PostgreSQL 14: Allow CLUSTER, VACUUM FULL and REINDEX to change tablespace on the fly

It is a common misunderstanding that VACUUM FULL saves you from running out of disk space if you already have space pressure. Running a VACUUM FULL temporarily requires at least double the space, as the table (and the indexes on the table) get completely re-written. PostgreSQL 14 will probably come with a solution for that as this patch introduces the possibility to move relations from one tablespace to another, when either CLUSTER, VACUUM FULL or REINDEX is executed.

As this is about moving relations from one tablespace to another we obviously need at least two tablespaces to play with:

postgres=# \! mkdir /var/tmp/tbs1
postgres=# \! mkdir /var/tmp/tbs2
postgres=# create tablespace tbs1 location '/var/tmp/tbs1';
CREATE TABLESPACE
postgres=# create tablespace tbs2 location '/var/tmp/tbs2';
CREATE TABLESPACE
postgres=# \db
          List of tablespaces
    Name    |  Owner   |   Location    
------------+----------+---------------
 pg_default | postgres | 
 pg_global  | postgres | 
 tbs1       | postgres | /var/tmp/tbs1
 tbs2       | postgres | /var/tmp/tbs2
(4 rows)

Lets assume we have a table in the first tablespace and we face space pressure on that file system:

postgres=# create table t1 ( a int, b date ) tablespace tbs1;
CREATE TABLE
postgres=# insert into t1 select x, now() from generate_series(1,1000000) x;
INSERT 0 1000000

Without that patch there is not much you can do, except for this (which blocks for the duration of the operation):

postgres=# alter table t1 set tablespace tbs2;
ALTER TABLE
postgres=# \d t1
                 Table "public.t1"
 Column |  Type   | Collation | Nullable | Default 
--------+---------+-----------+----------+---------
 a      | integer |           |          | 
 b      | text    |           |          | 
 c      | date    |           |          | 
Tablespace: "tbs2"

This will move the files of that table to the new tablespace (but not the indexes). If you really want to get back the space on disk with “vacuum full” you can now do that:

postgres=# vacuum (tablespace tbs1, full true)  t1;
VACUUM
postgres=# \d t1
                 Table "public.t1"
 Column |  Type   | Collation | Nullable | Default 
--------+---------+-----------+----------+---------
 a      | integer |           |          | 
 b      | date    |           |          | 
Tablespace: "tbs1"

The very same is possible with reindex:

postgres=# create index i1 on t1 (a);
CREATE INDEX
postgres=# reindex (tablespace tbs2) index i1;
REINDEX

… and cluster:

postgres=# cluster (tablespace tbs1, index_tablespace tbs1) t1 using i1;
CLUSTER
postgres=# \d t1
                 Table "public.t1"
 Column |  Type   | Collation | Nullable | Default 
--------+---------+-----------+----------+---------
 a      | integer |           |          | 
 b      | date    |           |          | 
Indexes:
    "i1" btree (a) CLUSTER, tablespace "tbs1"
Tablespace: "tbs1"

postgres=# 

Nice.

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