BUG #15582: ALTER TABLE/INDEX ... SET TABLESPACE does not free diskspace

The following bug has been logged on the website:

Bug reference:      15582
Logged by:          Andrew P
Email address:      ap@zip.com.au
PostgreSQL version: 11.1
Operating system:   Linux/Debian stretch
Description:

Hi,

I've added a tablespace recently to help deal with almost running out of
disk space.

I then tried to use it with:

ALTER TABLE schema.table SET TABLESPACE hydro2_tmp;

This worked in so far as disk space was used on hydro2_tmp but nothing was
ever freed in the default location.

I tried a VACUUM FULL but that did not help.

Then I tried moving the indexes belonging to the table but that also did not
help.

Everywhere I've read and everyone I spoke to said that this is a move
operation but it appears to be a copy so, unless I missed something, this
appears to be a bug.

The two table spaces are on separate drives and postgres is from postgres'
apt repository.