writeonce/database
shoney.arickathil 2cad84b7c6 feat(db2-chains): bound a keys-resident row's delta chain
TESTS DELIBERATELY HELD at the developer's instruction — logic only.
The existing suite passes (36 suites, 0 fail) but exercises NEITHER new
behaviour: nothing builds a 16-deep chain, and no checkpoint test uses a
log near 64 MiB. Green here means "did not break what existed".

- tier 1: wo_wal_fold_row_at gains hops_out. The walk already visits
  every hop, so the depth is free — this is the design's pd_prune_xid,
  a cheap "is work worth doing" hint taken from work already happening
- the update path branches on it: past WO_DELTA_MAX_HOPS (16) it writes
  a full-row image instead of a delta, terminating the chain. `r`
  already holds the complete post-update row because index maintenance
  required folding it, so flattening costs bytes, not an extra read
- wo_wal_append_row_image encodes from a caller-held row, as
  WO_WAL_INSERT: a chain's base must replay into a database where
  nothing precedes it, so replay/compaction/fold need no change
- tier 2: should_compact gains a TRIGGERING absolute term and a ceiling.
  Our `floor` SUPPRESSES on a small log — the opposite of postgres's
  vac_base_thresh, which triggers on a small absolute problem the
  proportion hides. We had the proportion and the suppressor and
  neither real guard
- verified by construction, not test: both update entry points converge
  on row_apply_field_keys; db.c captures next_offset BEFORE calling in,
  so the re-point is transparent to which record type was written

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
(cherry picked from commit 1b808abd5942de81c3a6416714d1302384103040)
2026-08-30 20:38:03 +02:00
..
src feat(db2-chains): bound a keys-resident row's delta chain 2026-08-30 20:38:03 +02:00