Help with list partitioning on expression

I have created table partitioned by list on expression using timestamp column, so for each timestamp value function returns int and each partition table is for single value (range of timestamp for which function result is the same). This helps me to simplify querying as I do not need to always write date ranges. Querying (partition pruning) works nice, however when I attach new partition it seems to always scan whole table, although I do have necessary check constraint on partitioned table. I have tried to make timestamp column both null and not null. Also, it takes longer to attach partition then to add constraint itself although per my understanding those 2 operations should do the same scan.

Thank you,
Dinko

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