-- PostgreSql
SELECT
pg_database.datname AS "Database Name",
pg_database_size(pg_database.datname) / 1024.0 / 1024.0 AS "Database Size (MB)"
FROM pg_database;
-- MySql
SELECT
table_schema "Database Name",
sum( data_length + index_length ) / 1024 / 1024 "Database Size (MB)"
FROM information_schema.TABLES
GROUP BY table_schema;