you will be a superstar - you will build a relatively untested, nonstandard data storage system with a tiny subset of PostgreSQL's features, but all of that will be worth it because the system will be fast. If you are wrong, you will work for a month or six and then end up installing PostgreSQL anyway. In fact, even if you are right, you will end up installing PostgreSQL after your customer changes the spec at the last minute to require some boring standard feature - like a shopping cart - which any CRUD jockey can build in a day but which your "simple and elegant" database doesn't support because it wasn't in the original spec.

So it's no surprise that the "de facto standard" is to build your site around an RDBMS, get the damn thing working, and optimize later - and that, as a result, the typical in-memory data structure ends up being backed by an RDBMS instead of by a "simple" log file.

As for your ire at SQL... if you think you're in pain, just imagine how the designers of SQL must have felt back in the 1970s, when string parsing and query planning were tens of thousands of times slower than they are now, an extra database server cost more than a coder's daily salary, and RDBMS software was very, very non-free. It was a dark time. And yet for some reason those guys abandoned their efficient hand-rolled binary databases for SQL. In fact, they did it so fast that Larry Ellison grew richer than the Beatles. Why did they do that, I wonder? It must have been the drugs.