$ cd pg_textsearch && git checkout 20313076f2f43cab
$ make install
$ psql
steve=# DROP EXTENSION IF EXISTS pg_textsearch;
steve=# create unlogged table test (content text);
steve=# create index idx_content on test using bm25 (content) with (text_config='english');
steve=# SELECT oid
FROM pg_class
WHERE relname = 'idx_content' AND relkind = 'i';
  oid  
-------
 36129
(1 row)

#-- exit psql

$ cd $PGDATA # cd to the PG data dir
$ find . -name '36129*'
./base/16384/36129_init
./base/16384/36129

$ ls -l ./base/16384/36129_init
.rw------- 0 steve 24 Dec 12:25 ./base/16384/36129_init