What's the difference between pg_table_size, pg_relation_size & pg_total_relation_size? (PostgreSQL)

Question

What\'s the difference between pg_table_size(), pg_relation_size() & pg_total_relation_size()?

I understand the basic diff

Answer 1

For a random table:

# select pg_relation_size(20306, 'main') as main,
  pg_relation_size(20306, 'fsm') as fsm,
  pg_relation_size(20306, 'vm') as vm,
  pg_relation_size(20306, 'init') as init,
  pg_table_size(20306), pg_indexes_size(20306) as indexes,
  pg_total_relation_size(20306) as total;
  main  |  fsm  |  vm  | init | pg_table_size | indexes |  total 
--------+-------+------+------+---------------+---------+--------
 253952 | 24576 | 8192 |    0 |        286720 |  196608 | 483328
(1 row)

From that, you can tell pg_table_size is the sum of all the return values of pg_relation_size. And pg_total_relation_size is the sum of pg_table_size and pg_indexes_size.

If you want to know how much space your tables are using, use pg_table_size and pg_total_relation_size to think about them -- one number is table-only, and one number is table + indexes.

Check the storage file layout for some info about what fsm, vm, and init mean, and how they're stored on disk.

Answer 2

pg_table_size: Disk space used by the specified table, excluding indexes (but including TOAST, free space map, and visibility map)

pg_relation_size: The size of the main data fork of the relation

select
      pg_size_pretty(pg_total_relation_size(relid)) as total_size,
      pg_size_pretty(pg_relation_size(relid, 'main')) as relation_size_main,
      pg_size_pretty(pg_relation_size(relid, 'fsm')) as relation_size_fsm,
      pg_size_pretty(pg_relation_size(relid, 'vm')) as relation_size_vm,
      pg_size_pretty(pg_relation_size(relid, 'init')) as relation_size_init,
      pg_size_pretty(pg_table_size(relid)) as table_size,
      pg_size_pretty(pg_total_relation_size(relid) - pg_relation_size(relid)) as external_size
 from 
      pg_catalog.pg_statio_user_tables
where 
      schemaname = 'XXXX'
  and relname like 'XXXXXX';

total_size         | 6946 MB
relation_size_main | 953 MB
relation_size_fsm  | 256 kB
relation_size_vm   | 32 kB
relation_size_init | 0 bytes
table_size         | 6701 MB
external_size      | 5994 MB

so pg_table_size is not only the sum of all the return values of pg_relation_size but you have to add toast size

toast_bytes         | 5748 MB