How to correctly specify the table name in the pg_relation_size function?

Question

PostgreSQL

There is such a query to display the sizes of indexes in the public schema:

SELECT
    t.tablename,
    indexname,
    c.reltuples AS num_rows,
    pg_size_pretty(pg_relation_size(quote_ident(t.tablename)::text)) AS table_size,
    pg_size_pretty(pg_relation_size(quote_ident(indexrelname)::text)) AS index_size,
    CASE WHEN indisunique THEN 'Y'
       ELSE 'N'
    END AS UNIQUE,
    idx_scan AS number_of_scans,
    idx_tup_read AS tuples_read,
    idx_tup_fetch AS tuples_fetched
FROM pg_tables t
LEFT OUTER JOIN pg_class c ON t.tablename=c.relname
LEFT OUTER JOIN
    ( SELECT c.relname AS ctablename, ipg.relname AS indexname, x.indnatts AS number_of_columns, idx_scan, idx_tup_read, idx_tup_fetch, indexrelname, indisunique FROM pg_index x
           JOIN pg_class c ON c.oid = x.indrelid
           JOIN pg_class ipg ON ipg.oid = x.indexrelid
           JOIN pg_stat_all_indexes psai ON x.indexrelid = psai.indexrelid )
    AS foo
    ON t.tablename = foo.ctablename
WHERE t.schemaname='public'
ORDER BY 1,2;

Works. But it is necessary, for example, to determine the sizes of indexes in another scheme, not public. I change the condition:
... WHERE t.schemaname='data' ...
I get the answer: relation "data_table" does not exist
where data_table is a table in the data schema. I'm trying to specify a schema:

... pg_size_pretty(pg_relation_size(quote_ident(t.schemaname || '.' || t.tablename)::text)) AS table_size ...

I get the answer: relation "data.data_table" does not exist.
What's wrong, why doesn't pg_relation_size see tables from another schema, even if you explicitly specify a schema?

Answer 1

Pass pg_class.oid and that's it.

pg_size_pretty(pg_table_size(с.oid)) AS size,
pg_size_pretty(pg_indexes_size(с.oid)) AS idxsize,
pg_size_pretty(pg_total_relation_size(с.oid)) as "total"

If you look at the definition of pg_tables, you will see that this is a view from pg_class with the filter relkind = 'r'. That is, joining with pg_tables is rather redundant.