compute: drop the duplicated instants from the hydration times relation [closed: not worth the churn] - #38427
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`installed_at` and `started_at` were maintained twice, in this relation and in `mz_compute_lifecycle_events_per_worker`, from the same handlers and the same event times, with the `installed_at <= started_at` invariant enforced in both representations. Keep them in one place: the lifecycle log's `installed` and `started` events. `time_ns` and `hydrated_at` stay. They are not duplicates of anything, because `hydrated_at` is the durability reading, taken when the reported output frontier passes the as-of. That frontier is the meet of the write and compute frontiers, so for a collection that sinks to persist it moves only once the output is durable, while for an index, which produces its output by writing its own trace, it coincides with computation. One column meant two things depending on the object. The lifecycle log answers that by using separate terms rather than redefining the column: its `hydrated` is always the dataflow-progress reading and its `written` is always the durability one, neither depending on the object type. Pivoting the log's `hydrated` into `hydrated_at` would therefore not be a refactor but a redefinition, and since `mz_compute_hydration_statuses.hydrated` is `time_ns IS NOT NULL` and feeds the blue-green readiness query, it would have readiness cut over on the earlier compute reading, before the output is durable. Comments at both relations record which reading each term carries. `handle_hydration_start` no longer writes to the hydration-time output, since nothing it changes is packed there any more; its retract-and-reinsert pair would have been a no-op. `started_at` survives as internal state only, to guard the handler and to back-fill `started` in `handle_hydration`. `test/testdrive/hydration-timestamps.td` moves its stage scenarios to the lifecycle relation and keeps the duration assertions here, so it now tests that the two relations agree rather than that one of them is self-consistent. The `time_ns` cross-check joins them on `(export_id, worker_id)`, both being per worker. No new migration step: the index key `(export_id, worker_id)` is unchanged, and the `mz_indexes` fingerprint move is covered by the existing replacement step at this dev version. Part of CPU-226 Co-Authored-By: Claude <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_018ZVCMBSLdxzGus78ZKWhZz
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mz_compute_hydration_times_per_workercarriestime_ns, a duration whose origin is the export's creation. Droppinginstalled_atmoves that origin into a different relation, so the relation no longer explains its own duration: interpreting it means either a join against the lifecycle log or the non-obvioushydrated_at - time_ns. Before, the instants and the duration cross-checked each other inside one row, which is what let the ordering invariant be asserted locally.Against that, the duplication costs two
Durationfields per export per worker and a few lines of packing, and is invisible to consumers. The removal gives no consumer a capability it did not have. Meanwhile it churns a downstream consumer that is being built against these relations right now."Remove the duplication" was the right instinct about an internal wart. The wart turns out to be cheaper than the surgery.
What survives from this PR: the reasoning about why
hydrated_atcannot be pivoted from the lifecycle log'shydrated— the first is the durability reading, taken when the reported output frontier passes the as-of, and the second is the dataflow-progress reading. Sincemz_compute_hydration_statuses.hydratedistime_ns IS NOT NULLand feeds blue-green readiness, redefining it would have readiness cut over before the output is durable. That is now recorded as a frozen commitment in the design doc's "What the timestamps promise, and what they do not", along with the rest of the consumer-facing compatibility contract.If this is ever revisited, the smaller and more defensible cut is to drop only
started_at, keepinginstalled_atsotime_ns's origin stays in the row.