Tag hit estimates from attribute search contexts as unknown when applicable.

22 files changed, 136 insertions, 65 deletions

diff --git a/searchcore/src/vespa/searchcore/proton/documentmetastore/search_context.cpp b/searchcore/src/vespa/searchcore/proton/documentmetastore/search_context.cpp
index 3b8759aac77..a93e7543ca1 100644
--- a/searchcore/src/vespa/searchcore/proton/documentmetastore/search_context.cpp
+++ b/searchcore/src/vespa/searchcore/proton/documentmetastore/search_context.cpp
@@ -113,10 +113,10 @@ SearchContext::onFind(DocId, int32_t ) const
     throw vespalib::IllegalStateException("The function is not implemented for documentmetastore::SearchContext");
 }
 
-unsigned int
-SearchContext::approximateHits() const
+search::attribute::HitEstimate
+SearchContext::calc_hit_estimate() const
 {
-    return _isWord ? 1 : search::attribute::SearchContext::approximateHits();
+    return _isWord ? search::attribute::HitEstimate(1) : search::attribute::SearchContext::calc_hit_estimate();
 }
 
 SearchIterator::UP
diff --git a/searchcore/src/vespa/searchcore/proton/documentmetastore/search_context.h b/searchcore/src/vespa/searchcore/proton/documentmetastore/search_context.h
index 9c868bb1454..d7bd2c7b630 100644
--- a/searchcore/src/vespa/searchcore/proton/documentmetastore/search_context.h
+++ b/searchcore/src/vespa/searchcore/proton/documentmetastore/search_context.h
@@ -20,7 +20,7 @@ private:
     document::GlobalId _gid;
     uint32_t _docid_limit;
 
-    unsigned int approximateHits() const override;
+    search::attribute::HitEstimate calc_hit_estimate() const override;
     int32_t onFind(DocId docId, int32_t elemId, int32_t &weight) const override;
     int32_t onFind(DocId docId, int32_t elemId) const override;
 
diff --git a/searchlib/src/tests/attribute/imported_search_context/imported_search_context_test.cpp b/searchlib/src/tests/attribute/imported_search_context/imported_search_context_test.cpp
index 79bdd83dc88..93ae9cb13cb 100644
--- a/searchlib/src/tests/attribute/imported_search_context/imported_search_context_test.cpp
+++ b/searchlib/src/tests/attribute/imported_search_context/imported_search_context_test.cpp
@@ -76,15 +76,17 @@ bool is_strict_hit_with_weight(Iterator& iter, TermFieldMatchData& match,
             EXPECT_EQUAL(weight, match.getWeight()));
 }
 
-TEST_F("approximateHits() returns document count of reference attribute when not using fast-search target attribute", Fixture) {
+TEST_F("calc_hit_estimate() returns document count of reference attribute when not using fast-search target attribute", Fixture) {
     add_n_docs_with_undefined_values(*f.target_attr, 10);
     add_n_docs_with_undefined_values(*f.reference_attr, 101);
 
     auto ctx = f.create_context(word_term("foo"));
-    EXPECT_EQUAL(101u, ctx->approximateHits());
+    auto est = ctx->calc_hit_estimate();
+    EXPECT_EQUAL(101u, est.est_hits());
+    EXPECT_TRUE(est.is_unknown());
 }
 
