How much storage does an empty table in Postgres take?
This is a post about Postgres tables that store ... well basically ... Nothing.
The idea for this post came from this tweet that hinted that an empty table on most databases today takes 16Kb of storage. Now admittedly Franck was probably reminiscing the good-old days so this is probably quite out of context, but it did get me thinking, and thus this post.
NB: Here's a video showing this in action ! - See video .
A "regular" empty table in Production
Here's a regular small table that could be found in Production. It has a Primary Key, a text column and a JSONB column. Let's check the table size using the
pg_total_relation_size() postgres function (you can read more about that function
here).
db1=# create table t(id bigint primary key, b text, c jsonb);
CREATE TABLE
db1=# select pg_total_relation_size('t');
pg_total_relation_size
------------------------
16384
(1 row)
Hmmm, so that tweet did have a point. Given how low-cost memory has become over the decades, it is easy to understand why databases today chose to optimize speed over memory efficiency (more on this later) and so even an empty table in Postgres, does consume 16 kb.
But "where" is the 16kb being used?
db1=# select pg_relation_size('t');
pg_relation_size
------------------
0
(1 row)
The relation itself isn't consuming any space! That is good (again more on this later) but then where is the space being used then?
db1=# \d t
Table "public.t"
Column | Type | Collation | Nullable | Default
--------+--------+-----------+----------+---------
id | bigint | | not null |
b | text | | |
c | jsonb | | |
Indexes:
"t_pkey" PRIMARY KEY, btree (id)
We see that the table has a Primary Key - and thus an index - t_pkey.
Does the index is consuming 16kb?
db1=# select pg_relation_size('t_pkey');
pg_relation_size
------------------
8192
(1 row)
So the index is using some of it - 8kb - so that is progress - but who's using the other 8kb?
Let's start cutting the table down
Let's start cutting down the columns and see if the disk-usage goes down.
db1=# DROP TABLE t; CREATE TABLE t (id BIGINT PRIMARY KEY, b JSONB); select pg_total_relation_size('t');
DROP TABLE
CREATE TABLE
pg_total_relation_size
------------------------
16384
(1 row)
db1=# DROP TABLE t; CREATE TABLE t (id BIGINT PRIMARY KEY, b TEXT); select pg_total_relation_size('t');
DROP TABLE
CREATE TABLE
pg_total_relation_size
------------------------
16384
(1 row)
Hmmm, that didn't help at all. Dropping either of the TEXT or JSONB column didn't help. Let's look at the expanded version of this table to see if there's any similarity in the two columns. (I've clipped the output to make it easier to read)
db1=# \d+ t
Table "public.t"
Column | Type | Collation | Nullable | Default | Storage | ...
--------+--------+-----------+----------+---------+----------+-...
id | bigint | | not null | | plain | ...
b | text | | | | extended | ...
c | jsonb | | | | extended | ...
Indexes:
"t_pkey" PRIMARY KEY, btree (id)
Access method: heap
Clearly, the two column's are "
extended" (you can read more about it
here), but basically what happened here is that an extended column type resulted in the creation of a TOAST table (you can read more about
TOAST here). Let's find out "how" to find the TOAST table for the table "
t", and check if that could be consuming the remaining 8kb?
db1=# select oid, relname, reltoastrelid, reltoastrelid::regclass from pg_class where relname = 't';
oid | relname | reltoastrelid | reltoastrelid
-------+---------+---------------+-------------------------
18300 | t | 18303 | pg_toast.pg_toast_18300
(1 row)
db1=# select pg_relation_size('pg_toast.pg_toast_18300');
pg_relation_size
------------------
0
(1 row)
Hmmm, it's not the TOAST table. But just like the main table "t", could it be that the TOAST table has supporting relations that are to blame?
db1=# select pg_total_relation_size('pg_toast.pg_toast_18300');
pg_total_relation_size
------------------------
8192
(1 row)
db1=# \d pg_toast.pg_toast_18300
TOAST table "pg_toast.pg_toast_18300"
Column | Type
------------+---------
chunk_id | oid
chunk_seq | integer
chunk_data | bytea
Owning table: "public.t"
Indexes:
"pg_toast_18300_index" PRIMARY KEY, btree (chunk_id, chunk_seq)
db1=# select pg_relation_size('pg_toast.pg_toast_18300_index');
pg_relation_size
------------------
8192
(1 row)
Yes! So we see above, that the TOAST table implicitly has a primary key (of it's own) that uses an index - which has 1 page (8kb) assigned to it.
