## Problem
Several display/grouping issues in the **Errors** feature, all rooted in
how the ClickHouse error materialized views (`errors_mv_v1`,
`error_occurrences_mv_v1`) read the stored error JSON produced by
`parseError`:
1. **Messageless errors show "Unknown error".** An empty message falls
straight through `coalesce(nullIf(message,''), 'Unknown error')` to the
literal, even though the error's class `name` is available (e.g. an
Effect tagged error `ListMessagesError` with no message).
2. **Unrelated errors collapse into one group.**
`calculateErrorFingerprint` keys on `type : message : stack`, where
`type` is always the union tag (`BUILT_IN_ERROR`, …), `message` is
empty, and the stack isn't read — so every messageless built-in error
(and every string/custom error) hashes to the same constant input → one
fingerprint.
3. **error_type shows the internal tag.** `coalesce(type, name, …)`
always resolves to `type` (always present), so the column shows
`BUILT_IN_ERROR` instead of the real class name.
4. **Stack traces never populate.** The MVs read `error.data.stack`, but
the serializer stores the trace under `stackTrace` — so the column is
always empty.
## Fix
All display changes are `ALTER TABLE … MODIFY QUERY` on the two views
(migration `035`); the fingerprint change is in the webapp.
- **Fingerprint** (`errorFingerprinting.ts`): fall back **message → name
→ raw**. Messageless errors now group by class name (or raw value for
non-Error throws); message-bearing errors are **unchanged**
(short-circuits at `message`), so existing groups don't split — only
currently-messageless errors get their own group going forward.
- **error_message**: same `message → name → raw` fallback before
`'Unknown error'`.
- **error_type**: coalesce `name → code → 'Error'` (drops the reliance
on the union tag). Built-in → class name, internal → `code`,
string/custom → `Error`.
- **stack trace**: read `error.data.stackTrace`. Bounded as before
(serializer caps 50 frames / 1024 chars per line; MV clips to 2000
chars).
## Migration notes
- `MODIFY QUERY` swaps the view query in place (no drop/recreate gap);
Down restores the previous query.
- **Existing rows are left unchanged** — changes apply only to rows
inserted after the migration. No backfill.
## Tests
`errorFingerprinting.test.ts` — 57 pass, incl. new cases for messageless
class names, string/custom raw values, and stability of message-bearing
fingerprints.
Fixes the display-derivation half of TRI-11938 (error_type + stack
trace); relates to TRI-9254 and TRI-9250.
Matt Aitken
2cac63f13a