External size of table still not released (as disk space) after issuing truncate command

Question

In PostgreSQL 9.3.10 (Ubuntu 14.04 OS), I have a table called groupseen whose size is as follows:

              Table               |  Size   | External Size 
----------------------------------+---------+---------------
          links_groupseen         | 394 MB  | 362 MB

I understand that External Size is the size that related objects of this table (like indices) take.

Upon issuing the command truncate links_groupseen;, only 394 MB disk space was released (I checked right after, via df -h). My two questions are:

1. What can I do to get rid of the space taken up by this table's indices (External Size) as well? Shouldn't truncate have taken care of everything?

2. I was using the following command to check the size of this table. Even though External Size hasn't yet been released, using this command now doesn't show the outstanding 362 MB. How do I monitor the outstanding External Size related to this table?

   SELECT
      relname as "Table",
      pg_size_pretty(pg_total_relation_size(relid)) As "Size",
      pg_size_pretty(pg_total_relation_size(relid) - pg_relation_size(relid)) as "External Size"
    FROM pg_catalog.pg_statio_user_tables
    ORDER BY pg_total_relation_size(relid) DESC;
   

In case required, here's how the table looks like:

                            Table "public.links_groupseen"
     Column     |           Type           |                          Modifiers                           
----------------+--------------------------+--------------------------------------------------------------
 id             | integer                  | not null default nextval('links_groupseen_id_seq'::regclass)
 seen_user_id   | integer                  | not null
 seen_at        | timestamp with time zone | not null
 which_reply_id | integer                  | not null
Indexes:
    "links_groupseen_pkey" PRIMARY KEY, btree (id)
    "links_groupseen_seen_user_id" btree (seen_user_id)
    "links_groupseen_which_reply_id" btree (which_reply_id)
Foreign-key constraints:
    "links_groupseen_seen_user_id_fkey" FOREIGN KEY (seen_user_id) REFERENCES auth_user(id) DEFERRABLE INITIALLY DEFERRED
    "links_groupseen_which_reply_id_fkey" FOREIGN KEY (which_reply_id) REFERENCES links_reply(id) DEFERRABLE INITIALLY DEFERRED

---

Note: this SO answer doesn't solve my problem because the OP in the question stated how disk space wasn't released for him at all. In my case, the subtlety is that only External Size is outstanding.

Answer 1

I think with a truncate even without a VACUUM FULL the space is released back to the OS. My guess is per the other question that you've got a backend process holding the underlying file open.

1. Find out what the file is. SELECT pg_relation_filepath('links_groupseen');
2. Find the data directory SHOW data_directory;
3. Run truncate.
4. See what's holding it open. lsof $data_directory/$relation_filepath

I think the most likely bet is something is holding it open if not, we'll have to know whether or not Pg is deleting the underlying relation_filepath before and after the truncate command and if anything has that file open (if it exists). You may have to run lsof before the truncate too.

Answer 2

The 394 MB is the total size, inclusive of external items like indexes and toast tables. If 394 MB is what you saw getting released, then that is all there is.