-TEST_F("approximateHits() estimates hits when using fast-search target attribute", Fixture(false, FastSearchConfig::ExplicitlyEnabled))
+TEST_F("calc_hit_estimate() estimates hits when using fast-search target attribute", Fixture(false, FastSearchConfig::ExplicitlyEnabled))
 {
     constexpr uint32_t target_docs = 1000;
     constexpr uint32_t docs = 10000;
@@ -115,16 +117,22 @@ TEST_F("approximateHits() estimates hits when using fast-search target attribute
     f.reference_attr->commit();
     auto ctx = f.create_context(word_term("10"));
     // Exact count: 0 target hits => 0
-    EXPECT_EQUAL(0u, ctx->approximateHits());
+    auto est = ctx->calc_hit_estimate();
+    EXPECT_EQUAL(0u, est.est_hits());
+    EXPECT_FALSE(est.is_unknown());
     TermFieldMatchData match;
     auto iter = f.create_iterator(*ctx, match, false);
     EXPECT_TRUE(iter->matches_any() == Trinary::False);
     ctx = f.create_context(word_term("20"));
     // Exact count: 2 target hits, 2 docs / target doc => 2 * 2 = 4
-    EXPECT_EQUAL(4u, ctx->approximateHits());
+    est = ctx->calc_hit_estimate();
+    EXPECT_EQUAL(4u, est.est_hits());
+    EXPECT_FALSE(est.is_unknown());
     ctx = f.create_context(word_term("30"));
     // Approximation: 110 target hits => 110 * 10001 / 1001 = 1099
-    EXPECT_EQUAL(1099u, ctx->approximateHits());
+    est = ctx->calc_hit_estimate();
+    EXPECT_EQUAL(1099u, est.est_hits());
+    EXPECT_FALSE(est.is_unknown());
 }
 
 TEST_F("attributeName() returns imported attribute name", Fixture) {
diff --git a/searchlib/src/tests/attribute/postinglistattribute/postinglistattribute_test.cpp b/searchlib/src/tests/attribute/postinglistattribute/postinglistattribute_test.cpp
index 3d3ff469546..ed91cefd35c 100644
--- a/searchlib/src/tests/attribute/postinglistattribute/postinglistattribute_test.cpp
+++ b/searchlib/src/tests/attribute/postinglistattribute/postinglistattribute_test.cpp
@@ -498,8 +498,10 @@ PostingListAttributeTest::checkSearch(bool useBitVector, bool need_unpack, bool
     SearchContextPtr sc = getSearch(vec, term, false, attribute::SearchContextParams().useBitVector(useBitVector));
     EXPECT_FALSE( ! sc );
     sc->fetchPostings(queryeval::ExecuteInfo::TRUE);
-    size_t approx = sc->approximateHits();
-    EXPECT_EQ(numHits, approx);
+    auto est = sc->calc_hit_estimate();
+    uint32_t est_hits = est.est_hits();
+    EXPECT_FALSE(est.is_unknown());
+    EXPECT_EQ(numHits, est_hits);
     if (docBegin == 0) {
         // Approximation does not know about the special 0
         // But the iterator does....
@@ -523,7 +525,7 @@ PostingListAttributeTest::checkSearch(bool useBitVector, bool need_unpack, bool
         hits++;
     }
     EXPECT_EQ(numHits, hits);
-    EXPECT_GE(approx, hits);
+    EXPECT_GE(est_hits, hits);
     EXPECT_EQ(docEnd, lastDocId+1);
 }
 
diff --git a/searchlib/src/tests/attribute/searchcontext/searchcontext_test.cpp b/searchlib/src/tests/attribute/searchcontext/searchcontext_test.cpp
index 741a86b0beb..85b13c20f88 100644
--- a/searchlib/src/tests/attribute/searchcontext/searchcontext_test.cpp
+++ b/searchlib/src/tests/attribute/searchcontext/searchcontext_test.cpp
@@ -60,6 +60,7 @@ using largeint_t = AttributeVector::largeint_t;
 using attribute::BasicType;
 using attribute::CollectionType;
 using attribute::Config;
+using attribute::HitEstimate;
 using attribute::SearchContextParams;
 using attribute::test::AttributeBuilder;
 using fef::MatchData;
@@ -87,6 +88,12 @@ public:
 DocSet::DocSet() noexcept = default;
 DocSet::~DocSet() = default;
 
