ALTER INDEX .. SET STATISTICS not documented

I'm sitting in Alexey Ermakov's talk and he just said that the command
in $SUBJECT is not documented ... do we need a separate blurb about it
somewhere?

I just tried it and it does do something useful, at least for expression
indexes, since we collect stats for those.  This is news to me.  There's
a small issue that the column names for them are rather awful, but since
they are unique there's not actually anything you can't do.

Note that it's allowed to change the stats target of non-expression
index columns too, but that doesn't seem to have any useful effect.

--
Álvaro Herrera       Peñalolén, Chile