Unexpected result from PostgreSQL information schema

I have written a simple query that should show all table sizes of any given schema in human-readable format:

select table_name, pg_relation_size(quote_ident(table_name))
from information_schema.tables
where table_schema = 'my_schema'
order by 2

When I run this query in PgAdmin, I get the following error:

ERROR:  relation "my_table" does not exist
SQL state: 42P01

How can this error even be possible? I have not changed the information_schema at all and, if a relation doesn't exist in the first place, why would it be in the information_schema ? Any idea how this could happen?

The only reason I can think of, is if my_schema is not part of your search_path.

As you pass a table name without schema qualifying it to the function pg_relation_size() the table is searched in the default search_path. If it's not found, you'll get that error.

Use a fully qualified name instead:

pg_relation_size(format('%I.%I', table_schema, table_name))

StackExchange Database Administrators Q#285365, answer score: 5