+bool is_flag_attribute(const Config& cfg) {
+    return cfg.fastSearch() &&
+        (cfg.basicType() == BasicType::INT8) &&
+        (cfg.collectionType() == CollectionType::ARRAY);
+}
+
 template <typename V, typename T>
 class PostingList
 {
@@ -104,6 +111,21 @@ public:
     DocSet & getHits() { return _hits; }
     const DocSet & getHits() const { return _hits; }
     uint32_t getHitCount() const { return _hits.size(); }
+    attribute::HitEstimate expected_hit_estimate() const {
+        if (getHitCount() == 0) {
+            return HitEstimate(0);
+        }
+        uint32_t docid_limit = _vec->getStatus().getNumDocs();
+        if (is_flag_attribute(_vec->getConfig())) {
+            return HitEstimate::unknown(docid_limit);
+        } else if (_vec->getConfig().fastSearch()) {
+            return HitEstimate(getHitCount());
+        } else if (_vec->getConfig().collectionType() == CollectionType::SINGLE) {
+            return HitEstimate::unknown(docid_limit);
+        } else {
+            return HitEstimate::unknown(std::max((uint64_t)docid_limit, _vec->getStatus().getNumValues()));
+        }
+    }
 };
 
 template <typename V, typename T>
@@ -166,7 +188,7 @@ private:
     void testSearchIteratorConformance();
     // test search functionality
     template <typename V, typename T>
-    void testFind(const PostingList<V, T> & first);
+    void testFind(const PostingList<V, T> & first, bool verify_hit_estimate);
 
     template <typename V, typename T>
     void testSearch(V & attribute, uint32_t numDocs, const std::vector<T> & values);
@@ -536,10 +558,16 @@ SearchContextTest::checkResultSet(const ResultSet & rs, const DocSet & expected,
 //-----------------------------------------------------------------------------
 template <typename V, typename T>
 void
-SearchContextTest::testFind(const PostingList<V, T> & pl)
+SearchContextTest::testFind(const PostingList<V, T> & pl, bool verify_hit_estimate)
 {
     { // strict search iterator
         SearchContextPtr sc = getSearch(pl.getAttribute(), pl.getValue());
+        if (verify_hit_estimate) {
+            auto act_est = sc->calc_hit_estimate();
+            auto exp_est = pl.expected_hit_estimate();
+            EXPECT_EQUAL(exp_est.est_hits(), act_est.est_hits());
+            EXPECT_EQUAL(exp_est.is_unknown(), act_est.is_unknown());
+        }
         sc->fetchPostings(queryeval::ExecuteInfo::TRUE);
         TermFieldMatchData dummy;
         SearchBasePtr sb = sc->createIterator(&dummy, true);
@@ -571,7 +599,7 @@ SearchContextTest::testSearch(V & attribute, uint32_t numDocs, const std::vector
 
     // test find()
     for (const auto & list : lists) {
-        testFind(list);
+        testFind(list, true);
     }
 }
 
@@ -591,7 +619,7 @@ SearchContextTest::testMultiValueSearchHelper(V & vec, const std::vector<T> & va
     for (const auto & list : lists) {
         //std::cout << "testFind(lists[" << i << "]): value = " << lists[i].getValue()
         //                                            << ", hit count = " << lists[i].getHitCount() << std::endl;
-        testFind(list);
+        testFind(list, false);
     }
 }
 
