If your database has a large number of small tables, you likely have a lot of wasted space. To demonstrate this, let’s create a table with a single record:
create table foo (str text); insert into foo values ('a'); VACUUM foo;
Now let’s find out the path of the file containing our data, relative to the data directory:
select pg_relation_filepath('foo'); pg_relation_filepath ---------------------- base/16384/16405 (1 row)
(For more information on PostgreSQL files and directories, see Craig Ringer’s article on the topic.)
Notice what happens when we drop to the system command line within the data directory and list the files, adding a wild card to the path above:
$ ls base/16384/16405* base/16384/16405 base/16384/16405_fsm base/16384/16405_vm
The file named 16405 is the heap of our table, but what about the others? These are auxiliary files, called “relation forks” that contain additional information to help PostgreSQL access and maintain the table.
- 16405_fsm is the free space map. Its job is to know which pages in the table have space available for inserting records.
- 16405_vm is the visibility map. Its job is to know which heap pages may need VACUUM-ing or freezing, and also which heap pages must be visited during index-only scans.
There are other files associated with this table, but to find them we have to use a query:
select pg_relation_filepath(c.reltoastrelid) as toast_table_path, pg_relation_filepath(i.indexrelid) as toast_index_path from pg_class c left outer join pg_index i on c.reltoastrelid=i.indrelid where c.relname = 'foo'; toast_table_path | toast_index_path ------------------+------------------ base/16384/16408 | base/16384/16410 (1 row)
This gives us the path of the toast table for our table, as well as that of the toast table’s index. If we insert any records with large enough strings, they will be compressed and stored here.
How much disk space do we need to store our one record?
\x select pg_relation_size(c.oid, 'main') as heap_size, pg_relation_size(c.oid, 'fsm') as fsm_size, pg_relation_size(c.oid, 'vm') as vm_size, pg_relation_size(c.reltoastrelid) as toast_table_size, pg_relation_size(i.indexrelid) as toast_index_size from pg_class c left outer join pg_index i on c.reltoastrelid=i.indrelid where c.relname = 'foo'; -[ RECORD 1 ]----+------ heap_size | 8192 fsm_size | 24576 vm_size | 8192 toast_table_size | 0 toast_index_size | 8192
To store a record with one string value, we need five files totalling 48kB of space! You may be thinking “So what? My tables are many gigabytes in size — why should I worry about a few kB?”. Well, consider the situation where an application uses schemas for multi-tenancy. Such a case could have thousands of schemas, each with hundreds of small tables, and the amount of space taken up by auxiliary files quickly becomes very high.
Is there anything we can do to reduce the amount of space consumed?
For starters, we can get rid of the toast table and its toast index by using a constraint on the length of the column:
create table foo_no_toast (str varchar(500)); insert into foo_no_toast values ('a'); VACUUM foo_no_toast; select pg_relation_size(c.oid, 'main') as heap_size, pg_relation_size(c.oid, 'fsm') as fsm_size, pg_relation_size(c.oid, 'vm') as vm_size, pg_relation_size(c.reltoastrelid) as toast_table_size, pg_relation_size(i.indexrelid) as toast_index_size from pg_class c left outer join pg_index i on c.reltoastrelid=i.indrelid where c.relname = 'foo_no_toast'; -[ RECORD 1 ]----+------ heap_size | 8192 fsm_size | 24576 vm_size | 8192 toast_table_size | toast_index_size |
So now we have 3 files totalling 40 kB. This works because internally Postgres has a threshold tuple size beyond which it has to store the tuple in the toast table. In our new table here, Postgres knows this table can’t have any tuples exceeding this threshold, so it doesn’t bother creating the toast table.
This isn’t a big improvement, however. For one, we only saved a little bit of space. For another, it’s difficult to ensure the constraint on the column type is large enough to allow any string your application might want to insert, yet small enough to prevent the creation of the toast table. I obtained 500 in my example with a little experimentation, but the result will vary depending on the number of columns, the database encoding, and possibly on the major version of PostgreSQL.
Wouldn’t it be nice if we could get rid of the free space map? After all, it’s 3x bigger than the heap it’s keeping track of! It turns out there will soon be an easy way to do that.
Note: A previous version of this article mentioned that in PG12 small tables would not have a free space map. This feature has been reverted last-minute due to architectural concerns. I will update this article if it reappears in a future release.