In a nutshell, the empty table 't' above, consumes 16kb and the storage allocation takes this shape:
- Main relation - 0 bytes - 't'
- Main relation Index - 8kb - 't_pkey'
- Toast relation - 0 bytes - 'pg_toast_18300'
- Toast relation Index - 8kb - 'pg_toast_18300_index'
Cut Cut Cut
Okay, lets see if we can reduce the table size further, by dropping both the Extended columns.
db1=# DROP TABLE t; CREATE TABLE t (id BIGINT PRIMARY KEY); select pg_total_relation_size('t');
DROP TABLE
CREATE TABLE
pg_total_relation_size
------------------------
8192
(1 row)
Okay, that makes sense. Now the main table (heap) is still not consuming anything, but the index still does.
Let's reduce further.
db1=# DROP TABLE t; CREATE TABLE t (id BIGINT); select pg_total_relation_size('t');
DROP TABLE
CREATE TABLE
pg_total_relation_size
------------------------
0
(1 row)
Nice! But seriously - 0 bytes?
It kind of makes sense, that since Postgres doesn't yet have anything to store - besides the metadata of the table (and since the metadata is stored in the system catalogs - for e.g. pg_catalog schema) - there isn't anything to store in the main relation (heap) as yet.
Disk Usage - Check filesystem
Hmmm - Nah, what if I don't trust Postgres?
Let's skip Postgres functions and ask the filesystem directly - and see if the table is actually 0 bytes. Here we first find the file path of the table in question using the postgres function pg_relation_filepath() and then ask the file-system for the file-size.
db1=# select pg_relation_filepath('t');
pg_relation_filepath
----------------------
base/17727/18356
(1 row)
db1=# \! ls -la /home/robins/proj/localpg/data/base/17727/18356
-rw------- 1 robins robins 0 Sep 23 10:19 /home/robins/proj/localpg/data/base/17727/18356
So the file corresponding to the table, actually is using 0 bytes. Nice!
Now, when a table is created, some entries are added to the system catalog. Let see if the database size is signficantly more than a blank database?
postgres=# create database db1;
CREATE DATABASE
postgres=# \c db1
You are now connected to database "db1" as user "robins".
db1=# CREATE TABLE t (id BIGINT);
CREATE TABLE
db1=# select pg_database_size('db1');
pg_database_size
------------------
7482515
(1 row)
db1=# create database db2;
CREATE DATABASE
db1=# \c db2
You are now connected to database "db2" as user "robins".
db2=# select pg_database_size('db2');
pg_database_size
------------------
7482515
(1 row)
Good. So this somewhat confirms that a blank database and a database with an "empty" table use the same disk-space.
Postgres is hiding something
Technically though, I'm lying. Well actually Postgres is lying hiding something from the file-system (i.e. every new table does make the database grow logically - a tad little - just that most of the times, the filesystem doesn't get the memo).
Under the cover, the way postgres stores data in a table (catalog table, or any user table), although it consumes a page of disk-space (often 8Kb), logically it may be consuming only a small part of that page. This is very helpful when more rows need to be stored in the table. When more rows come in, Postgres is able to reuse the same (first) page to now logically store more data - although for the filesystem - no extra pages were requested. This is what I was hinting at earlier that today's database use disk space (and thus memory cache) with larger (8kb) chunks and continue to keep using that page (until a new page is needed). Further below, I show a brief example of how all of this works.
But to summarize, it is then unfair to say that the system catalog did not grow at all (when a new table was added) - since some catalogs are guaranteed to have grown (for e.g. metadata of the new table is stored as an extra row in pg_class etc.) within the disk blocks already allocated as in-use for that catalog.
