Yes. For example, in Postgres you can sort tables arbitrarily large, not constrained by main memory. Postgres uses external merge sort when the tables are really large. There are other situations where the working data are disk-based when they are too large in Postgres. We will eventually be able to do that in Cozo as well, but no timetable is available yet.
For your second question, say you have a relation with lots of fields, one of them particularly large. As long as you don't use that field in your query, it will not impact memory usage. The query may be slower though since the RocksDB storage engine needs to read more pages from disk, but the fields that are loaded by RocksDB but not needed will be promptly evicted from memory.
For your second question, say you have a relation with lots of fields, one of them particularly large. As long as you don't use that field in your query, it will not impact memory usage. The query may be slower though since the RocksDB storage engine needs to read more pages from disk, but the fields that are loaded by RocksDB but not needed will be promptly evicted from memory.