diff --git a/searchlib/src/vespa/searchcommon/attribute/hit_estimate.h b/searchlib/src/vespa/searchcommon/attribute/hit_estimate.h
new file mode 100644
index 00000000000..b9063bec1f6
--- /dev/null
+++ b/searchlib/src/vespa/searchcommon/attribute/hit_estimate.h
@@ -0,0 +1,31 @@
+// Copyright Vespa.ai. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
+
+#pragma once
+
+#include <stdint.h>
+
+namespace search::attribute {
+
+/**
+ * Class encapsulating the estimated number of hits an attribute search context will provide.
+ *
+ * E.g. for attributes without fast-search an estimate is not known.
+ * Instead the total number of values to match against is returned as the estimate.
+ * This is always at least the docid limit space.
+ */
+class HitEstimate {
+private:
+    uint32_t _est_hits;
+    bool _unknown;
+
+    HitEstimate(uint32_t est_hits_in, bool unknown_in) : _est_hits(est_hits_in), _unknown(unknown_in) {}
+
+public:
+    explicit HitEstimate(uint32_t est_hits_in) : HitEstimate(est_hits_in, false) {}
+    static HitEstimate unknown(uint32_t total_value_count) { return HitEstimate(total_value_count, true); }
+    uint32_t est_hits() const { return _est_hits; }
+    bool is_unknown() const { return _unknown; }
+};
+
+}
+
diff --git a/searchlib/src/vespa/searchcommon/attribute/i_search_context.h b/searchlib/src/vespa/searchcommon/attribute/i_search_context.h
index 67c88b25d83..8ea6f0f60af 100644
--- a/searchlib/src/vespa/searchcommon/attribute/i_search_context.h
+++ b/searchlib/src/vespa/searchcommon/attribute/i_search_context.h
@@ -2,6 +2,7 @@
 
 #pragma once
 
+#include "hit_estimate.h"
 #include <vespa/searchcommon/common/range.h>
 #include <vespa/vespalib/stllike/string.h>
 #include <memory>
@@ -27,7 +28,7 @@ private:
 public:
     virtual ~ISearchContext() = default;
 
-    virtual unsigned int approximateHits() const = 0;
+    virtual HitEstimate calc_hit_estimate() const = 0;
 
     /**
      * Creates an attribute search iterator associated with this
diff --git a/searchlib/src/vespa/searchlib/attribute/attribute_blueprint_factory.cpp b/searchlib/src/vespa/searchlib/attribute/attribute_blueprint_factory.cpp
index d5c67664a5e..8742f53bc8e 100644
--- a/searchlib/src/vespa/searchlib/attribute/attribute_blueprint_factory.cpp
+++ b/searchlib/src/vespa/searchlib/attribute/attribute_blueprint_factory.cpp
@@ -184,7 +184,7 @@ AttributeFieldBlueprint::AttributeFieldBlueprint(FieldSpecBase field, const IAtt
       _search_context(attribute.createSearchContext(std::move(term), params)),
       _type(OTHER)
 {
-    uint32_t estHits = _search_context->approximateHits();
+    uint32_t estHits = _search_context->calc_hit_estimate().est_hits();
     HitEstimate estimate(estHits, estHits == 0);
     setEstimate(estimate);
     if (attribute.isFloatingPointType()) {
@@ -235,7 +235,7 @@ public:
             query::SimpleRangeTerm rt(qr, "", 0, query::Weight(0));
             string stack(StackDumpCreator::create(rt));
             _rangeSearches.push_back(attr.createSearchContext(QueryTermDecoder::decodeTerm(stack), scParams));
-            estHits += _rangeSearches.back()->approximateHits();
+            estHits += _rangeSearches.back()->calc_hit_estimate().est_hits();
             LOG(debug, "Range '%s' estHits %" PRId64, qr.getRangeString().c_str(), estHits);
         }
         if (estHits > attr.getNumDocs()) {
diff --git a/searchlib/src/vespa/searchlib/attribute/attribute_weighted_set_blueprint.cpp b/searchlib/src/vespa/searchlib/attribute/attribute_weighted_set_blueprint.cpp
index d0353ab8947..a5003ba542b 100644
--- a/searchlib/src/vespa/searchlib/attribute/attribute_weighted_set_blueprint.cpp
+++ b/searchlib/src/vespa/searchlib/attribute/attribute_weighted_set_blueprint.cpp
@@ -110,7 +110,7 @@ AttributeWeightedSetBlueprint::~AttributeWeightedSetBlueprint()
 void
 AttributeWeightedSetBlueprint::addToken(std::unique_ptr<ISearchContext> context, int32_t weight)
 {
-    _estHits = std::min(_estHits + context->approximateHits(), _numDocs);
+    _estHits = std::min(_estHits + context->calc_hit_estimate().est_hits(), _numDocs);
     setEstimate(HitEstimate(_estHits, (_estHits == 0)));
     _weights.push_back(weight);
     _contexts.push_back(context.release());
diff --git a/searchlib/src/vespa/searchlib/attribute/empty_search_context.cpp b/searchlib/src/vespa/searchlib/attribute/empty_search_context.cpp
index 379118226a8..795cbc3d6f0 100644
--- a/searchlib/src/vespa/searchlib/attribute/empty_search_context.cpp
+++ b/searchlib/src/vespa/searchlib/attribute/empty_search_context.cpp
@@ -24,10 +24,10 @@ EmptySearchContext::onFind(DocId, int32_t) const
     return -1;
 }
 
