On 2018/11/29 19:54, David Rowley wrote: > The problem is only made worse in PG11 from PG10 > because generating the custom plan has become faster than it > previously was due to the new partition pruning code which might make > it appear we can handle more partitions than we could previously, Actually, PG 11's pruning improvements don't change plancache.c's equation of custom plan cost, that is, even if pruning may have gotten faster it doesn't change the value cached_plan_cost comes up with.
Although, you're certainly right that users are well advised to trust the documentation to not go beyond hundreds of partitions, even if they may not care about all the internal details that make partitioning slow. Thanks, Amit
