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## Summary A NIP-01 `limit: 0` REQ asks for no stored events: it is a live-only subscription. The relay still acquired a pool connection for it, then ran a `LIMIT 0` query. Desktop is moving live subscriptions to `limit: 0`, so these acquires add up during a reconnect burst. The existing match-nothing shortcuts in `query_events_on` (an empty `kinds`, `authors`, `ids` or `#e` list) had the same cost, because they ran after the caller had already acquired a connection. This PR moves the query validation and the match-nothing checks into one function, `answer_without_db`. It adds a zero page (`limit: 0`, after the page clamp) to those checks, and every entry point now calls the function before it acquires a connection: - `query_events_with_operation`, which covers `Db::query_events` and its writer variants - `Db::query_events_routed` - `Db::query_events_routed_bounded` - `query_events_on`, for the session (aux) path that already holds a connection Invalid queries still return the same errors, in the same order, and match-nothing queries still return an empty page. What changes besides the connection: - A routed match-nothing read now returns before `route_read`, so it emits no `buzz_db_route_decision` and runs no replica proof. As desktop moves live subscriptions to `limit: 0`, writer/replica route-decision ratios will shift. Fewer reads are routed; no read changes destination. - During a pool outage, an invalid query now returns its `InvalidData` error instead of the pool error. - A zero page skips checks that only SQL applies, such as rejecting a negative `offset`. Current callers only build non-negative offsets. **Simpler?** Yes. The four separate early returns become one predicate, and the page-limit clamp is computed in one place (`page_limit`), shared by the shortcut and the SQL builder. ### Related issue N/A. This is a follow-up from https://github.com/block/buzz/pull/8651. ### Testing - New unit test, `runtime::postgres_tests::match_nothing_reads_take_no_connection`. Both pools point at a closed port, replica routing is on, and the fence is forced open, so a routed read that reached `route_read` would try the reader first. Each match-nothing shape (`limit: 0`, `max_limit: 0`, and an empty `kinds`, `authors`, `ids` or `#e` list) must return an empty page through the writer, routed and routed-bounded entry points. Invalid queries that would also match nothing must return `InvalidData` through each entry point. None of these may record a `buzz_db_pool_acquire_attempts_total`. A `limit: 1` routed control must fail after trying both the reader and the writer. The test runs in the `cargo nextest run -p buzz-db --lib` unit lane. 🤖 Generated with [Claude Code](https://claude.com/claude-code) --------- Signed-off-by: Luke Tornquist <tornquist@squareup.com> Co-authored-by: Elrond <28d6302a099e5225b02c4155ac4236e4912603df2ab08dbfc2f4fef08ce598c8@buzz.block.builderlab.xyz> Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>