-unsigned int
-EmptySearchContext::approximateHits() const
+HitEstimate
+EmptySearchContext::calc_hit_estimate() const
 {
-    return 0u;
+    return HitEstimate(0);
 }
 
 uint32_t
diff --git a/searchlib/src/vespa/searchlib/attribute/empty_search_context.h b/searchlib/src/vespa/searchlib/attribute/empty_search_context.h
index 23e981f18a8..f946288df61 100644
--- a/searchlib/src/vespa/searchlib/attribute/empty_search_context.h
+++ b/searchlib/src/vespa/searchlib/attribute/empty_search_context.h
@@ -13,7 +13,7 @@ class EmptySearchContext : public SearchContext
 {
     int32_t onFind(DocId, int32_t, int32_t&) const override;
     int32_t onFind(DocId, int32_t) const override;
-    unsigned int approximateHits() const override;
+    HitEstimate calc_hit_estimate() const override;
     std::unique_ptr<queryeval::SearchIterator> createIterator(fef::TermFieldMatchData*, bool) override;
     std::unique_ptr<queryeval::SearchIterator> createFilterIterator(fef::TermFieldMatchData*, bool) override;
 public:
diff --git a/searchlib/src/vespa/searchlib/attribute/enumhintsearchcontext.cpp b/searchlib/src/vespa/searchlib/attribute/enumhintsearchcontext.cpp
index 9c5218be16e..22362231cc1 100644
--- a/searchlib/src/vespa/searchlib/attribute/enumhintsearchcontext.cpp
+++ b/searchlib/src/vespa/searchlib/attribute/enumhintsearchcontext.cpp
@@ -54,12 +54,12 @@ EnumHintSearchContext::createPostingIterator(TermFieldMatchData *, bool )
 }
 
 
-unsigned int
-EnumHintSearchContext::approximateHits() const
+HitEstimate
+EnumHintSearchContext::calc_hit_estimate() const
 {
     return (_uniqueValues == 0u)
-        ? 0u
-        : std::max(uint64_t(_docIdLimit), _numValues);
+        ? HitEstimate(0u)
+        : HitEstimate::unknown(std::max(uint64_t(_docIdLimit), _numValues));
 }
 
 }
diff --git a/searchlib/src/vespa/searchlib/attribute/enumhintsearchcontext.h b/searchlib/src/vespa/searchlib/attribute/enumhintsearchcontext.h
index ee07f852af5..23a0a6c560b 100644
--- a/searchlib/src/vespa/searchlib/attribute/enumhintsearchcontext.h
+++ b/searchlib/src/vespa/searchlib/attribute/enumhintsearchcontext.h
@@ -40,7 +40,7 @@ protected:
     createPostingIterator(fef::TermFieldMatchData *matchData, bool strict) override;
 
     void fetchPostings(const queryeval::ExecuteInfo & execInfo) override;
-    unsigned int approximateHits() const override;
+    HitEstimate calc_hit_estimate() const override;
     uint32_t get_committed_docid_limit() const noexcept { return _docIdLimit; }
 };
 
diff --git a/searchlib/src/vespa/searchlib/attribute/imported_search_context.cpp b/searchlib/src/vespa/searchlib/attribute/imported_search_context.cpp
index b27a67e893b..79a021989fc 100644
--- a/searchlib/src/vespa/searchlib/attribute/imported_search_context.cpp
+++ b/searchlib/src/vespa/searchlib/attribute/imported_search_context.cpp
@@ -97,21 +97,21 @@ ImportedSearchContext::calc_exact_hits() const
     return sum_hits;
 }
 
