0350 — PartitionSize is immutable by omission from AlterConfig; dynamic partition bounds deferred

Context. Partitions are created FOR VALUES FROM (n*size) TO ((n+1)*size), and the runtime computes partition identity from head/partitionSize at every hot site — producer ensureCoveringPartition, consumer EnsureNextPartition, DropExpiredPartitions, dropPartition’s delivery-cleanup range, sweep batching. Name and bounds are the same number viewed through size; change size mid-life and head/size names a partition whose real on-disk bounds no longer match — wrong partition dropped, overlapping-partition CREATE errors, a message routed to a table that does not cover its id. Dated approximately; built across July 2026.

Decision. PartitionSize is simply absent from AlterConfig — the type system enforces immutability, not a runtime “cannot change this” check.

Consequences. The settled future unlock, deliberately deferred: stop computing bounds and read the real ones from pg_inherits + pg_get_expr(relpartbound); keep _<n> naming but never reconstruct a name from math, using the (relname, lower, upper) triples as handed back; mint each new partition at from = max existing upper bound, to = from + current size. That yields Kafka segment.bytes semantics — altering size affects only future partitions — and is fully backward-compatible since existing bounds already sit in the catalog. Deferred because it rewrites every hot partition-math path for an admin nicety. Rejected: a runtime immutability check — an absent field cannot even be misused.