Batch facet searches into a single multi_search (per-facet engine.search fan-out)

[ddeboer]

Context

The keyed DatasetFacets GraphQL surface (@lde/search-api-graphql) resolves each facet through its own context.engine.search(...) call, with that facet’s own where-filter removed (skip-own-filter). A typical listing page selects ~7 facets, so one page load fans out to 1 listing search + 7 facet searches against Typesense, and the reference facets additionally re-resolve labels. The pre-migration direct-Typesense path issued a single multi_search for the whole sidebar.

Problem

• ~4–5× the Typesense round-trips per page load (and per keystroke) – more latency, more load, and a higher chance any single sub-query flakes.
• In the common unfiltered browse, skip-own-filter removes nothing, so the 7 facet searches are identical except for facet_by; they could be a single faceted search.

Proposal

Batch the selected facet computations into one Typesense multi_search (the client’s multiSearch is already used for label lookups):

• Group the selected facets by their effective where (after skip-own-filter). Facets whose own field is not filtered share the same where → one search faceting all of them; the unfiltered case collapses to a single search.
• Expose a batch entry point on the SearchEngine port (e.g. a searchFacets(queries) / multi-facet method) so the surface dispatches once instead of per field. The per-field GraphQL resolvers then read from a per-request batched result (dataloader-style) rather than each calling the engine.

Acceptance

• A typical page load issues ≤ 2 Typesense round-trips in the common case (listing + one batched facet search), more only when distinct skip-own-filter wheres are genuinely needed.
• Facet counts, skip-own-filter semantics, and reference-facet labels are unchanged.

Notes

• Surfaced during the Dataset Register search-migration code review.
• Related follow-up: an in-memory label cache (TTL + single-flight, like the former browser createLabelResolver) would further cut the per-search label lookups.