-unsigned int
-ImportedSearchContext::approximateHits() const
+HitEstimate
+ImportedSearchContext::calc_hit_estimate() const
 {
-    uint32_t target_approx_hits = _target_search_context->approximateHits();
-    if (target_approx_hits == 0) {
+    uint32_t target_est_hits = _target_search_context->calc_hit_estimate().est_hits();
+    if (target_est_hits == 0) {
         _zero_hits.store(true, std::memory_order_relaxed);
-        return 0;
+        return HitEstimate(0);
     }
     if (!_target_attribute.getIsFastSearch()) {
-        return _reference_attribute.getNumDocs();
+        return HitEstimate::unknown(_reference_attribute.getNumDocs());
     }
-    if (target_approx_hits >= MIN_TARGET_HITS_FOR_APPROXIMATION) {
-        return calc_approx_hits(target_approx_hits);
+    if (target_est_hits >= MIN_TARGET_HITS_FOR_APPROXIMATION) {
+        return HitEstimate(calc_approx_hits(target_est_hits));
     } else {
-        return calc_exact_hits();
+        return HitEstimate(calc_exact_hits());
     }
 }
 
diff --git a/searchlib/src/vespa/searchlib/attribute/imported_search_context.h b/searchlib/src/vespa/searchlib/attribute/imported_search_context.h
index 6a0c43f8578..305a8bf2379 100644
--- a/searchlib/src/vespa/searchlib/attribute/imported_search_context.h
+++ b/searchlib/src/vespa/searchlib/attribute/imported_search_context.h
@@ -63,10 +63,9 @@ public:
                           const attribute::IAttributeVector &target_attribute);
     ~ImportedSearchContext() override;
 
-
     std::unique_ptr<queryeval::SearchIterator>
     createIterator(fef::TermFieldMatchData* matchData, bool strict) override;
-    unsigned int approximateHits() const override;
+    HitEstimate calc_hit_estimate() const override;
     void fetchPostings(const queryeval::ExecuteInfo &execInfo) override;
     bool valid() const override;
     Int64Range getAsIntegerTerm() const override;
diff --git a/searchlib/src/vespa/searchlib/attribute/ipostinglistsearchcontext.h b/searchlib/src/vespa/searchlib/attribute/ipostinglistsearchcontext.h
index 38dfefb878b..ac1abc56329 100644
--- a/searchlib/src/vespa/searchlib/attribute/ipostinglistsearchcontext.h
+++ b/searchlib/src/vespa/searchlib/attribute/ipostinglistsearchcontext.h
@@ -2,6 +2,7 @@
 
 #pragma once
 
+#include <vespa/searchcommon/attribute/hit_estimate.h>
 #include <memory>
 
 namespace search::queryeval {
@@ -30,7 +31,7 @@ protected:
 public:
     virtual void fetchPostings(const queryeval::ExecuteInfo & execInfo) = 0;
     virtual std::unique_ptr<queryeval::SearchIterator> createPostingIterator(fef::TermFieldMatchData *matchData, bool strict) = 0;
-    virtual unsigned int approximateHits() const = 0;
+    virtual HitEstimate calc_hit_estimate() const = 0;
 };
 
