What's the right way to calculate table size in postgres?

Question

I'm working on a project where calculations are being made for certain tables in the postgres DB, and in different parts of code, there are 2 different calculations being made:

select pg_total_relation_size ('TABLENAME');

and the second one:

select (relpages*8/1024) from pg_class where relname='TABLENAME' (Size in MB)

I know that pg_total_relation_size is what I need to use, but I was wondering what does this other calculation represents, and if the second one is also some kind of size calculation, why am I getting different results for the same table?

Answer 1

You can use pg_size_pretty(pg_total_relation_side(oid)) to pretty print table size:

select  nspname
,       relname
,       pg_size_pretty(pg_total_relation_size(c.oid)) as "size"
from    pg_class c
left join
        pg_namespace n 
on      n.oid = c.relnamespace
where   nspname not in ('pg_catalog', 'information_schema')
order by
        pg_relation_size(c.oid) desc;

Answer 2

pg_total_relation_size - is the right way to see full table size because it includes disk space utilized by indices and TOAST data.

When you do the second-one select - you get table size only (relpages is table size in 8Kb pages). You can get TOAST and index sizes from pg_class as well but you need to do some more sql queries as described in here and sum all the sizes you've got.