What is the size of metadata in a postgres table? - sql

Question

There is a table in postgres 9.4 with following types of columns:
NAME TYPE TYPE SIZE
id | integer | 4 bytes
timestamp | timestamp with time zone | 8 bytes
num_seconds | double precision | 8 bytes
count | integer | 4 bytes
total | double precision | 8 bytes
min | double precision | 8 bytes
max | double precision | 8 bytes
local_counter | integer | 4 bytes
global_counter | integer | 4 bytes
discrete_value | integer | 4 bytes
Giving in total: 60 bytes per row
The size of a table(with toast) returned by pg_table_size(table) is: 49 152 bytes
Number of rows in the table: 97
Taking into account that a table is split into pages of 8kB, we can fit 49 152/8 192 = 6 pages into this table.
Each page and each row has some meta-data...
Looking at the pure datatype size we should expect something around 97 * 60 = 5 820 bytes of row data and adding approximately the same amount of metadata to it, we are not landing even close to the result returned by pg_table_size: 49 152 bytes.
Does metadata really take ~9x space compared to the pure data in postgres?

Answer 1

A factor 9 is clearly more wasted space ("bloat") than there should be:
Each page has a 16-byte header.
Each row has a 23-byte "tuple header".
There will be four bytes of padding between id and timestamp and between count and total for alignment reasons (you can avoid that by reordering the columns).
Moreover, each tuple has a "line pointer" of two bytes in the data page.
See this answer for some details.
To see exactly how the space in your table is used, install the pgstattuple extension:
CREATE EXTENSION pgstattuple;
and use the pgstattuple function on the table:
SELECT * FROM pgstattuple('tablename');