Table sizes

[luis . roberto]

Hi! 

I'm trying to use this query to get table sizes, however I'm getting a strange 
error: 

select tablename,pg_relation_size(tablename::text) 
from pg_tables; 

In PG 13: 

SQL Error [42P01]: ERROR: relation "sql_implementation_info" does not exist 

In PG 12: 

SQL Error [42P01]: ERROR: relation "sql_parts" does not exist