The builder assumes the vector leg always runs #2597

Current behavior

• Whatever the request is, build() always creates two slots: one for a vector read and one for a lexical (BM25) read ([FindAndRerankOperationBuilder.java#L89-L102](

  data-api/src/main/java/io/stargate/sgv2/jsonapi/service/resolver/FindAndRerankOperationBuilder.java

  Lines 89 to 102 in 02d0423

  	// Step 1 - we need a reranking task and the deferrable actions to do the intermediate reads
  	// Making the deferrables here so we can associate them with read types that will fill them
  	var deferredVectorRead =
  	new RerankingTask.DeferredCommandWithSource(
  	Rank.RankSource.VECTOR, new DeferredCommandResult());
  	var deferredBM25Read =
  	new RerankingTask.DeferredCommandWithSource(
  	Rank.RankSource.BM25, new DeferredCommandResult());
  	var rerankTasksAndDeferrables = rerankTasks(List.of(deferredBM25Read, deferredVectorRead));
  	// Step 2 - we need to read the data from the collections, we are wrapping the old collections
  	// in the new tasks so we do not change the collection code
  	var readTasksAndDeferrables = readTasks(deferredVectorRead, deferredBM25Read);

  )).

• When the lexical read isn't needed, the slot is filled with a fake empty result ([#L344](

  data-api/src/main/java/io/stargate/sgv2/jsonapi/service/resolver/FindAndRerankOperationBuilder.java

  Line 344 in 02d0423

  deferredAction.setEmptyMultiDocumentResponse();

  ), [#L357](

  data-api/src/main/java/io/stargate/sgv2/jsonapi/service/resolver/FindAndRerankOperationBuilder.java

  Line 357 in 02d0423

  deferredAction.setEmptyMultiDocumentResponse();

  )).

• The vector read has no "not needed" option:

  • it always runs ([#L333](

    data-api/src/main/java/io/stargate/sgv2/jsonapi/service/resolver/FindAndRerankOperationBuilder.java

    Line 333 in 02d0423

    var vectorReadAndDeferrables = buildVectorRead(deferredVectorReadAction);

    ));
  • it throws when there is no vector ([#L410](

    data-api/src/main/java/io/stargate/sgv2/jsonapi/service/resolver/FindAndRerankOperationBuilder.java

    Line 410 in 02d0423

    throw new IllegalArgumentException("buildVectorRead() - no vector or vectorize");

    )).

  As a result, {"$hybrid": {"$lexical": "..."}} returns a 500 even when rerankQuery and rerankOn are set ([stargate/data-api#2577](IllegalArgumentException / UNEXPECTED_SERVER_ERROR in FindAndRerank when omitting $vectorize in $hybrid #2577)).

Why fix it

• A single lexical leg can't run, because the vector read would throw.
• The filter leg has no slot at all: it is neither a vector read nor a lexical read.
• Patching would also work: make the vector slot optional and add a third slot for the filter leg. But with 4 new modes, every rule would need if branches in 5–6 places of a 495-line file, and there would be even more fake empty results. The rules in question:
  • which collection features are required;
  • how many documents to read;
  • the default rerank query and passage;
  • how to build each read.