SERVER-60675 Remove 4-byte element count from BSONColumn binary

Instead, it is placed under 'control.count' for v2 compressed buckets.

10 files changed, 214 insertions, 166 deletions

diff --git a/src/mongo/bson/util/bsoncolumn.cpp b/src/mongo/bson/util/bsoncolumn.cpp
index c902c1e9487..3d11465dbbc 100644
--- a/src/mongo/bson/util/bsoncolumn.cpp
+++ b/src/mongo/bson/util/bsoncolumn.cpp
@@ -752,14 +752,14 @@ BSONColumn::BSONColumn(BSONElement bin) {
             "Invalid BSON type for column",
             bin.type() == BSONType::BinData && bin.binDataType() == BinDataType::Column);
     _binary = bin.binData(_size);
-    uassert(ErrorCodes::BadValue, "Invalid BSON Column encoding", _size > kElementCountBytes);
+    uassert(ErrorCodes::BadValue, "Invalid BSON Column encoding", _size > 0);
     _elementCount = ConstDataView(_binary).read<LittleEndian<uint32_t>>();
-    _maxDecodingStartPos._control = _binary + kElementCountBytes;
+    _maxDecodingStartPos._control = _binary;
     _name = bin.fieldNameStringData().toString();
 }
 
 BSONColumn::Iterator BSONColumn::begin() {
-    Iterator it{*this, _binary + kElementCountBytes, _binary + _size};
+    Iterator it{*this, _binary, _binary + _size};
     it._initialize(0);
     return it;
 }
@@ -796,6 +796,22 @@ BSONElement BSONColumn::operator[](size_t index) {
     return *it;
 }
 
+size_t BSONColumn::size() {
+    if (_fullyDecompressed)
+        return _decompressed.size();
+
+    // We can begin iterating from last known literal
+    Iterator it{*this, _maxDecodingStartPos._control, _binary + _size};
+    it._initialize(_maxDecodingStartPos._index);
+
+    // Traverse until we reach end
+    for (auto e = end(); it != e; ++it) {
+    }
+
+    invariant(_fullyDecompressed);
+    return _decompressed.size();
+}
+
 void BSONColumn::DecodingStartPosition::setIfLarger(size_t index, const char* control) {
     if (_index < index) {
         _control = control;
diff --git a/src/mongo/bson/util/bsoncolumn.h b/src/mongo/bson/util/bsoncolumn.h
index 45316af7a6d..65dc012f2ac 100644
--- a/src/mongo/bson/util/bsoncolumn.h
+++ b/src/mongo/bson/util/bsoncolumn.h
@@ -237,11 +237,10 @@ public:
     /**
      * Number of elements stored in this BSONColumn
      *
-     * O(1) time complexity
+     * O(1) time complexity if BSONColumn is fully decompressed (iteration reached end).
+     * O(N) time complexity otherwise, will fully decompress BSONColumn.
      */
-    size_t size() const {
-        return _elementCount;
-    }
+    size_t size();
 
     /**
      * Field name that this BSONColumn represents.
diff --git a/src/mongo/bson/util/bsoncolumn_test.cpp b/src/mongo/bson/util/bsoncolumn_test.cpp
index 12772fb478b..5673cc97b3c 100644
--- a/src/mongo/bson/util/bsoncolumn_test.cpp
+++ b/src/mongo/bson/util/bsoncolumn_test.cpp
@@ -277,10 +277,6 @@ public:
                                              prev.Date().toMillisSinceEpoch());
     }
 
-    static void appendElementCount(BufBuilder& builder, uint32_t count) {
-        builder.appendNum(count);
-    }
-
     static void appendLiteral(BufBuilder& builder, BSONElement elem) {
         // BSON Type byte
         builder.appendChar(elem.type());
@@ -466,7 +462,6 @@ TEST_F(BSONColumnTest, BasicValue) {
     cb.append(elem);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, elem);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock64(expected, 0);
@@ -485,7 +480,6 @@ TEST_F(BSONColumnTest, BasicSkip) {
     cb.skip();
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, elem);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock64(expected, boost::none);
@@ -502,7 +496,6 @@ TEST_F(BSONColumnTest, OnlySkip) {
     cb.skip();
 
     BufBuilder expected;
-    appendElementCount(expected, 1);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock64(expected, boost::none);
     appendEOO(expected);
@@ -520,7 +513,6 @@ TEST_F(BSONColumnTest, ValueAfterSkip) {
     cb.append(elem);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock64(expected, boost::none);
     appendLiteral(expected, elem);
@@ -542,7 +534,6 @@ TEST_F(BSONColumnTest, LargeDeltaIsLiteral) {
     cb.append(second);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, first);
     appendLiteral(expected, second);
     appendEOO(expected);
@@ -564,7 +555,6 @@ TEST_F(BSONColumnTest, LargeDeltaIsLiteralAfterSimple8b) {
     cb.append(large);
 
     BufBuilder expected;
-    appendElementCount(expected, 4);
     appendLiteral(expected, zero);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock64(expected, deltaInt64(zero, zero));
@@ -594,7 +584,6 @@ TEST_F(BSONColumnTest, OverBlockCount) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendLiteral(expected, elems.front());
     appendSimple8bControl(expected, 0b1000, 0b1111);
 
@@ -622,7 +611,6 @@ TEST_F(BSONColumnTest, TypeChangeAfterLiteral) {
     cb.append(elemInt64);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, elemInt32);
     appendLiteral(expected, elemInt64);
     appendEOO(expected);
@@ -643,7 +631,6 @@ TEST_F(BSONColumnTest, TypeChangeAfterSimple8b) {
     cb.append(elemInt64);
 
     BufBuilder expected;
-    appendElementCount(expected, 3);
     appendLiteral(expected, elemInt32);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock64(expected, 0);
@@ -666,7 +653,6 @@ TEST_F(BSONColumnTest, Simple8bAfterTypeChange) {
     cb.append(elemInt64);
 
     BufBuilder expected;
-    appendElementCount(expected, 3);
     appendLiteral(expected, elemInt32);
     appendLiteral(expected, elemInt64);
     appendSimple8bControl(expected, 0b1000, 0b0000);
@@ -687,7 +673,6 @@ TEST_F(BSONColumnTest, BasicDouble) {
     cb.append(d2);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, d1);
     appendSimple8bControl(expected, 0b1001, 0b0000);
     appendSimple8bBlock64(expected, deltaDouble(d2, d1, 1));
@@ -711,7 +696,6 @@ TEST_F(BSONColumnTest, DoubleSameScale) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendLiteral(expected, elems.front());
     appendSimple8bControl(expected, 0b1001, 0b0000);
     appendSimple8bBlocks64(
@@ -732,7 +716,6 @@ TEST_F(BSONColumnTest, DoubleIncreaseScaleFromLiteral) {
     cb.append(d2);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, d1);
     appendSimple8bControl(expected, 0b1010, 0b0000);
     appendSimple8bBlock64(expected, deltaDouble(d2, d1, 10));
@@ -753,7 +736,6 @@ TEST_F(BSONColumnTest, DoubleLiteralAndScaleAfterSkip) {
     cb.append(d2);
 
