Corpus Chain

corpus-chain — walk the knowledge graph

The pg-claude corpus is a graph. This skill is the query engine.

scenario ──(§Files)──> file ──(backlinks:auto)──> subsystem
   │                    │
   │  (§Idioms invoked) │  (idiom Call sites)
   ▼                    ▼
 idiom ──(Call sites)──> file
   ▲
   └──(Scenarios that use me)── scenario

When to use

• Before a brainstorm — resolve keywords into candidate scenarios + idioms + past runs before framing.
• Before pg-feature-plan §3 — a scenario chain gives you the initial file list, invoked idioms, adjacent scenarios (things you might also need), and past features with analogous file footprints.
• When you land on an unfamiliar file — see which idioms apply, which scenarios touch it, which subsystems own it.
• When investigating a pattern — start from an idiom, see its call sites + which scenarios invoke it + sibling idioms sharing files.
• When the user asks "have we done anything like this before?" — keyword mode surfaces past planning + sessions.

Skip when

• The task is a one-file bug fix at a known location.
• You've already loaded and internalized the chain for this area in the current session.
• The task is non-PG (frontend, DevOps, other databases).

How to invoke

scripts/corpus-chain.py --scenario <slug>     # e.g. add-new-wal-record
scripts/corpus-chain.py --idiom <slug>         # e.g. memory-contexts
scripts/corpus-chain.py --file <src-path>      # e.g. src/backend/access/heap/heapam.c
scripts/corpus-chain.py --keywords "..."       # free-text search
scripts/corpus-chain.py --scenario <slug> --depth 2  # pattern-siblings + 2nd-order context

Output is markdown to stdout — read it, don't dump it wholesale into a plan.

What the output tells you

For a --scenario chain:

• Files touched — the initial §3 seed for pg-feature-plan.
• Idioms invoked — for each, tagged direct (named in scenario prose) or transitive (found because of file overlap). Direct-only for pure evidence; use transitive as "you'll probably also touch these patterns".
• Adjacent scenarios — related_scenarios frontmatter + scenarios sharing many files. If a scenario shares 5+ files, expect the two changes to conflict at review time; either bundle or sequence.
• Subsystems — which subsystem docs already cite these files. Cross-cutting features hit ≥ 2 subsystems.
• Data structures involved — DS docs whose Call sites overlap this scenario's files.
• Likely reviewers (personas) — from progress/persona-scenario-matrix.md. Reflect who might catch this on hackers-list.
• Depth-2 pattern-siblings (only with --depth 2) — idioms whose Call sites share ≥2 files with any invoked idiom. Second-order context ("you'll probably also touch these patterns while implementing the primary"). Useful for planning + risk enumeration.
• Analogous past features — planning slugs + session logs with keyword or file overlap. Read the top-2 before starting; they carry the design decisions you'll re-litigate.

For an --idiom chain:

• Call sites — files that already apply this pattern. Pick 1-2 as your reference implementations.
• Sibling idioms — patterns co-located with yours. If both apply, cite both in the plan.
• Scenarios that use me — reverse index; which change-classes need this idiom.

For a --file chain:

• Idioms applying here — before touching the file, know which patterns already live in it.
• Scenarios touching this file — if a scenario touches it, your feature might trigger the scenario's downstream effects.

For --keywords:

• A shallow scan of scenario titles + idiom titles + planning + sessions.
• Use to discover the anchor, then re-run with --scenario / --idiom / --file for the full chain.

Integration with the planner suite

In pg-feature-brainstorm §0 / §5:

Run --keywords with the user's language first to find candidate scenarios and past work. Read the top-2 past-feature docs before writing §0's usage surface — they encode what worked and what got rejected. Then run --scenario <picked> to see the file scope you're inheriting.

In pg-feature-plan before §3:

Run --scenario <slug> for the picked scenario. The output is the DRAFT §3 file table. Cite each file with [via knowledge/files/<path>.md] when the file doc exists. Idioms in the chain populate §8's per-phase "≥1 idiom" requirement automatically.

In pg-implement per-phase:

For each phase's staged files, run --file <path> to see which idioms should be respected. If a phase touches a file where memory-contexts applies but the phase's edit doesn't use MemoryContexts, that's a smell — R7 escalate or reconsider.

What the tool does NOT do

• It does not invent claims. Every edge is derived from an already-written link in the corpus (idiom's Call sites table, scenario's Files section, scenario's Idioms-invoked block, frontmatter references). If the graph is thin in some area, so is the answer.
• It does not verify the anchor at the current commit. Cite drift is possible if the corpus hasn't refreshed since the last upstream anchor bump — check the last_verified_commit in the target doc's frontmatter if the edge feels stale.
• It does not rank quality. Ordering is by shared-file count / hit count, not by prose quality. Read the source doc, don't rely on ranks alone.

Refresh cadence

Chain results are only as good as the graph edges. Refresh order after any bulk corpus change:

1. scripts/populate-idiom-callsites.py — updates idiom → file edges.
2. scripts/build-scenario-idiom-matrix.py — rebuilds the scenario ↔ idiom join.
3. scripts/corpus-chain.py --scenario/--idiom/--file/--keywords — query.

Steps 1 and 2 are idempotent and safe to schedule from a cloud routine on anchor bumps.

Examples

"I want to add a new WAL record type."

scripts/corpus-chain.py --scenario add-new-wal-record

Returns: 16 files, 9 idioms (5 direct + 4 transitive via xlog.c), 3 adjacent scenarios (add-new-index-am, bump-catversion, add-new-table-am), 2 subsystems, 1 past session.

"Where is the memory-contexts pattern already applied?"

scripts/corpus-chain.py --idiom memory-contexts

Returns: 4 canonical call sites, 8 scenarios that invoke it, 2 sibling idioms, past planning: jsonpath_leak / memory-hunt / sp2-pgstr-maxalloc.

"Have we done anything with jsonpath before?"

scripts/corpus-chain.py --keywords "jsonpath memory"

Returns: matching scenarios (fix-memory-leak), matching idioms, planning: jsonpath_leak (score 4), pgstat_progress_leak, session: 2026-06-23-memory-hunt-calibration.

Failure modes

• "anchor not found" — the slug doesn't exist. Check ls knowledge/scenarios/ or ls knowledge/idioms/.
• Empty "Subsystems" section on a file — the file's owner subsystem doc doesn't cite it with source/<path> inline. Path-prefix inference (owner = <top-dir>/<second-dir>) is a v2 improvement.
• Fewer idioms than expected — the scenario's ## Idioms invoked block is stale; re-run scripts/build-scenario-idiom-matrix.py first.