Let's squeeze a little more?
db1=# create table a();
CREATE TABLE
Right off the bat, that might seem completely wrong. Does Postgres allow a table with no Columns?
Yes !! 😎
All databases allow creation of an empty table (obviously), but Postgres allows a new table even if there are no columns! Let's verify this from the Postgres Documentation. Although suttle, we can see that in the syntax section of the
CREATE TABLE page, the
column_name data_type is enclosed with a square braces
[] - which implies that columns are in fact, optional. What's more, this "feature" is a part of Postgres
at least for the past 20 years!
Now the utility of this table is arguable (we'll explore that below), but it is now clear that this syntax is legal and works just fine.
Let's see how such a table looks like with psql \d
db1=# \d a
Table "public.a"
Column | Type | Collation | Nullable | Default
--------+------+-----------+----------+---------
That's it! That's the complete output - Since the table has no columns, the output above (rightly) doesn't show anything.
Let's go a little deeper
The obvious next question is - What on earth could a table like this be used for? That is a perfectly good question, and the answer is probably not much. However if you instead ask whether "Squeeze a little more" mean that a no column table takes less storage space? The answer (depends on input data but) is most probably yes. Let's see how does it help Postgres if it knows that you don't want to store any column in the table.
db1=# create table a();
CREATE TABLE
db1=# \dt+ a
List of relations
Schema | Name | Type | Owner | Persistence | Access method | Size | Description
--------+------+-------+--------+-------------+---------------+---------+-------------
public | a | table | robins | permanent | heap | 0 bytes |
(1 row)
db1=# insert into a select;
INSERT 0 1
db1=# \dt+ a
List of relations
Schema | Name | Type | Owner | Persistence | Access method | Size | Description
--------+------+-------+--------+-------------+---------------+------------+-------------
public | a | table | robins | permanent | heap | 8192 bytes |
(1 row)
Nothing new here. We see that although an empty (0 column) table consumes
0 byte for storage. And like a regular table, as soon as the first row is inserted, the table uses 1 page - which in my test database (and probably 99.99% of postgres databases world-wide) consumes
8192 bytes. This is expected, but do note that the storage of logical rows in a postgres page, is a little oddly done (and for good reason). There is
lllllooooottttt of detail here - but I wouldn't blame you if you'd want to keep that aside for a cold winter morning - when armed with a cup of hot coffee.
For now, we see below that each row that is inserted into the table, consumes 24 bytes - in that 8kb page.
db1=# create extension pageinspect ;
CREATE EXTENSION
db1=# truncate table a;
TRUNCATE TABLE
db1=# insert into a select FROM generate_series(1,3);
INSERT 0 3
db1=# SELECT * FROM heap_page_items(get_raw_page('a', 0));
lp | lp_off | lp_flags | lp_len | t_xmin | t_xmax | t_field3 | t_ctid | t_infomask2 | t_infomask | t_hoff | t_bits
| t_oid | t_data
----+--------+----------+--------+--------+--------+----------+--------+-------------+------------+--------+--------
+-------+--------
1 | 8168 | 1 | 24 | 85105 | 0 | 0 | (0,1) | 0 | 2048 | 24 |
| | \x
2 | 8144 | 1 | 24 | 85105 | 0 | 0 | (0,2) | 0 | 2048 | 24 |
| | \x
3 | 8120 | 1 | 24 | 85105 | 0 | 0 | (0,3) | 0 | 2048 | 24 |
| | \x
(3 rows)
Still going Deeper - but on a Tangent