     BufBuilder expected;
-    appendElementCount(expected, 3);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock64(expected, boost::none);
     appendLiteral(expected, d1);
@@ -777,7 +759,6 @@ TEST_F(BSONColumnTest, DoubleIncreaseScaleFromLiteralAfterSkip) {
     cb.append(d2);
 
     BufBuilder expected;
-    appendElementCount(expected, 4);
     appendLiteral(expected, d1);
     appendSimple8bControl(expected, 0b1010, 0b0000);
 
@@ -805,7 +786,6 @@ TEST_F(BSONColumnTest, DoubleIncreaseScaleFromDeltaWithRescale) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendLiteral(expected, elems.front());
     appendSimple8bControl(expected, 0b1010, 0b0000);
     appendSimple8bBlocks64(
@@ -832,7 +812,6 @@ TEST_F(BSONColumnTest, DoubleIncreaseScaleFromDeltaNoRescale) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendLiteral(expected, elems.front());
 
     auto deltaBegin = elems.begin() + 1;
@@ -864,7 +843,6 @@ TEST_F(BSONColumnTest, DoubleDecreaseScaleAfterBlock) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendLiteral(expected, elems.front());
 
     auto deltaBegin = elems.begin() + 1;
@@ -901,7 +879,6 @@ TEST_F(BSONColumnTest, DoubleDecreaseScaleAfterBlockUsingSkip) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendLiteral(expected, elems.front());
 
     auto deltaBegin = elems.begin() + 1;
@@ -934,7 +911,6 @@ TEST_F(BSONColumnTest, DoubleDecreaseScaleAfterBlockThenScaleBackUp) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendLiteral(expected, elems.front());
     appendSimple8bControl(expected, 0b1101, 0b0001);
     appendSimple8bBlocks64(
@@ -965,7 +941,6 @@ TEST_F(BSONColumnTest, DoubleDecreaseScaleAfterBlockUsingSkipThenScaleBackUp) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendLiteral(expected, elems.front());
     appendSimple8bControl(expected, 0b1101, 0b0001);
     appendSimple8bBlocks64(
@@ -990,7 +965,6 @@ TEST_F(BSONColumnTest, DoubleUnscalable) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendLiteral(expected, elems.front());
 
     std::vector<boost::optional<uint64_t>> expectedVals;
@@ -1015,7 +989,6 @@ TEST_F(BSONColumnTest, DoubleSignalingNaN) {
     cb.append(nan);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, elem);
 
     if (auto delta = deltaDoubleMemory(nan, elem); simple8bPossible(delta)) {
@@ -1042,7 +1015,6 @@ TEST_F(BSONColumnTest, DoubleQuietNaN) {
     cb.append(nan);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, elem);
     if (auto delta = deltaDoubleMemory(nan, elem); simple8bPossible(delta)) {
         appendSimple8bControl(expected, 0b1000, 0b0000);
@@ -1067,7 +1039,6 @@ TEST_F(BSONColumnTest, DoubleInfinity) {
     cb.append(inf);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, elem);
     if (auto delta = deltaDoubleMemory(inf, elem); simple8bPossible(delta)) {
         appendSimple8bControl(expected, 0b1000, 0b0000);
@@ -1092,7 +1063,6 @@ TEST_F(BSONColumnTest, DoubleDenorm) {
     cb.append(denorm);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, elem);
     if (auto delta = deltaDoubleMemory(denorm, elem); simple8bPossible(delta)) {
         appendSimple8bControl(expected, 0b1000, 0b0000);
@@ -1121,7 +1091,6 @@ TEST_F(BSONColumnTest, DoubleIntegerOverflow) {
     cb.append(e2);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, e1);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock64(expected, deltaDoubleMemory(e2, e1));
@@ -1140,7 +1109,6 @@ TEST_F(BSONColumnTest, Decimal128Base) {
     cb.append(elemDec128);
 
     BufBuilder expected;
-    appendElementCount(expected, 1);
     appendLiteral(expected, elemDec128);
     appendEOO(expected);
 
@@ -1158,7 +1126,6 @@ TEST_F(BSONColumnTest, Decimal128Delta) {
     cb.append(elemDec128);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, elemDec128);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock128(expected, deltaDecimal128(elemDec128, elemDec128));
@@ -1178,7 +1145,6 @@ TEST_F(BSONColumnTest, DecimalNonZeroDelta) {
     cb.append(elemDec128Max);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, elemDec128Zero);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock128(expected, deltaDecimal128(elemDec128Max, elemDec128Zero));
@@ -1198,7 +1164,6 @@ TEST_F(BSONColumnTest, DecimalMaxMin) {
     cb.append(elemDec128Max);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, elemDec128Zero);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock128(expected, deltaDecimal128(elemDec128Max, elemDec128Zero));
@@ -1221,7 +1186,6 @@ TEST_F(BSONColumnTest, DecimalMultiElement) {
     cb.append(elemDec128One);
 
     BufBuilder expected;
-    appendElementCount(expected, 5);
     appendLiteral(expected, elemDec128Zero);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     std::vector<boost::optional<uint128_t>> valuesToAppend = {
@@ -1252,7 +1216,6 @@ TEST_F(BSONColumnTest, DecimalMultiElementSkips) {
     cb.append(elemDec128One);
 
     BufBuilder expected;
-    appendElementCount(expected, 7);
     appendLiteral(expected, elemDec128Zero);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     std::vector<boost::optional<uint128_t>> valuesToAppend = {
@@ -1291,7 +1254,6 @@ TEST_F(BSONColumnTest, BasicObjectId) {
     cb.append(third);
 
     BufBuilder expected;
-    appendElementCount(expected, 4);
     appendLiteral(expected, first);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     std::vector<boost::optional<uint64_t>> expectedDeltas{
@@ -1314,7 +1276,6 @@ TEST_F(BSONColumnTest, ObjectIdDifferentProcessUnique) {
     cb.append(second);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, first);
     appendLiteral(expected, second);
     appendEOO(expected);
@@ -1339,7 +1300,6 @@ TEST_F(BSONColumnTest, ObjectIdAfterChangeBack) {
     cb.append(second);
 
     BufBuilder expected;
-    appendElementCount(expected, 5);
     appendLiteral(expected, first);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock64(expected, deltaObjectId(second, first));
@@ -1368,7 +1328,6 @@ TEST_F(BSONColumnTest, Simple8bTimestamp) {
     cb.append(second);
 
