There are a couple of factors to consider:
- Sorting is a N.log(N) operation.
- The sort for 14M rows might well fit in main memory; the sort with 500M rows probably doesn’t, so the sort spills to disk, which slows things up enormously.
Since the factor is about 30 in size, the nominal sort time for the big data set would be of the order of 50 times as long – under two hours. However, you need 8 bytes per data value and about another 8 bytes of overhead (that’s a guess – tune to mySQL if you know more about what it stores in an index). So, 14M × 16 ≈ 220 MB main memory. But 500M × 16 ≈ 8 GB main memory. Unless your machine has that much memory to spare (and MySQL is configured to use it), then the big sort is spilling to disk and that accounts for a lot of the rest of the time.