PostgreSQL: How to show table sizes

When you have a large PG database, you may want to find out which tables are consuming the most disk space.
You can easily check this using the following SQL statement from the PostgreSQL wiki.

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SELECT nspname || '.' || relname AS "relation",
    pg_size_pretty(pg_total_relation_size(C.oid)) AS "total_size"
  FROM pg_class C
  LEFT JOIN pg_namespace N ON (N.oid = C.relnamespace)
  WHERE nspname NOT IN ('pg_catalog', 'information_schema')
    AND C.relkind <> 'i'
    AND nspname !~ '^pg_toast'
  ORDER BY pg_total_relation_size(C.oid) DESC
  LIMIT 5;

Example output:

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       relation       | total_size 
----------------------+------------
 public.snapshots     | 823 MB
 public.invoice_items | 344 MB
 public.messages      | 267 MB
 public.topics        | 40 MB
 public.invoices      | 35 MB
(5 rows)

See the PostgreSQL wiki Data Usage Archive page for more information on calculating sizes of databases or tables.