So does the above imply that adding more columns to a table would mean Postgres consumes more bytres-per-row? Let's verify:
db1=# drop table t;
DROP TABLE
db1=# create table t(id bigint);
CREATE TABLE
db1=# truncate table t; insert into t select generate_series(1,3); vacuum full t; \dt+ t
TRUNCATE TABLE
INSERT 0 3
VACUUM
List of relations
Schema | Name | Type | Owner | Persistence | Access method | Size | Description
--------+------+-------+--------+-------------+---------------+------------+-------------
public | t | table | robins | permanent | heap | 8192 bytes |
(1 row)
db1=# SELECT * FROM heap_page_items(get_raw_page('t', 0));
lp | lp_off | lp_flags | lp_len | t_xmin | t_xmax | t_field3 | t_ctid | t_infomask2 | t_infomask | t_hoff | t_bits
| t_oid | t_data
----+--------+----------+--------+--------+--------+----------+--------+-------------+------------+--------+--------
+-------+--------------------
1 | 8160 | 1 | 32 | 85100 | 0 | 0 | (0,1) | 1 | 2816 | 24 |
| | \x0100000000000000
2 | 8128 | 1 | 32 | 85100 | 0 | 0 | (0,2) | 1 | 2816 | 24 |
| | \x0200000000000000
3 | 8096 | 1 | 32 | 85100 | 0 | 0 | (0,3) | 1 | 2816 | 24 |
| | \x0300000000000000
(3 rows)
Here we see that each row is now consuming
32 bytes - which is an
extra 8 bytes from earlier. Good chances the only column we've added is the reason for the extra 8 bytes. Let's verify that using the
pg_column_size() function (you can read more about it
here):
db1=# select pg_column_size(1::bigint);
pg_column_size
----------------
8
(1 row)
But wait, there's one more twist here:
db1=# INSERT INTO t SELECT;
INSERT 0 1
db1=# SELECT * FROM heap_page_items(get_raw_page('t', 0));
lp | lp_off | lp_flags | lp_len | t_xmin | t_xmax | t_field3 | t_ctid | t_infomask2 | t_infomask | t_hoff | t_bits
| t_oid | t_data
----+--------+----------+--------+--------+--------+----------+--------+-------------+------------+--------+--------
--+-------+--------------------
1 | 8160 | 1 | 32 | 85111 | 0 | 0 | (0,1) | 1 | 2816 | 24 |
| | \x0100000000000000
2 | 8128 | 1 | 32 | 85111 | 0 | 0 | (0,2) | 1 | 2816 | 24 |
| | \x0200000000000000
3 | 8096 | 1 | 32 | 85111 | 0 | 0 | (0,3) | 1 | 2816 | 24 |
| | \x0300000000000000
4 | 8072 | 1 | 24 | 85113 | 0 | 0 | (0,4) | 1 | 2049 | 24 | 0000000
0 | | \x
(4 rows)
So wait, see row 4. Although the table has a column, just because the column didn't have a value, the row actually consumed only 24 bytes (the minimum)?
Is this scalable? I mean can I have a 5 column table and still Postgres stores a row, but only consume the bare minimum 24 bytes? Let's see:
db1=# drop table h;
DROP TABLE
db1=# create table h(c1 bigint, c2 bigint, c3 bigint, c4 bigint, c5 bigint);
CREATE TABLE
db1=# insert into h select;
INSERT 0 1
db1=# SELECT * FROM heap_page_items(get_raw_page('h', 0));
lp | lp_off | lp_flags | lp_len | t_xmin | t_xmax | t_field3 | t_ctid | t_infomask2 | t_infomask | t_hoff | t_bits
| t_oid | t_data
----+--------+----------+--------+--------+--------+----------+--------+-------------+------------+--------+--------
--+-------+--------
1 | 8168 | 1 | 24 | 86262 | 0 | 0 | (0,1) | 5 | 2049 | 24 | 0000000
0 | | \x
(1 row)
So yes, that does work and it does scale for "many" columns - but with a minor variation. There's more detail
in code, but basically for regular columns the header contains 1 bit per column which expands in 8 byte chunks - and so say for 100 column table (with no data) - Postgres consumes ~40 bytes per row.
db1=# drop table h; create table h(); select 'alter table h add column c' || n || ' bigint;' from generate_series(1,100) e(n); \gexec
.
.
