[BUG] — `ProxySelectorServiceImpl.listByPage`: 4×N+1 per row

- severity: High; files: `shenyu-admin/src/main/java/org/apache/shenyu/admin/service/impl/ProxySelectorServiceImpl.java:113-148`; SQL `discovery-rel-sqlmap.xml:54-59`, `discovery-upstream-sqlmap.xml:145-150`
- description: Inside the `forEach` over `proxySelectorMapper.selectByQuery` results, for EACH row the method issues 4 separate mapper calls (`discoveryRelMapper.selectByProxySelectorId`, `discoveryHandlerMapper.selectById`, `discoveryMapper.selectById`, `discoveryUpstreamMapper.selectByDiscoveryHandlerId`). `discovery_rel` has no index on `proxy_selector_id`/`discovery_handler_id`.
- impact: List latency grows linearly with page size × 4 round-trips; seconds on large inventories.
- suggested_fix: Batch-fetch `discovery_rel` by `proxy_selector_id IN (...)` once, then batch the referenced handlers/discoveries/upstreams; or rewrite as a single JOIN.
- confidence: High

---
_Identified during the 2026-08-02 deep re-scan; full list in [`docs/scan2-2026-08-02/00-consolidated-critical-high.md`](docs/scan2-2026-08-02/00-consolidated-critical-high.md)._

Start in shenyu-admin/src/main/java/org/apache/shenyu/admin/service/impl/ProxySelectorServiceImpl.java:113-148 and trace the four mapper calls inside the proxy-selector result loop. Then read discovery-rel-sqlmap.xml:54-59 and discovery-upstream-sqlmap.xml:145-150 to understand the current queries. Done means listByPage avoids the per-row round trips through batching or a JOIN while preserving its results and reducing query growth with page size.

Written by the indexing model from the issue text.