Postgresql Physical Reads and Buffer Reads
If you want to know it per database:
SELECT datname, blks_read, blks_hit
FROM pg_stat_database;

If you are interested in values per table, use
SELECT * FROM pg_statio_all_tables;

If you want to have statistics per query, install pg_stat_statements and use
SELECT query, shared_blks_read, shared_blks_hit
FROM pg_stat_statements;

Don't forget that since PostgreSQL uses buffered I/O, a “block read” might come from the kernel cache rather than from disk.