Wow, thats a convoluted solution and I don't know if it will allow the use of upsert. I hope this solution is in the works :(
I have a workaround for this in pg_partman. It uses a template table to apply the unique indexes to the child tables. Note that it doesn't enforce the unique constraint across all the child tables, only each child table. Since you're doing ON CONFLICT DO NOTHING, this may work ok for your situation tho. I don't think it would work if you needed it to move the rows between partitions on conflict tho, unless the constraint was part of the partition key.
The real solution will have to wait on a global index in a future version.
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