[mjb]

> (Single query tag lookup; naively joins the entire taggings and tags tables before limiting with WHERE)

Really? Which modern RDBMS database actually works like this when indexes are available?

Index selection and optimization is a standard feature of even the simplest relational database system. JOINs on keys with high selectivity (especially unique keys) are extremely efficient in modern database systems - in some common cases just as efficient as pulling two columns out of the same table. Subselects are something that several database systems (MySQL, for example) really suck at - but they are easily avoided in most schemas.

> Try the "when all tags match" case ($all in MongoDB), and you'll go grey a few years earlier.

Depending on your database engine, even very large AND chains can be extremely efficiently computed on an indexed column. For larger use-cases, there are good ways to avoid this problem using careful schema design.

There are legitimate reasons why relational databases aren't the solution to every database problem. Claiming issues which clearly do not exist is not a good way to get points for the other side.