     BufBuilder expected;
-    appendElementCount(expected, 3);
     appendLiteral(expected, first);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     std::vector<boost::optional<uint64_t>> expectedDeltaOfDeltas{
@@ -1393,7 +1352,6 @@ TEST_F(BSONColumnTest, Simple8bTimestampNegativeDeltaOfDelta) {
     cb.append(third);
 
     BufBuilder expected;
-    appendElementCount(expected, 3);
     appendLiteral(expected, first);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     std::vector<boost::optional<uint64_t>> expectedDeltaOfDeltas{
@@ -1420,7 +1378,6 @@ TEST_F(BSONColumnTest, Simple8bTimestampAfterChangeBack) {
     cb.append(second);
 
     BufBuilder expected;
-    appendElementCount(expected, 5);
     appendLiteral(expected, first);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock64(expected, deltaOfDeltaTimestamp(second, first));
@@ -1448,7 +1405,6 @@ TEST_F(BSONColumnTest, LargeDeltaOfDeltaTimestamp) {
     cb.append(second);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, first);
     appendLiteral(expected, second);
     appendEOO(expected);
@@ -1474,7 +1430,6 @@ TEST_F(BSONColumnTest, LargeDeltaOfDeltaIsLiteralAfterSimple8bTimestamp) {
     cb.append(semiLarge);
 
     BufBuilder expected;
-    appendElementCount(expected, 5);
     appendLiteral(expected, zero);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock64(expected, deltaOfDeltaTimestamp(zero, zero));
@@ -1500,7 +1455,6 @@ TEST_F(BSONColumnTest, DateBasic) {
     cb.append(second);
 
     BufBuilder expected;
-    appendElementCount(expected, 3);
     appendLiteral(expected, first);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     std::vector<boost::optional<uint64_t>> expectedDeltaOfDeltas{deltaDate(second, first),
@@ -1524,7 +1478,6 @@ TEST_F(BSONColumnTest, DateAfterChangeBack) {
     cb.append(date);
 
     BufBuilder expected;
-    appendElementCount(expected, 3);
     appendLiteral(expected, elemInt32);
     appendLiteral(expected, date);
     appendSimple8bControl(expected, 0b1000, 0b0000);
@@ -1547,7 +1500,6 @@ TEST_F(BSONColumnTest, DateLargeDelta) {
     cb.append(second);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, first);
     appendLiteral(expected, second);
     appendEOO(expected);
@@ -1568,7 +1520,6 @@ TEST_F(BSONColumnTest, BoolBasic) {
     cb.append(trueBson);
 
     BufBuilder expected;
-    appendElementCount(expected, 4);
     appendLiteral(expected, trueBson);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     std::vector<boost::optional<uint64_t>> expectedDeltaOfDeltas{deltaBool(trueBson, trueBson),
@@ -1593,7 +1544,6 @@ TEST_F(BSONColumnTest, BoolAfterChangeBack) {
     cb.append(trueBson);
 
     BufBuilder expected;
-    appendElementCount(expected, 3);
     appendLiteral(expected, elemInt32);
     appendLiteral(expected, trueBson);
     appendSimple8bControl(expected, 0b1000, 0b0000);
@@ -1614,7 +1564,6 @@ TEST_F(BSONColumnTest, UndefinedBasic) {
     cb.append(first);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, first);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock64(expected, kDeltaForBinaryEqualValues);
@@ -1636,7 +1585,6 @@ TEST_F(BSONColumnTest, UndefinedAfterChangeBack) {
     cb.append(undefined);
 
     BufBuilder expected;
-    appendElementCount(expected, 3);
     appendLiteral(expected, elemInt32);
     appendLiteral(expected, undefined);
     appendSimple8bControl(expected, 0b1000, 0b0000);
@@ -1657,7 +1605,6 @@ TEST_F(BSONColumnTest, NullBasic) {
     cb.append(first);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, first);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock64(expected, kDeltaForBinaryEqualValues);
@@ -1679,7 +1626,6 @@ TEST_F(BSONColumnTest, NullAfterChangeBack) {
     cb.append(null);
 
     BufBuilder expected;
-    appendElementCount(expected, 3);
     appendLiteral(expected, elemInt32);
     appendLiteral(expected, null);
     appendSimple8bControl(expected, 0b1000, 0b0000);
@@ -1702,7 +1648,6 @@ TEST_F(BSONColumnTest, RegexBasic) {
     cb.append(second);
 
     BufBuilder expected;
-    appendElementCount(expected, 3);
     appendLiteral(expected, first);
     appendLiteral(expected, second);
     appendSimple8bControl(expected, 0b1000, 0b0000);
@@ -1725,7 +1670,6 @@ TEST_F(BSONColumnTest, RegexAfterChangeBack) {
     cb.append(regex);
 
     BufBuilder expected;
-    appendElementCount(expected, 3);
     appendLiteral(expected, elemInt32);
     appendLiteral(expected, regex);
     appendSimple8bControl(expected, 0b1000, 0b0000);
@@ -1749,7 +1693,6 @@ TEST_F(BSONColumnTest, DBRefBasic) {
     cb.append(second);
 
     BufBuilder expected;
-    appendElementCount(expected, 3);
     appendLiteral(expected, first);
     appendLiteral(expected, second);
     appendSimple8bControl(expected, 0b1000, 0b0000);
@@ -1773,7 +1716,6 @@ TEST_F(BSONColumnTest, DBRefAfterChangeBack) {
     cb.append(dbRef);
 
     BufBuilder expected;
-    appendElementCount(expected, 3);
     appendLiteral(expected, elemInt32);
     appendLiteral(expected, dbRef);
     appendSimple8bControl(expected, 0b1000, 0b0000);
@@ -1796,7 +1738,6 @@ TEST_F(BSONColumnTest, CodeWScopeBasic) {
     cb.append(second);
 
     BufBuilder expected;
-    appendElementCount(expected, 3);
     appendLiteral(expected, first);
     appendLiteral(expected, second);
     appendSimple8bControl(expected, 0b1000, 0b0000);
@@ -1819,7 +1760,6 @@ TEST_F(BSONColumnTest, CodeWScopeAfterChangeBack) {
     cb.append(codeWScope);
 
     BufBuilder expected;
-    appendElementCount(expected, 3);
     appendLiteral(expected, elemInt32);
     appendLiteral(expected, codeWScope);
     appendSimple8bControl(expected, 0b1000, 0b0000);
@@ -1842,7 +1782,6 @@ TEST_F(BSONColumnTest, SymbolBasic) {
     cb.append(second);
 