ALTER TABLE
ALTER TABLE
ALTER TABLE
ALTER TABLE
db1=# select count(*) from pg_attribute where attrelid = 'h'::regclass;
count
-------
106
(1 row)
db1=# insert into h select; vacuum full h; SELECT * FROM heap_page_items(get_raw_page('h', 0));
INSERT 0 1
VACUUM
lp | lp_off | lp_flags | lp_len | t_xmin | t_xmax | t_field3 | t_ctid | t_infomask2 | t_infomask | t_hoff |
t_bits | t_oid | t_data
----+--------+----------+--------+--------+--------+----------+--------+-------------+------------+--------+--------
--------------------------------------------------------------------------------------------------+-------+--------
1 | 8152 | 1 | 40 | 88376 | 0 | 0 | (0,1) | 100 | 2817 | 40 | 0000000
0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000 | | \x
(1 row)
Given the task at hand, I'd say that's still decently crisp.
So does a zero column table, squeeze max rows per page?
Yes, and No. So going back to a 0 column table - let's try to fill the whole page with rows and see how many can be stuffed on a single page.
If you're running low on coffee - the page-header is 24 bytes and so back-of-the-envelope math suggests that number of rows possible to squeeze into a page should be - ( 8192 bytes in a page - some bytes for page header & footer ) / 24 bytes per row = ~340 rows.
db1=# truncate table a; insert into a select FROM generate_series(1,340); vacuum full a; \dt+ a
TRUNCATE TABLE
INSERT 0 340
VACUUM
List of relations
Schema | Name | Type | Owner | Persistence | Access method | Size | Description
--------+------+-------+--------+-------------+---------------+-------+-------------
public | a | table | robins | permanent | heap | 16 kB |
(1 row)
Something's not right! That size should have stayed 8Kb. Why did Postgres use 16Kb (two pages - instead of one)?
That's because of this
tiny bit of trivia that the maximum number of tuples that Postgres can squeeze onto a page, is hard-coded to 291 (for an 8kb page) - which was interesting to know - but to clarify,
Heap-Only-Tuples (HOT feature) can effectively force a few more rows on a running database, but we'll go deeper into that some other day.
So let's go back and confirm if that understanding is correct - that Postgres can in fact squeeze at max only 291 rows onto the same page.
db1=# truncate table a; insert into a select FROM generate_series(1,291); vacuum full a; \dt+ a
TRUNCATE TABLE
INSERT 0 291
VACUUM
List of relations
Schema | Name | Type | Owner | Persistence | Access method | Size | Description
--------+------+-------+--------+-------------+---------------+------------+-------------
public | a | table | robins | permanent | heap | 8192 bytes |
(1 row)
db1=# truncate table a; insert into a select FROM generate_series(1,292); vacuum full a; \dt+ a
TRUNCATE TABLE
INSERT 0 292
VACUUM
List of relations
Schema | Name | Type | Owner | Persistence | Access method | Size | Description
--------+------+-------+--------+-------------+---------------+-------+-------------
public | a | table | robins | permanent | heap | 16 kB |
(1 row)
Here we see that:
- When 291 rows are inserted (blue), the table stays at 1 page (8kb)
- Whereas when 292 (291+1) rows are inserted (green), the table expands to 2 pages (16 kb)
Utility
So, that's all fine, but what's this table good for?
Well beyond understanding Postgres :) not much. This table is unhelpful for most database tasks. It can't store values - it wouldn't allow columns / indexes / selective deletes (let's avoid ctid hacks for now) etc.
But if I was forced to conjure an idea, a high-contention ticker app (that only needs to store +1s) via postgres functions - may be (and that's a BIG may be) this table could be used to store +1s - with a back-off algorithm on the application side. Good chances if there's a good DBA - this is done much better in many other ways (simplest of which is at the application end, or as a value in a column etc.) but it'd be better than nothing.
There are few other possible use-cases
discussed here - for e.g. if (for some reason) you'd want to add columns to a table in a programmatic fashion after a base table is created - like we did above in the 100 column table test, OR, if you want to reserve a table name (in a multi-user setup) months / years in advance etc.
Finally
Much Ado About Nothing... This was a good exercise where we learnt something new about how Postgres stores table data - when ironically - there's nothing to store :)
Hope you had fun! Comments more than welcome.
