What's the most efficient way to copy a large amount of rows from a table in one database to another table in a different database that has the exact same structure?
If your log IO subsystem allows it, then:
INSERT INTO target(field1, field2, field3) SELECT field1, field2, field3 FROM source;
But having the entire transfer occur in one single statement means one single transaction, which means that x2.5 data size log has to be generated and kept during the entire statement. Ie. if you transfer 50Gb, your target db log will grow to 250Gb (even if the recovery mode is set to simple!).
If you are concerned about log, then you have to transfer in batches. You can still do the INSERT ... SELECT trick, but your SELECT has to use some key range and batch acceptable number of rows.