 }
diff --git a/searchlib/src/vespa/searchlib/attribute/postinglistsearchcontext.h b/searchlib/src/vespa/searchlib/attribute/postinglistsearchcontext.h
index 3e0794835ae..41a12dce756 100644
--- a/searchlib/src/vespa/searchlib/attribute/postinglistsearchcontext.h
+++ b/searchlib/src/vespa/searchlib/attribute/postinglistsearchcontext.h
@@ -119,7 +119,7 @@ protected:
     createPostingIterator(fef::TermFieldMatchData *matchData, bool strict) override;
 
     unsigned int singleHits() const;
-    unsigned int approximateHits() const override;
+    HitEstimate calc_hit_estimate() const override;
     void applyRangeLimit(long rangeLimit);
     struct FillPart;
 };
@@ -217,11 +217,11 @@ private:
     void getIterators(bool shouldApplyRangeLimit);
     bool valid() const override { return this->isValid(); }
 
-    unsigned int approximateHits() const override {
-        const unsigned int estimate = PostingListSearchContextT<DataT>::approximateHits();
+    HitEstimate calc_hit_estimate() const override {
+        HitEstimate estimate = PostingListSearchContextT<DataT>::calc_hit_estimate();
         const unsigned int limit = std::abs(this->getRangeLimit());
-        return ((limit > 0) && (limit < estimate))
-            ? limit
+        return ((limit > 0) && (limit < estimate.est_hits()))
+            ? HitEstimate(limit)
             : estimate;
     }
     void fetchPostings(const ExecuteInfo & execInfo) override {
diff --git a/searchlib/src/vespa/searchlib/attribute/postinglistsearchcontext.hpp b/searchlib/src/vespa/searchlib/attribute/postinglistsearchcontext.hpp
index f937d567588..77f773d0469 100644
--- a/searchlib/src/vespa/searchlib/attribute/postinglistsearchcontext.hpp
+++ b/searchlib/src/vespa/searchlib/attribute/postinglistsearchcontext.hpp
@@ -284,8 +284,8 @@ PostingListSearchContextT<DataT>::singleHits() const
 }
 
 template <typename DataT>
-unsigned int
-PostingListSearchContextT<DataT>::approximateHits() const
+HitEstimate
+PostingListSearchContextT<DataT>::calc_hit_estimate() const
 {
     size_t numHits = 0;
     if (_uniqueValues == 0u) {
@@ -294,9 +294,9 @@ PostingListSearchContextT<DataT>::approximateHits() const
     } else if (_dictionary.get_has_btree_dictionary()) {
         numHits = estimated_hits_in_range();
     } else {
-        numHits = _docIdLimit;
+        return HitEstimate::unknown(_docIdLimit);
     }
-    return std::min(numHits, size_t(std::numeric_limits<uint32_t>::max()));
+    return HitEstimate(std::min(numHits, size_t(std::numeric_limits<uint32_t>::max())));
 }
 
 template <typename DataT>
diff --git a/searchlib/src/vespa/searchlib/attribute/search_context.cpp b/searchlib/src/vespa/searchlib/attribute/search_context.cpp
index e59326bc808..0c0318bf1e3 100644
--- a/searchlib/src/vespa/searchlib/attribute/search_context.cpp
+++ b/searchlib/src/vespa/searchlib/attribute/search_context.cpp
@@ -10,20 +10,20 @@ using search::queryeval::SearchIterator;
 
 namespace search::attribute {
 
-unsigned int
-SearchContext::approximateHits() const
+HitEstimate
+SearchContext::calc_hit_estimate() const
 {
     if (_plsc != nullptr) {
-        return _plsc->approximateHits();
+        return _plsc->calc_hit_estimate();
     }
-    return std::max(uint64_t(_attr.getNumDocs()), _attr.getStatus().getNumValues());
+    return HitEstimate::unknown(std::max(uint64_t(_attr.getNumDocs()), _attr.getStatus().getNumValues()));
 }
 
