> primary key index will be mostly cached so the cost of the indirection is much smaller than it may first appear
Not sure I follow. If it's in-memory you save having to read from disk, but you still have to walk the b-tree to go from PK to data.
Yes, this is true, but they framed this as "the Postgres approach is better when you have a good schema design", but that's not true. There are plenty of ways the MySQL approach is better even when you have a really good schema.
> Not sure I follow. If it's in-memory you save having to read from disk, but you still have to walk the b-tree to go from PK to data.
The point I was trying to make is that going to disk is going to be orders of magnitude slower than doing an in-memory B-tree traversal. Because of that, the cost of doing an extra b-tree traversal to find the page you're looking for is a relatively small cost compared to reading the page in the first place
To be fair this probably covered probably 9x% of production queries, like high-selectivity index-covered filters and sorts (with a limit). Even today most planners will still use an inner loop.
It's been interesting to watch how different planners have evolved over the years. SQL Server 7 in 1998 launched with features that MySQL and Postgres wouldn't catch up to until the late 2010s, but they had other features and quality-of-life improvements that made these edge-case optimizations hardly noticable.