Lately there have been couple of discussions on IRC, Slack, and Reddit that showed that people assume that by using int4/integer they use 4 bytes less than they would in case of int8/bigint. This is not really the case. Let me explain why.
On 20th of March, Andres Freund committed patch:
Use 128-bit math to accelerate some aggregation functions. On platforms where we support 128bit integers, use them to implement faster transition functions for sum(int8), avg(int8), var_*(int2/int4),stdev_*(int2/int4). Where not supported continue to use numeric as a transition type. In some synthetic benchmarks this has been shown to provide significant speedups. Bumps catversion. Discussion: 544BB5F1.email@example.com Author: Andreas Karlsson Reviewed-By: Peter Geoghegan, Petr Jelinek, Andres Freund, Oskari Saarenmaa, David Rowley