I tried removing the index from the part_master table and got the same
result
Since all the data is in the partitions, the part_master table is empty,
so the index is not useful for your query.
myDB=# explain SELECT min(logdate) FROM part_master;
Proposals :
1- Use plpgsql to parse the system catalogs, get the list of partitions,
and issue a min() query against each
2- Since dates tend to be incrementing, I guess the minimum date must not
be changing that often (unless you delete rows) ; therefore if you need
that information often I suggest a trigger that updates a separate table
which keeps the min_date (perhaps global or for each client, you choose).
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