     BufBuilder expected;
-    appendElementCount(expected, 3);
     appendLiteral(expected, first);
     appendLiteral(expected, second);
     appendSimple8bControl(expected, 0b1000, 0b0000);
@@ -1865,7 +1804,6 @@ TEST_F(BSONColumnTest, SymbolAfterChangeBack) {
     cb.append(symbol);
 
     BufBuilder expected;
-    appendElementCount(expected, 3);
     appendLiteral(expected, elemInt32);
     appendLiteral(expected, symbol);
     appendSimple8bControl(expected, 0b1000, 0b0000);
@@ -1886,7 +1824,6 @@ TEST_F(BSONColumnTest, BinDataBase) {
     cb.append(elemBinData);
 
     BufBuilder expected;
-    appendElementCount(expected, 1);
     appendLiteral(expected, elemBinData);
     appendEOO(expected);
 
@@ -1903,7 +1840,6 @@ TEST_F(BSONColumnTest, BinDataOdd) {
     cb.append(elemBinData);
 
     BufBuilder expected;
-    appendElementCount(expected, 1);
     appendLiteral(expected, elemBinData);
     appendEOO(expected);
 
@@ -1921,7 +1857,6 @@ TEST_F(BSONColumnTest, BinDataDelta) {
     cb.append(elemBinData);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, elemBinData);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock128(expected, deltaBinData(elemBinData, elemBinData));
@@ -1944,7 +1879,6 @@ TEST_F(BSONColumnTest, BinDataDeltaShouldFail) {
     cb.append(elemBinDataLong);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, elemBinData);
     appendLiteral(expected, elemBinDataLong);
     appendEOO(expected);
@@ -1968,7 +1902,6 @@ TEST_F(BSONColumnTest, BinDataDeltaCheckSkips) {
     cb.append(elemBinData);
 
     BufBuilder expected;
-    appendElementCount(expected, 4);
     appendLiteral(expected, elemBinData);
     appendSimple8bControl(expected, 0b1000, 0b0001);
     std::vector<boost::optional<uint128_t>> expectedValues = {
@@ -1997,7 +1930,6 @@ TEST_F(BSONColumnTest, BinDataLargerThan16) {
     cb.append(elemBinDataLong);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, elemBinData);
     appendLiteral(expected, elemBinDataLong);
     appendEOO(expected);
@@ -2021,7 +1953,6 @@ TEST_F(BSONColumnTest, BinDataEqualTo16) {
     cb.append(elemBinDataLong);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, elemBinData);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock128(expected, deltaBinData(elemBinDataLong, elemBinData));
@@ -2042,7 +1973,6 @@ TEST_F(BSONColumnTest, BinDataLargerThan16SameValue) {
     cb.append(elemBinData);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, elemBinData);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock128(expected, deltaBinData(elemBinData, elemBinData));
@@ -2059,7 +1989,6 @@ TEST_F(BSONColumnTest, StringBase) {
     cb.append(elem);
 
     BufBuilder expected;
-    appendElementCount(expected, 1);
     appendLiteral(expected, elem);
     appendEOO(expected);
 
@@ -2075,7 +2004,6 @@ TEST_F(BSONColumnTest, StringDeltaSame) {
     cb.append(elemString);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, elemString);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock128(expected, deltaString(elemString, elemString));
@@ -2094,7 +2022,6 @@ TEST_F(BSONColumnTest, StringDeltaDiff) {
     cb.append(elemString2);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, elemString);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock128(expected, deltaString(elemString2, elemString));
@@ -2115,7 +2042,6 @@ TEST_F(BSONColumnTest, StringDeltaLarge) {
     cb.append(elemString2);
 
     BufBuilder expected;
-    appendElementCount(expected, 2);
     appendLiteral(expected, elemString);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock128(expected, deltaString(elemString2, elemString));
@@ -2138,7 +2064,6 @@ TEST_F(BSONColumnTest, StringDeltaAfterInvalid) {
     cb.append(elem2);
 
     BufBuilder expected;
-    appendElementCount(expected, 3);
     appendLiteral(expected, elem);
     appendLiteral(expected, elemInvalid);
     appendSimple8bControl(expected, 0b1000, 0b0000);
@@ -2175,7 +2100,6 @@ TEST_F(BSONColumnTest, StringMultiType) {
     cb.append(elemString2);
 
