Vacuum full does a index rebuild automatically. Since a vacuum full builds an entire new heap table, the old indexs are all pointing to the incorrect locations for all tuples, so it has no choice but to rebuild.

Excellent there you go.

Is there a way to just do the index build part, short of dropping an index and adding it back?

This is described about a quarter the way into TFA.

> REINDEX INDEX CONCURRENTLY index_name;

(A vanilla REINDEX will lock the table, preventing writes, while it runs. The CONCURRENT creates a new index, replicates any updates to the original while it does so, and then does an atomic switcheroo at the end.)

I believe pg_repack can do that.