A few years ago, PostgreSQL added a feature where if (1) you ran a query (and there was no "order by" clause); and (2) before it finished, you started an identical query, the second query would simply piggyback off of the first -- until the first finished. After the original query finishes, the second query will retrieve any rows it missed.
When they added this optimization, they issued a warning about it, because identical queries without "order by" clauses will return the same results, but without a guarantee of how they're sorted.
When they added this optimization, they issued a warning about it, because identical queries without "order by" clauses will return the same results, but without a guarantee of how they're sorted.