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SELECT
sum(pg_column_size(the_text_column)) as total_size,
avg(pg_column_size(the_text_column)) as average_size,
sum(pg_column_size(the_text_column)) * 100.0 / pg_relation_size('tablename') as percentage
FROM tablename;
xxxxxxxxxx
-- Database size
SELECT pg_size_pretty( pg_database_size('dbname') );
-- Table size
SELECT pg_size_pretty( pg_total_relation_size('tablename') );