Set in a configuration the number of of row to prefetch
For now we prefetch IN_MAX rows (and make a second query if necessary). But indeed this value may be too small as the cost of reading more data depend on the power/memory and speed between the server and the database. Also this value should depend on the number of column prefetched. The more there are, the less must be prefetched. Probably a default value of 10_000 / len(columns) should be better default.