     BufBuilder expected;
-    appendElementCount(expected, 7);
     appendLiteral(expected, elemDec128Zero);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     std::vector<boost::optional<uint128_t>> valuesToAppend = {
@@ -2209,7 +2133,6 @@ TEST_F(BSONColumnTest, ObjectUncompressed) {
                                       createElementObj(BSON("x" << 1 << "y" << 3))};
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     for (auto elem : elems) {
         appendLiteral(expected, elem);
     }
@@ -2230,7 +2153,6 @@ TEST_F(BSONColumnTest, ObjectEqual) {
     std::vector<BSONElement> elems = {elemObj, elemObj};
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendLiteral(expected, elemObj);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock64(expected, kDeltaForBinaryEqualValues);
@@ -2253,7 +2175,6 @@ TEST_F(BSONColumnTest, ArrayUncompressed) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     for (auto elem : elems) {
         appendLiteral(expected, elem);
     }
@@ -2275,7 +2196,6 @@ TEST_F(BSONColumnTest, ArrayEqual) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendLiteral(expected, elemObj);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock64(expected, kDeltaForBinaryEqualValues);
@@ -2304,7 +2224,6 @@ TEST_F(BSONColumnTest, Interleaved) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendInterleavedStart(expected, elems.front().Obj());
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlocks64(expected,
@@ -2348,7 +2267,6 @@ TEST_F(BSONColumnTest, InterleavedAfterNonInterleaved) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendLiteral(expected, elems.front());
     appendInterleavedStart(expected, elems[1].Obj());
     appendSimple8bControl(expected, 0b1000, 0b0000);
@@ -2386,7 +2304,6 @@ TEST_F(BSONColumnTest, InterleavedLevels) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendInterleavedStart(expected, elems.front().Obj());
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlocks64(expected,
@@ -2427,7 +2344,6 @@ TEST_F(BSONColumnTest, InterleavedDoubleDifferentScale) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendInterleavedStart(expected, elems.front().Obj());
     appendSimple8bControl(expected, 0b1010, 0b0000);
     appendSimple8bBlocks64(expected,
@@ -2474,7 +2390,6 @@ TEST_F(BSONColumnTest, InterleavedMix64And128Bit) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendInterleavedStart(expected, elems.front().Obj());
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlocks64(expected,
@@ -2516,7 +2431,6 @@ TEST_F(BSONColumnTest, InterleavedWithEmptySubObj) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendInterleavedStart(expected, elems.front().Obj());
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlocks64(expected,
@@ -2548,7 +2462,6 @@ TEST_F(BSONColumnTest, InterleavedRemoveEmptySubObj) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendInterleavedStart(expected, elems.front().Obj());
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlocks64(
@@ -2584,7 +2497,6 @@ TEST_F(BSONColumnTest, InterleavedAddEmptySubObj) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendInterleavedStart(expected, elems.front().Obj());
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlocks64(expected, {kDeltaForBinaryEqualValues}, 1);
@@ -2618,7 +2530,6 @@ TEST_F(BSONColumnTest, InterleavedSchemaChange) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendInterleavedStart(expected, elems.front().Obj());
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlocks64(expected,
@@ -2659,7 +2570,6 @@ TEST_F(BSONColumnTest, InterleavedObjectSchemaChange) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendInterleavedStart(expected, elems.front().Obj());
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlocks64(
@@ -2701,7 +2611,6 @@ TEST_F(BSONColumnTest, InterleavedObjectNameChange) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendInterleavedStart(expected,
                            BSON("x" << 1 << "y" << BSON("z" << 2) << "y2" << BSON("z" << 3)));
     appendSimple8bControl(expected, 0b1000, 0b0000);
@@ -2738,7 +2647,6 @@ TEST_F(BSONColumnTest, InterleavedObjectEmptyObjChange) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendInterleavedStart(expected, elems.front().Obj());
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlocks64(
@@ -2781,7 +2689,6 @@ TEST_F(BSONColumnTest, ReenterInterleaved) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendInterleavedStart(expected, elems.front().Obj());
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlocks64(
@@ -2837,7 +2744,6 @@ TEST_F(BSONColumnTest, InterleavedAlternatingMergeRight) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendInterleavedStart(expected,
                            BSON("x" << elems[0].Obj().firstElement().Int() << "y"
                                     << elems[1].Obj().firstElement().Int() << "z"
@@ -2892,7 +2798,6 @@ TEST_F(BSONColumnTest, InterleavedAlternatingMergeLeftThenRight) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendInterleavedStart(expected,
                            BSON("y" << elems[1].Obj().firstElement().Int() << "x"
                                     << elems[2].Obj().firstElement().Int() << "z"
@@ -2930,7 +2835,6 @@ TEST_F(BSONColumnTest, InterleavedIncompatibleMerge) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendInterleavedStart(
         expected,
         BSON("x" << elems[0].Obj().firstElement().Int() << "y" << elems[1].Obj()["y"_sd].Int()));
@@ -2974,7 +2878,6 @@ TEST_F(BSONColumnTest, InterleavedIncompatibleAfterDeterminedReference) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendInterleavedStart(expected, elems.front().Obj());
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlocks64(expected,
@@ -3014,7 +2917,6 @@ TEST_F(BSONColumnTest, ObjectEmpty) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendLiteral(expected, elems.front());
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock64(expected, kDeltaForBinaryEqualValues);
@@ -3039,7 +2941,6 @@ TEST_F(BSONColumnTest, ObjectEmptyAfterNonEmpty) {
     }
 
     BufBuilder expected;
-    appendElementCount(expected, elems.size());
     appendInterleavedStart(expected, elems.front().Obj());
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock64(expected, kDeltaForBinaryEqualValues);
@@ -3056,7 +2957,6 @@ TEST_F(BSONColumnTest, InvalidControlByte) {
     auto elem = createElementInt32(0);
 
     BufBuilder expected;
-    appendElementCount(expected, 0);
     appendLiteral(expected, elem);
     appendSimple8bControl(expected, 0b0010, 0b0000);
     appendSimple8bBlock64(expected, deltaInt32(elem, elem));
@@ -3076,7 +2976,6 @@ TEST_F(BSONColumnTest, InvalidSize) {
     auto elem = createElementInt32(0);
 
     BufBuilder expected;
-    appendElementCount(expected, 0);
     appendLiteral(expected, elem);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     appendSimple8bBlock64(expected, deltaInt32(elem, elem));
@@ -3097,7 +2996,6 @@ TEST_F(BSONColumnTest, InvalidDoubleScale) {
     auto d2 = createElementDouble(1.12);
 
     BufBuilder expected;
-    appendElementCount(expected, 0);
     appendLiteral(expected, d1);
     appendSimple8bControl(expected, 0b1001, 0b0000);
     appendSimple8bBlock64(expected, deltaDouble(d2, d1, 100));
@@ -3117,7 +3015,6 @@ TEST_F(BSONColumnTest, MissingEOO) {
     auto elem = createElementInt32(0);
 
     BufBuilder expected;
-    appendElementCount(expected, 0);
     appendLiteral(expected, elem);
 
     try {
@@ -3150,7 +3047,6 @@ TEST_F(BSONColumnTest, InvalidSimple8b) {
     auto elem = createElementInt32(0);
 
     BufBuilder expected;
-    appendElementCount(expected, 0);
     appendLiteral(expected, elem);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     uint64_t invalidSimple8b = 0;
@@ -3170,7 +3066,6 @@ TEST_F(BSONColumnTest, NoLiteralStart) {
     auto elem = createElementInt32(0);
 
     BufBuilder expected;
-    appendElementCount(expected, 0);
     appendLiteral(expected, elem);
     appendSimple8bControl(expected, 0b1000, 0b0000);
     uint64_t invalidSimple8b = 0;
@@ -3187,7 +3082,6 @@ TEST_F(BSONColumnTest, AppendMinKey) {
     ASSERT_THROWS_CODE(cb.append(createElementMinKey()), DBException, ErrorCodes::InvalidBSONType);
 
     BufBuilder expected;
-    appendElementCount(expected, 0);
     appendEOO(expected);
 
     verifyBinary(cb.finalize(), expected);
@@ -3198,7 +3092,6 @@ TEST_F(BSONColumnTest, AppendMaxKey) {
     ASSERT_THROWS_CODE(cb.append(createElementMaxKey()), DBException, ErrorCodes::InvalidBSONType);
 
     BufBuilder expected;
-    appendElementCount(expected, 0);
     appendEOO(expected);
 
     verifyBinary(cb.finalize(), expected);
@@ -3217,7 +3110,6 @@ TEST_F(BSONColumnTest, AppendMinKeyInSubObj) {
         cb.append(createElementObj(obj.obj())), DBException, ErrorCodes::InvalidBSONType);
 
