If you specify the rebuild without specifying the partition, then you're getting a full index rebuild. It's not done one partition at a time, it's done as one operation on the entire index in one go. If you want to rebuild one partition at time, you need to specify the index rebuilds one partition at a time.
Typically tables that are partitioned across months have static data in the older partitions. If that's the case here, why are you wasting time by rebuilding the entire thing rather than just rebuilding the partitions that need it.
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