I think the point is that git isn't "so much better" than mercurial, while pgsql has had a lead on mysql for quite some time on a lot of technical measurements.
I would say that that's pretty dubious claim with modern versions of Postgres and MySQL/InnoDB, running on modern hardware. See for example this recent comparative Benchmark from Mark Callaghan, a well known member of the MySQL community:
I'm not claiming that this benchmark justifies the claim that Postgres broadly performs better than MySQL/InnoDB these days -- that would be highly simplistic. Just as it would be simplistic to claim that MySQL is clearly well ahead with OLTP stuff in some kind of broad and entrenched way. It's highly dependent on workload.
Note that Postgres really comes out ahead on a test called "update-index", which involves updates that modify indexed columns -- the write amplification is much worse on MySQL there. This is precisely the opposite of what most commentators would have predicted. Including (and perhaps even especially) Postgres community people.
"Is table a heap with indexes on the side or is table a tree with other indexes on the side (i.e. 'clustered index')" is a more complicated discussion.
The former makes it possible to have MVCC (and thus gives you snapshot isolation and serializability) and makes secondary indexes perform faster, at the cost of vacuum or Oracle-style redo/undo/rollback segments with associated "Snapshot too old" issues.
The latter pretty much forces use of locking even for read so queries block each other (but don't require vacuum or something), makes clustering key selective queries perform faster than secondary index ones and makes you think really hard about the clustering key.
It's not really a feature you would have, but a complicated design tradeoff.
