MB-28163: Provide an API to estimate the RAM needed for SearchResult

exported API: MemoryNeededForSearchResult(req *SearchRequest)

search.go

@@ -22,8 +22,10 @@ import (
 
 	"github.com/blevesearch/bleve/analysis"
 	"github.com/blevesearch/bleve/analysis/datetime/optional"
+	"github.com/blevesearch/bleve/document"
 	"github.com/blevesearch/bleve/registry"
 	"github.com/blevesearch/bleve/search"
+	"github.com/blevesearch/bleve/search/collector"
 	"github.com/blevesearch/bleve/search/query"
 	"github.com/blevesearch/bleve/size"
 )
@@ -518,3 +520,44 @@ func (sr *SearchResult) Merge(other *SearchResult) {
 
 	sr.Facets.Merge(other.Facets)
 }
+
+// MemoryNeededForSearchResult is an exported helper function to determine the RAM
+// needed to accommodate the results for a given search request.
+func MemoryNeededForSearchResult(req *SearchRequest) uint64 {
+	if req == nil {
+		return 0
+	}
+
+	numDocMatches := req.Size + req.From
+	if req.Size+req.From > collector.PreAllocSizeSkipCap {
+		numDocMatches = collector.PreAllocSizeSkipCap
+	}
+
+	estimate := 0
+
+	// overhead from the SearchResult structure
+	var sr SearchResult
+	estimate += sr.Size()
+
+	var dm search.DocumentMatch
+	sizeOfDocumentMatch := dm.Size()
+
+	// overhead from results
+	estimate += numDocMatches * sizeOfDocumentMatch
+
+	// overhead from facet results
+	if req.Facets != nil {
+		var fr search.FacetResult
+		estimate += len(req.Facets) * fr.Size()
+	}
+
+	// highlighting, store
+	var d document.Document
+	if len(req.Fields) > 0 || req.Highlight != nil {
+		for i := 0; i < (req.Size + req.From); i++ {
+			estimate += (req.Size + req.From) * d.Size()
+		}
+	}
+
+	return uint64(estimate)
+}

search_test.go

@@ -399,3 +399,18 @@ func TestSearchResultFacetsMerge(t *testing.T) {
 		t.Errorf("expected %#v, got %#v", expected, l)
 	}
 }
+
+func TestMemoryNeededForSearchResult(t *testing.T) {
+	query := NewTermQuery("blah")
+	req := NewSearchRequest(query)
+
+	var sr SearchResult
+	expect := sr.Size()
+	var dm search.DocumentMatch
+	expect += 10 * dm.Size()
+
+	estimate := MemoryNeededForSearchResult(req)
+	if estimate != uint64(expect) {
+		t.Errorf("estimate not what is expected: %v != %v", estimate, expect)
+	}
+}