     BufBuilder expected;
-    appendElementCount(expected, 0);
     appendEOO(expected);
 
     verifyBinary(cb.finalize(), expected);
diff --git a/src/mongo/bson/util/bsoncolumn_util.h b/src/mongo/bson/util/bsoncolumn_util.h
index 9ec93bf98e1..3233abbb1d4 100644
--- a/src/mongo/bson/util/bsoncolumn_util.h
+++ b/src/mongo/bson/util/bsoncolumn_util.h
@@ -33,8 +33,6 @@
 #include "mongo/platform/int128.h"
 
 namespace mongo::bsoncolumn {
-// Number of bytes for element count at the beginning of BSON Column binary
-static constexpr uint8_t kElementCountBytes = 4;
 static constexpr char kInterleavedStartControlByte = (char)0xF0;
 
 inline bool isLiteralControlByte(char control) {
diff --git a/src/mongo/bson/util/bsoncolumnbuilder.cpp b/src/mongo/bson/util/bsoncolumnbuilder.cpp
index 58e1f2ef9e8..1cc3bacd152 100644
--- a/src/mongo/bson/util/bsoncolumnbuilder.cpp
+++ b/src/mongo/bson/util/bsoncolumnbuilder.cpp
@@ -257,11 +257,7 @@ BSONColumnBuilder::BSONColumnBuilder(StringData fieldName)
 
 BSONColumnBuilder::BSONColumnBuilder(StringData fieldName, BufBuilder&& builder)
     : _state(&_bufBuilder, nullptr), _bufBuilder(std::move(builder)), _fieldName(fieldName) {
-    // Leave space for element count at the beginning
-    static_assert(sizeof(_elementCount) == kElementCountBytes,
-                  "Element count for BSONColumn should be 4 bytes");
     _bufBuilder.reset();
-    _bufBuilder.skip(kElementCountBytes);
 }
 
 BSONColumnBuilder& BSONColumnBuilder::append(BSONElement elem) {
@@ -276,14 +272,12 @@ BSONColumnBuilder& BSONColumnBuilder::append(BSONElement elem) {
             _flushSubObjMode();
         }
         _state.append(elem);
-        ++_elementCount;
         return *this;
     }
 
 
     if (_mode == Mode::kRegular) {
         _startDetermineSubObjReference(elem.Obj());
-        ++_elementCount;
         return *this;
     }
 
@@ -310,7 +304,6 @@ BSONColumnBuilder& BSONColumnBuilder::append(BSONElement elem) {
                 _referenceSubObj = obj.getOwned();
                 _bufferedObjElements.push_back(_referenceSubObj);
                 _mode = Mode::kSubObjDeterminingReference;
-                ++_elementCount;
                 return *this;
             }
             _referenceSubObj = merged;
@@ -320,7 +313,6 @@ BSONColumnBuilder& BSONColumnBuilder::append(BSONElement elem) {
         // compression so use the currently built reference.
         if (numElements * 2 >= _bufferedObjElements.size()) {
             _bufferedObjElements.push_back(obj.getOwned());
-            ++_elementCount;
             return *this;
         }
 
@@ -329,13 +321,11 @@ BSONColumnBuilder& BSONColumnBuilder::append(BSONElement elem) {
 
     // Reference already determined for sub-object compression, try to add this new object.
     _appendSubElements(elem.Obj());
-    ++_elementCount;
     return *this;
 }
 
 
 BSONColumnBuilder& BSONColumnBuilder::skip() {
-    ++_elementCount;
     if (_mode == Mode::kRegular) {
         _state.skip();
     } else if (_mode == Mode::kSubObjDeterminingReference) {
@@ -359,9 +349,6 @@ BSONBinData BSONColumnBuilder::finalize() {
     // Write EOO at the end
     _bufBuilder.appendChar(EOO);
 
-    // Write element count at the beginning
-    DataView(_bufBuilder.buf()).write<LittleEndian<uint32_t>>(_elementCount);
-
     return {_bufBuilder.buf(), _bufBuilder.len(), BinDataType::Column};
 }
 
diff --git a/src/mongo/bson/util/bsoncolumnbuilder.h b/src/mongo/bson/util/bsoncolumnbuilder.h
index 83796ad5fe3..545cda8c492 100644
--- a/src/mongo/bson/util/bsoncolumnbuilder.h
+++ b/src/mongo/bson/util/bsoncolumnbuilder.h
@@ -73,13 +73,6 @@ public:
     }
 
     /**
-     * Returns the number of BSONElements added or skipped so far.
-     */
-    size_t size() const {
-        return _elementCount;
-    }
-
-    /**
      * Finalizes the BSON Column and returns the BinData binary.
      *
      * The BSONColumnBuilder must remain in scope for the pointer to be valid.
@@ -192,7 +185,6 @@ private:
     enum class Mode { kRegular, kSubObjDeterminingReference, kSubObjAppending };
     Mode _mode = Mode::kRegular;
 
-    uint32_t _elementCount = 0;
     std::string _fieldName;
 };
 
diff --git a/src/mongo/db/exec/bucket_unpacker.cpp b/src/mongo/db/exec/bucket_unpacker.cpp
index 59e522b96b2..ba7d5554630 100644
--- a/src/mongo/db/exec/bucket_unpacker.cpp
+++ b/src/mongo/db/exec/bucket_unpacker.cpp
@@ -205,7 +205,7 @@ void BucketUnpackerV1::extractSingleMeasurement(MutableDocument& measurement,
 // Unpacker for V2 compressed buckets
 class BucketUnpackerV2 : public BucketUnpacker::UnpackingImpl {
 public:
-    BucketUnpackerV2(const BSONElement& timeField);
+    BucketUnpackerV2(const BSONElement& timeField, int elementCount);
 
     void addField(const BSONElement& field) override;
     int measurementCount(const BSONElement& timeField) const override;
@@ -239,16 +239,24 @@ private:
     // Iterators used to unpack the columns of the above bucket that are populated during the reset
     // phase according to the provided 'Behavior' and 'BucketSpec'.
     std::vector<ColumnStore> _fieldColumns;
+
+    // Element count
+    int _elementCount;
 };
 
-BucketUnpackerV2::BucketUnpackerV2(const BSONElement& timeField) : _timeColumn(timeField) {}
+BucketUnpackerV2::BucketUnpackerV2(const BSONElement& timeField, int elementCount)
+    : _timeColumn(timeField), _elementCount(elementCount) {
+    if (_elementCount == -1) {
+        _elementCount = _timeColumn.column.size();
+    }
+}
 
 void BucketUnpackerV2::addField(const BSONElement& field) {
     _fieldColumns.emplace_back(field);
 }
 
 int BucketUnpackerV2::measurementCount(const BSONElement& timeField) const {
-    return _timeColumn.column.size();
+    return _elementCount;
 }
 
 bool BucketUnpackerV2::getNext(MutableDocument& measurement,
@@ -287,12 +295,17 @@ void BucketUnpackerV2::extractSingleMeasurement(MutableDocument& measurement,
                                                 const BSONElement& metaValue,
                                                 bool includeTimeField,
                                                 bool includeMetaField) {
+    if (includeTimeField) {
+        BSONElement val = _timeColumn.column[j];
+        uassert(6067500, "Bucket unexpectedly contained fewer values than count", !val.eoo());
+        measurement.addField(_timeColumn.column.name(), Value{val});
+    }
+
     if (includeMetaField && !metaValue.isNull()) {
         measurement.addField(*spec.metaField, Value{metaValue});
     }
 
