Andrei Lepikhov runs some tests:
PostgreSQL users often employ analytical queries that sort and group data by different rules. Optimising these operators can significantly reduce the time and cost of query execution. In this post, I will discuss one such optimisation: choosing the order of columns in the GROUP BY expression.
Postgres can already reshuffle the list of grouped expressions according to the ORDER BY condition to eliminate additional sorting and save computing resources. We went further and implemented an additional strategy of group-by-clause list permutation in a series of patches (the first attempt and the second one) for discussion with the Postgres community, expecting it to be included in the next version of PostgreSQL core. You can also try it in action in the commercial Postgres Pro Enterprise fork.
From a “this is a 4th generation language” perspective, there should never be a performance difference in the ordering of grouped columns in a GROUP BY clause. The database optimizer should always determine the optimal ordering. Read Andrei’s article to see how much of a challenge that can be in practice.