GET /api/sync/changes?start=now answers an empty page whose cursor sits at
serverNow in every domain, in the same opaque format the feed hands out.
A client that only needs what changes from now on, such as the Apple
Health write-back, pages forward from it instead of draining the whole
history first. start and cursor together are refused with 422.
Refs #1339
`/api/sync/changes` read the `measurementMirror` domain with one query
that ordered every token-linked reading by the joined token's
`mirror_changed_at`. No index on `measurements` can serve that order, so
every poll read and sorted every row of every token ever switched, even
when nothing had changed past the client's watermark.
The walk now reads the account's tokens with `mirror_changed_at` at or
past the watermark first. Nearly every poll finds none and stops there.
Otherwise it walks each change stamp's readings by
`(ingest_token_id, id)`, the order the index holds, with tokens that
share a stamp walked together so rows interleave by id as before. Rows,
order, page boundaries and the cursor are unchanged; the wire contract
is untouched.
EXPLAIN (ANALYZE, BUFFERS) on a seeded Postgres 16: one account, a
token with 300,000 readings switched at T0, a second token with 50
readings switched at T1, 900,000 untokened readings, page size 200.
steady-state poll (watermark past every switch)
before: 53 ms, Merge Join over ~300,000 index rows, 10,827 buffers
after: 0.07 ms token lookup + 0.02 ms resume probe, 10 buffers
first page after the switch
before: 51 ms, top-N heapsort over ~300,000 rows, 10,948 buffers
after: 0.08 ms + 0.17 ms index scan on (ingest_token_id, id)
resume in the middle of the big token
before: 53 ms
after: 0.02 ms + 0.20 ms, Index Cond on (ingest_token_id, id)
A new integration file pages both forms side by side (tied stamps, a
watermark inside a tie, soft-deleted rows, another account's token,
page sizes from 1 to 100) and asserts identical pages, and that a poll
with nothing switched past its watermark reads no reading at all.
Measurement upserts in /api/sync/changes carry appleHealthMirror. A new
measurementMirror domain delivers { id, appleHealthMirror } for the live
readings of every token whose setting changed past the cursor, keyed on
(token.mirrorChangedAt, measurement.id). Switching the setting writes the
token row only, so the readings keep their updatedAt and syncVersion and
nothing keyed on them recomputes. The domain is additive inside the v2
cursor and is left out of the retention expiry, since its watermark is a
setting's age, not a deletion's.
A '/*' inside a line comment opens a block for every guard that strips
block comments first, and the code after it disappears from that guard's
view. Four server comments and one new one did this; reword them.
Marc-André Bombeck