     if (includeTimeField) {
-        measurement.addField(_timeColumn.column.name(), Value{_timeColumn.column[j]});
         for (auto& fieldColumn : _fieldColumns) {
             measurement.addField(fieldColumn.column.name(), Value{fieldColumn.column[j]});
         }
@@ -430,7 +443,12 @@ void BucketUnpacker::reset(BSONObj&& bucket) {
     if (version == 1) {
         _unpackingImpl = std::make_unique<BucketUnpackerV1>(timeFieldElem);
     } else if (version == 2) {
-        _unpackingImpl = std::make_unique<BucketUnpackerV2>(timeFieldElem);
+        auto countField = controlField.Obj()[timeseries::kBucketControlCountFieldName];
+        _unpackingImpl =
+            std::make_unique<BucketUnpackerV2>(timeFieldElem,
+                                               countField && isNumericBSONType(countField.type())
+                                                   ? static_cast<int>(countField.Number())
+                                                   : -1);
     } else {
         uasserted(5857900, "Invalid bucket version");
     }
diff --git a/src/mongo/db/exec/bucket_unpacker_test.cpp b/src/mongo/db/exec/bucket_unpacker_test.cpp
index 85e2bfb45d5..bd2bb8289b0 100644
--- a/src/mongo/db/exec/bucket_unpacker_test.cpp
+++ b/src/mongo/db/exec/bucket_unpacker_test.cpp
@@ -102,6 +102,32 @@ public:
         BSONObj obj = root.obj();
         return {obj, "time"_sd};
     }
+
+    // Modifies the 'control.count' field for a v2 compressed bucket. Zero delta removes the
+    // 'control.count' field, positive increases the count and negative delta decreases the count.
+    BSONObj modifyCompressedBucketElementCount(BSONObj compressedBucket, int delta) {
+        BSONObjBuilder root;
+        for (auto&& elem : compressedBucket) {
+            if (elem.fieldNameStringData() != "control"_sd) {
+                root.append(elem);
+                continue;
+            }
+
+            BSONObjBuilder controlBuilder(root.subobjStart("control"_sd));
+            for (auto&& controlElem : elem.Obj()) {
+                if (controlElem.fieldNameStringData() != "count"_sd) {
+                    controlBuilder.append(controlElem);
+                    continue;
+                }
+
+                if (delta != 0) {
+                    int count = controlElem.Number();
+                    controlBuilder.append("count"_sd, count + delta);
+                }
+            }
+        }
+        return root.obj();
+    }
 };
 
 TEST_F(BucketUnpackerTest, UnpackBasicIncludeAllMeasurementFields) {
@@ -726,5 +752,109 @@ TEST_F(BucketUnpackerTest, ComputeMeasurementCountInLargerIntervals) {
     testMeasurementCount(11111);
     testMeasurementCount(449998);
 }
+
+TEST_F(BucketUnpackerTest, TamperedCompressedCountLess) {
+    std::set<std::string> fields{
+        "_id", kUserDefinedMetaName.toString(), kUserDefinedTimeName.toString(), "a", "b"};
+
+    auto bucket = fromjson(
+        "{control: {'version': 1}, meta: {'m1': 999, 'm2': 9999}, data: {_id: {'0':1, '1':2}, "
+        "time: {'0':1, '1':2}, "
+        "a:{'0':1, '1':2}, b:{'1':1}}}");
+
+    auto compressedBucket = timeseries::compressBucket(bucket, "time"_sd);
+    // Reduce the count by one to be 1.
+    auto modifiedCompressedBucket = modifyCompressedBucketElementCount(*compressedBucket, -1);
+
+    auto unpacker = makeBucketUnpacker(std::move(fields),
+                                       BucketUnpacker::Behavior::kInclude,
+                                       std::move(modifiedCompressedBucket),
+                                       kUserDefinedMetaName.toString());
+
+    auto doc0 = Document{fromjson("{time: 1, myMeta: {m1: 999, m2: 9999}, _id: 1, a: 1}")};
+    auto doc1 = Document{fromjson("{time: 2, myMeta: {m1: 999, m2: 9999}, _id: 2, a :2, b: 1}")};
+
+    // 1 is reported when asking for numberOfMeasurements()
+    ASSERT_EQ(unpacker.numberOfMeasurements(), 1);
+    ASSERT_DOCUMENT_EQ(unpacker.extractSingleMeasurement(0), doc0);
+
+    // Iterating returns both documents
+    ASSERT_TRUE(unpacker.hasNext());
+    assertGetNext(unpacker, doc0);
+
+    ASSERT_TRUE(unpacker.hasNext());
+    assertGetNext(unpacker, doc1);
+    ASSERT_FALSE(unpacker.hasNext());
+}
+
+TEST_F(BucketUnpackerTest, TamperedCompressedCountMore) {
+    std::set<std::string> fields{
+        "_id", kUserDefinedMetaName.toString(), kUserDefinedTimeName.toString(), "a", "b"};
+
+    auto bucket = fromjson(
+        "{control: {'version': 1}, meta: {'m1': 999, 'm2': 9999}, data: {_id: {'0':1, '1':2}, "
+        "time: {'0':1, '1':2}, "
+        "a:{'0':1, '1':2}, b:{'1':1}}}");
+
+    auto compressedBucket = timeseries::compressBucket(bucket, "time"_sd);
+    // Increase the count by one to be 3.
+    auto modifiedCompressedBucket = modifyCompressedBucketElementCount(*compressedBucket, 1);
+
+    auto unpacker = makeBucketUnpacker(std::move(fields),
+                                       BucketUnpacker::Behavior::kInclude,
+                                       std::move(modifiedCompressedBucket),
+                                       kUserDefinedMetaName.toString());
+
+    auto doc0 = Document{fromjson("{time: 1, myMeta: {m1: 999, m2: 9999}, _id: 1, a: 1}")};
+    auto doc1 = Document{fromjson("{time: 2, myMeta: {m1: 999, m2: 9999}, _id: 2, a :2, b: 1}")};
+
+    ASSERT_EQ(unpacker.numberOfMeasurements(), 3);
+    ASSERT_DOCUMENT_EQ(unpacker.extractSingleMeasurement(0), doc0);
+    ASSERT_DOCUMENT_EQ(unpacker.extractSingleMeasurement(1), doc1);
+    ASSERT_THROWS_CODE(unpacker.extractSingleMeasurement(2), AssertionException, 6067500);
+
+    ASSERT_TRUE(unpacker.hasNext());
+    assertGetNext(unpacker, doc0);
+
+    ASSERT_TRUE(unpacker.hasNext());
+    assertGetNext(unpacker, doc1);
+    ASSERT_FALSE(unpacker.hasNext());
+}
+
+TEST_F(BucketUnpackerTest, TamperedCompressedCountMissing) {
+    std::set<std::string> fields{
+        "_id", kUserDefinedMetaName.toString(), kUserDefinedTimeName.toString(), "a", "b"};
+
+    auto bucket = fromjson(
+        "{control: {'version': 1}, meta: {'m1': 999, 'm2': 9999}, data: {_id: {'0':1, '1':2}, "
+        "time: {'0':1, '1':2}, "
+        "a:{'0':1, '1':2}, b:{'1':1}}}");
+
+    auto compressedBucket = timeseries::compressBucket(bucket, "time"_sd);
+    // Remove the count field
+    auto modifiedCompressedBucket = modifyCompressedBucketElementCount(*compressedBucket, 0);
+
+    auto unpacker = makeBucketUnpacker(std::move(fields),
+                                       BucketUnpacker::Behavior::kInclude,
+                                       std::move(modifiedCompressedBucket),
+                                       kUserDefinedMetaName.toString());
+
+    auto doc0 = Document{fromjson("{time: 1, myMeta: {m1: 999, m2: 9999}, _id: 1, a: 1}")};
+    auto doc1 = Document{fromjson("{time: 2, myMeta: {m1: 999, m2: 9999}, _id: 2, a :2, b: 1}")};
+
+    // Missing count field will make the unpacker measure the number of time fields for an accurate
+    // count
+    ASSERT_EQ(unpacker.numberOfMeasurements(), 2);
+    ASSERT_DOCUMENT_EQ(unpacker.extractSingleMeasurement(0), doc0);
+    ASSERT_DOCUMENT_EQ(unpacker.extractSingleMeasurement(1), doc1);
+
+    ASSERT_TRUE(unpacker.hasNext());
+    assertGetNext(unpacker, doc0);
+
+    ASSERT_TRUE(unpacker.hasNext());
+    assertGetNext(unpacker, doc1);
+    ASSERT_FALSE(unpacker.hasNext());
+}
+
 }  // namespace
 }  // namespace mongo
diff --git a/src/mongo/db/timeseries/bucket_compression.cpp b/src/mongo/db/timeseries/bucket_compression.cpp
index 06f65f5252e..669a24be224 100644
--- a/src/mongo/db/timeseries/bucket_compression.cpp
+++ b/src/mongo/db/timeseries/bucket_compression.cpp
@@ -54,36 +54,22 @@ boost::optional<BSONObj> compressBucket(const BSONObj& bucketDoc, StringData tim
     std::vector<std::pair<StringData, BSONObjIterator>>
         columns;  // Iterators to read data fields from uncompressed bucket
 
