## Summary
Three changes from the 2026-04-27 incident:
1. **Cursor SQL fix** — `addCursorCondition` rewritten to use a
row-constructor comparison `(server_name, version) > ($1, $2)` instead
of the OR-decomposed form. Postgres can index-seek on the row
constructor; it can't on the OR form, which scanned from the start of
the index and filtered rows before the cursor. Cost grew linearly with
cursor depth.
2. **Per-phase publish timings restored** — `createServerInTransaction`
was simplified during #1211 review to log only `validate_ms`. The
incident showed publishes spending 50+ s in `acquire_lock` /
`version_checks` / `db_create` while `validate_ms` reported a few
hundred ms — the diagnostic signal was hidden. Restored timings for
every phase, refactored into a small `runPhase` helper.
3. **`pg_stat_statements` enabled** in the CNPG cluster spec. We had no
aggregate query visibility during the incident — could only EXPLAIN
queries we happened to suspect.
## Cursor fix evidence
Local benchmark, 100K rows, cold cache:
| Form | Rows filtered | Buffer reads | Time |
|------|--------------:|-------------:|-----:|
| OLD | 80,001 | 7,679 | 31.6 ms |
| NEW | 2 | 1 | **0.05 ms** |
Maps to prod's measured 8,911 buffer hits → 760 ms. End-to-end API:
`/v0/servers?limit=100&cursor=…` returns in 4–7 ms regardless of depth.
Risk check: confirmed `(server_name, version)` index exists in that
column order (both `servers_pkey` and `idx_servers_name_version`), and
both columns are `NOT NULL` so row-constructor comparison can't silently
drop rows.
## Tests
- New
`TestPostgreSQL_PerformanceScenarios/compound_cursor_across_versions_of_same_server`
pins multi-version pagination semantics. The existing cursor tests only
exercised the fallback (single-component cursor) and degenerate (one
version per server) cases.
- `make lint` clean, `go test -race ./internal/... ./cmd/...` green.
## Deployment
The cursor + slog changes are zero-downtime. The CNPG spec change
triggers a brief PG restart (single-instance cluster). After the
restart, run once on prod:
```bash
kubectl exec -i registry-pg-1 -c postgres \
--context gke_mcp-registry-prod_us-central1-b_mcp-registry-prod \
-- psql -U postgres -c "CREATE EXTENSION IF NOT EXISTS pg_stat_statements"
```
Time the merge for a low-traffic UTC window. v1.7.1's DB
retry-with-backoff covers the brief PG restart.
## Out of scope
`MaxConns` bump, per-IP rate limiting at nginx, response caching, the
pre-existing `superfluous WriteHeader` warnings — separate follow-ups.
🤖 Generated with [Claude Code](https://claude.com/claude-code)
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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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