 std::unique_ptr<SearchIterator>
 SearchContext::createIterator(fef::TermFieldMatchData* matchData, bool strict)
 {
     if (_plsc != nullptr) {
-        std::unique_ptr<SearchIterator> res = _plsc->createPostingIterator(matchData, strict);
+        auto res = _plsc->createPostingIterator(matchData, strict);
         if (res) {
             return res;
         }
diff --git a/searchlib/src/vespa/searchlib/attribute/search_context.h b/searchlib/src/vespa/searchlib/attribute/search_context.h
index a7e87ee86fc..a18e15ae8f4 100644
--- a/searchlib/src/vespa/searchlib/attribute/search_context.h
+++ b/searchlib/src/vespa/searchlib/attribute/search_context.h
@@ -32,7 +32,7 @@ public:
     SearchContext& operator=(SearchContext&&) noexcept = delete;
     ~SearchContext() override = default;
 
-    unsigned int approximateHits() const override;
+    HitEstimate calc_hit_estimate() const override;
     std::unique_ptr<queryeval::SearchIterator> createIterator(fef::TermFieldMatchData* matchData, bool strict) override;
     void fetchPostings(const queryeval::ExecuteInfo& execInfo) override;
     bool valid() const override { return false; }
diff --git a/searchlib/src/vespa/searchlib/attribute/singleboolattribute.cpp b/searchlib/src/vespa/searchlib/attribute/singleboolattribute.cpp
index c3e47548f85..4713f452adc 100644
--- a/searchlib/src/vespa/searchlib/attribute/singleboolattribute.cpp
+++ b/searchlib/src/vespa/searchlib/attribute/singleboolattribute.cpp
@@ -18,6 +18,7 @@
 namespace search {
 
 using attribute::Config;
+using attribute::HitEstimate;
 
 SingleBoolAttribute::
 SingleBoolAttribute(const vespalib::string &baseFileName, const GrowStrategy & grow, bool paged)
@@ -131,7 +132,7 @@ public:
     createFilterIterator(fef::TermFieldMatchData * matchData, bool strict) override;
     void fetchPostings(const queryeval::ExecuteInfo &execInfo) override;
     std::unique_ptr<queryeval::SearchIterator> createPostingIterator(fef::TermFieldMatchData *matchData, bool strict) override;
-    unsigned int approximateHits() const override;
+    HitEstimate calc_hit_estimate() const override;
     uint32_t get_committed_docid_limit() const noexcept override;
 };
 
@@ -169,13 +170,13 @@ BitVectorSearchContext::createPostingIterator(fef::TermFieldMatchData *matchData
     return createFilterIterator(matchData, strict);
 }
 
-unsigned int
-BitVectorSearchContext::approximateHits() const {
+HitEstimate
+BitVectorSearchContext::calc_hit_estimate() const {
     return valid()
-        ? (_invert)
-            ? (_bv.size() - _bv.countTrueBits())
-            : _bv.countTrueBits()
-        : 0;
+        ? (_invert
+            ? HitEstimate(_bv.size() - _bv.countTrueBits())
+            : HitEstimate(_bv.countTrueBits()))
+        : HitEstimate(0);
 }
 
 uint32_t
diff --git a/searchlib/src/vespa/searchlib/queryeval/leaf_blueprints.cpp b/searchlib/src/vespa/searchlib/queryeval/leaf_blueprints.cpp
index 930f6ed3a21..6c619ee085b 100644
--- a/searchlib/src/vespa/searchlib/queryeval/leaf_blueprints.cpp
+++ b/searchlib/src/vespa/searchlib/queryeval/leaf_blueprints.cpp
@@ -107,7 +107,7 @@ struct FakeContext : attribute::ISearchContext {
         int32_t ignore_weight;
         return onFind(docid, elem, ignore_weight);
     }
-    unsigned int approximateHits() const override { return 0; }
+    attribute::HitEstimate calc_hit_estimate() const override { return attribute::HitEstimate(0); }
     std::unique_ptr<SearchIterator> createIterator(fef::TermFieldMatchData *, bool) override { abort(); }
     void fetchPostings(const ExecuteInfo &) override { }
     bool valid() const override { return true; }