+    BSONElement controlElement;
+    std::vector<BSONElement> otherElements;
+
     // Read everything from the uncompressed bucket
     for (auto& elem : bucketDoc) {
-        // Control field is left as-is except for the version field.
+        // Record control element, we need to parse the uncompressed bucket before writing new
+        // control block.
         if (elem.fieldNameStringData() == kBucketControlFieldName) {
-            BSONObjBuilder control(builder.subobjStart(kBucketControlFieldName));
-
-            // Set right version, leave other control fields unchanged
-            bool versionSet = false;
-            for (const auto& controlField : elem.Obj()) {
-                if (controlField.fieldNameStringData() == kBucketControlVersionFieldName) {
-                    control.append(kBucketControlVersionFieldName,
-                                   kTimeseriesControlCompressedVersion);
-                    versionSet = true;
-                } else {
-                    control.append(controlField);
-                }
-            }
-
-            // Set version if it was missing from uncompressed bucket
-            if (!versionSet) {
-                control.append(kBucketControlVersionFieldName, kTimeseriesControlCompressedVersion);
-            }
-
+            controlElement = elem;
             continue;
         }
 
-        // Everything that's not under data or control is left as-is
+        // Everything that's not under data or control is left as-is, record elements so we can
+        // write later (we want control to be first).
         if (elem.fieldNameStringData() != kBucketDataFieldName) {
-            // Skip any updates to non-data fields.
-            builder.append(elem);
+            otherElements.push_back(elem);
             continue;
         }
 
@@ -159,6 +145,35 @@ boost::optional<BSONObj> compressBucket(const BSONObj& bucketDoc, StringData tim
                   return lhs.timeField.timestamp() < rhs.timeField.timestamp();
               });
 
+    // Write control block
+    {
+        BSONObjBuilder control(builder.subobjStart(kBucketControlFieldName));
+
+        // Set right version, leave other control fields unchanged
+        bool versionSet = false;
+        for (const auto& controlField : controlElement.Obj()) {
+            if (controlField.fieldNameStringData() == kBucketControlVersionFieldName) {
+                control.append(kBucketControlVersionFieldName, kTimeseriesControlCompressedVersion);
+                versionSet = true;
+            } else {
+                control.append(controlField);
+            }
+        }
+
+        // Set version if it was missing from uncompressed bucket
+        if (!versionSet) {
+            control.append(kBucketControlVersionFieldName, kTimeseriesControlCompressedVersion);
+        }
+
+        // Set count
+        control.append(kBucketControlCountFieldName, static_cast<int32_t>(measurements.size()));
+    }
+
+    // Write non control or data elements that are left as-is.
+    for (auto&& elem : otherElements) {
+        builder.append(elem);
+    }
+
     // Last, compress elements and build compressed bucket
     {
         BSONObjBuilder dataBuilder = builder.subobjStart(kBucketDataFieldName);
diff --git a/src/mongo/db/timeseries/timeseries_constants.h b/src/mongo/db/timeseries/timeseries_constants.h
index 43c63b15a46..24e69bc1837 100644
--- a/src/mongo/db/timeseries/timeseries_constants.h
+++ b/src/mongo/db/timeseries/timeseries_constants.h
@@ -42,6 +42,7 @@ static constexpr StringData kBucketDataFieldName = "data"_sd;
 static constexpr StringData kBucketMetaFieldName = "meta"_sd;
 static constexpr StringData kBucketControlFieldName = "control"_sd;
 static constexpr StringData kBucketControlVersionFieldName = "version"_sd;
+static constexpr StringData kBucketControlCountFieldName = "count"_sd;
 static constexpr StringData kBucketControlMinFieldName = "min"_sd;
 static constexpr StringData kBucketControlMaxFieldName = "max"_sd;
 static constexpr StringData kControlMaxFieldNamePrefix = "control.max."_sd;