Re: Size functions inconsistent results

On Fri, 25 Feb 2022 at 22:58, Fabrízio de Royes Mello <fabriziomello(at)gmail(dot)com> wrote:
> Hi all,
>
> While doing some work using our functions [1] for calculate relations size
> I noticed an inconsistency between pg_total_relation_size and calculate
> everything separately, have a look in this example:
>
> fabrizio=# create table test_size (id bigserial primary key, toast_column
> text);
> CREATE TABLE
>
> fabrizio=# insert into test_size (toast_column)
>
> select repeat('X'::text, pg_size_bytes('1MB')::integer)
> from generate_series(1,1000);
> INSERT 0 1000
>
> fabrizio=# with relations as (
> select schemaname, relname, relid
> from pg_stat_user_tables
> where relname = 'test_size'
> ),
> sizes as (
> select
> schemaname,
> r.relname,
>
> pg_total_relation_size(relid) AS total_bytes,
>
> pg_relation_size(relid, 'main') +
> pg_relation_size(relid, 'init') +
> pg_relation_size(relid, 'fsm') +
> pg_relation_size(relid, 'vm') AS heap_bytes,
> pg_indexes_size(relid) AS index_bytes,
> pg_table_size(reltoastrelid) AS toast_bytes
> from relations r
> join pg_class on pg_class.oid = r.relid
> )
> select
> total_bytes, heap_bytes, index_bytes, toast_bytes,
> (total_bytes = (heap_bytes+index_bytes+toast_bytes)) as "Equal?",
> (total_bytes - (heap_bytes+index_bytes+toast_bytes)) as "Diff"
> from sizes;
> total_bytes | heap_bytes | index_bytes | toast_bytes | Equal? | Diff
> -------------+------------+-------------+-------------+--------+--------
> 14000128 | 90112 | 40960 | 13688832 | f | 180224
> (1 row)
>
> I want to calculate separately HEAP, INDEXES and TOAST (including indexes)
> sizes but it seems it's a bit inconsistent with pg_total_relation_size.
>
> Is it correct or am I missing something?
>

I think, you forget the index size of toast table.

with relations as (
select schemaname, relname, relid
from pg_stat_user_tables
where relname = 'test_size'
),
sizes as (
select
schemaname,
r.relname,

pg_total_relation_size(relid) AS total_bytes,

pg_relation_size(relid, 'main') +
pg_relation_size(relid, 'init') +
pg_relation_size(relid, 'fsm') +
pg_relation_size(relid, 'vm') AS heap_bytes,
pg_indexes_size(relid) AS index_bytes,
pg_table_size(reltoastrelid) + pg_indexes_size(reltoastrelid) AS toast_bytes
from relations r
join pg_class on pg_class.oid = r.relid
)
select
total_bytes, heap_bytes, index_bytes, toast_bytes,
(total_bytes = (heap_bytes+index_bytes+toast_bytes)) as "Equal?",
(total_bytes - (heap_bytes+index_bytes+toast_bytes)) as "Diff"
from sizes;

--
Regrads,
Japin Li.
ChengDu WenWu Information Technology Co.,Ltd.