Ask the database for distinct edge connections instead of scanning edges

[kmcginnes]

Description

The sampled query from #2279 can miss a rare edge connection. This adds an exact strategy and picks between them automatically.

• Complete runs groupCount() keyed by a named project() over every edge, which returns one entry per distinct combination rather than one per edge. Sampled is the per-type branching from Sample each edge type with its own limit, 10 types per request #2279.
• A pure planDiscovery picks between them from the edge type count and the edge total. The total is now part of the query key, so an answer from a smaller graph isn't reused. Refresh Schema fetches edge connections under the key built from the refreshed schema, so a changed total costs one request, not a stale one plus a fresh one. Requests are bounded by edges read and by characters of query text, and sampled batches by the worst case of every branch reaching its limit.
• The 50,000-edge scan budget is the largest volume that never failed in testing, not a prediction. When a complete scan turns out too large, it's abandoned and redone as sampled. Each request carries its own query id and abort signal, so the database stops working too.
• When nothing cheaper is left, discovery throws EdgeConnectionDiscoveryError, which keeps the original error as the cause and says what to change:
  • Fetch timeout: "Raise the Fetch Timeout in this connection's advanced options, or clear it, since this request may simply need longer than that allows. Until then, the Schema view shows node types without the edge connections between them."
  • Database timeout or memory limit: "Raise the query timeout in the database configuration, such as the DB cluster parameter group for Neptune, or use an instance with more memory. Until then, the Schema view shows node types without the edge connections between them."
• In the Schema view's popover, this error's recovery text is the next step, and the popover drops its duplicate title.
• Neptune's MemoryLimitExceededException gets its own display message. The ADR records every rejected query shape and what it actually returned.

How to read

1. docs/adr/20260918-ask-the-database-for-distinct-edge-connections.md, including every rejected shape. Start here.
2. edgeConnectionsTemplate.ts, the two shapes.
3. discoveryPlan.ts, the decision, pure and testable without a database.
4. index.ts, running the plan, degrading and cancelling.
5. discoveryError.ts, what the user is told.

Validation

• Planner tests cover budgets, label-length chunking, the branch cap, and the unknown-total rule.
• A test refreshes with a changed edge total and asserts one fetch under the new total. It saw the stale total before the fix.
• Discovery tests cover degrading from complete to sampled on fetch timeouts, database timeouts and memory limits, a user cancel propagating untouched, and abandoning in-flight requests.
• The complete shape is native on every Neptune version tested and on TinkerPop 3.6.2. On Neptune 1.3.5.0, discovery took 53 requests and 119s, against 520 requests and 516s for one request per edge type.
• pnpm checks and pnpm test clean on this layer.

Related Issues

• Part of [3.2.2][Gremlin] Relationship discovery fails with MemoryLimitExceededException for high-cardinality edge labels #2141
• Related to Edge-connections discovery fans out one request per edge type across all three query languages #2085 (the request storm this must not bring back)
• Split out of Ask the database for distinct edge connections instead of scanning edges #2244
• Layer 3 of 4. Stack, bottom first: Sample each edge type with its own limit, 10 types per request #2279 → Show the Schema view without edge connections when their discovery fails #2277 → Ask the database for distinct edge connections instead of scanning edges #2280 → Let a connection choose how edge connections are discovered #2282

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I have verified pnpm checks passes with no errors.
• I have verified pnpm test passes with no failures.
• I have covered new added functionality with unit tests if necessary.
• I have updated documentation if necessary.