TCP K5: Add QBit(T, N) support

[alex-clickhouse]

Stacked on tcp/epic-o-sessions (#578). Implements K5 QBit(T, N) (read + write) for the native/TCP client — the last real codec in the type epic. Not an alias and not a refusal: it has a layout of its own that the client has to transpose in both directions.

What QBit is

QBit(T, N) stores an N-element vector with its bit planes transposed, which is what lets a vector search read only the high-order planes and compute an approximate distance at reduced precision (L2DistanceTransposed, cosineDistanceTransposed). Before this, the type resolved to nothing and a SELECT returning one failed the whole query.

T is BFloat16, Float32 or Float64 only — the server rejects everything else. The three-argument stride form documented in the binary type encoding is rejected by 26.6 ("must have exactly two argument"), so only the two-argument form is built.

Wire layout

No state prefix. The body is bits(T) planes, most significant bit first; each plane holds one ceil(N/8)-byte bitmap per row, rows contiguous within a plane. So the body is plane-major and exactly bits(T) * num_rows * ceil(N/8) bytes — every row the same width.

Within a row's bitmap the bits run LSB-first, but the bytes run in the reverse of the element order: element i is at bit i % 8 of byte ceil(N/8) - 1 - i / 8. Equivalently the bitmap is the big-endian encoding of a ceil(N/8)-byte integer whose bit i is element i.

That byte order is the one thing here worth reviewing carefully — see below.

CLR surface

QBit(Float32, N) and QBit(BFloat16, N) surface as IColumn<float[]>, QBit(Float64, N) as IColumn<double[]>. BFloat16 is widened to float and narrowed by dropping the low 16 bits, consistent with BFloat16ColumnCodec (G2).

The decoded column keeps the blob transposed rather than de-transposing on read, so a column read from the server and inserted straight back is a plane copy with no transposition at all. That is the common shape for a vector workload — the distance is computed server-side and the client never looks at a vector — so the per-row vector view is a lazily materialized pooled cache, never eager. De-transposing one row costs bits × ceil(N/8) byte fetches, 4 KiB for a 1024-dimension Float32 embedding.

IQBitColumn is new public API: Dimension, BitWidth, BytesPerRow, and GetPlane(bit). It is indexed by bit significance (31 is the sign bit), not wire order, because the high planes are what a reduced-precision distance wants; the accessor hides the MSB-first storage. It is non-generic — planes are raw bits, so plane access does not depend on the element type.

Write paths

• Dense (a QBitColumn of the same geometry): one copy per plane. Not one copy for the whole range — the body is plane-major, so a row range is contiguous within a plane but the planes are strided by the source's row count.
• Ergonomic (IColumn<float[]>): transposed into a rented scratch. Plane-major output cannot be streamed a row at a time, since plane 0 needs every row before plane 1 begins, so the scratch is the size of the slice's wire bytes. The dense path avoids it entirely.

SIMD, write direction only

Vector256<uint>.ExtractMostSignificantBits() gathers the top bit of 8 lanes into a byte, which is one plane byte for 8 Float32 elements in the order the wire wants. So a plane costs one extract plus one shift instead of 8 test-and-sets, and walking planes most significant first is just shifting the vector left. BFloat16 needs no narrowing on this path — its 16 planes are the float's top 16. Float64 takes two extracts per byte, since a Vector256 holds only 4 lanes.

Guarded on Vector256.IsHardwareAccelerated, hoisted out of the row loop, with the scalar path as both the fallback and the handler for the sub-8 tail — the UuidColumnCodec (G9) shape. No #if: the project floors at net8.0.

Ad-hoc timing, 200 rows, Release, net9.0, AVX2 on vs off:

dimension	scalar	vector
1024	85.9 ms	7.3 ms	11.7×
768	55.1 ms	3.3 ms	16.5×

The read direction is deliberately left scalar. Movemask has no cheap inverse before AVX-512, the portable sequence is only ~1.5× on op count, and by the type's own usage model the read path only runs when a caller materializes a vector — which most never do. Filed as a follow-up behind a real benchmark project rather than guessed at here.

The byte order — and why the round-trip corpus could not catch it

The first commit had element i at byte i / 8, matching what the wire notes said. That is wrong for N > 8, and it is worth understanding how it survived: every earlier verification used N = 4 or N = 8, where a row is a single byte and the order is unobservable.

An insert round-trip cannot catch it either. It writes and reads with the same client mapping, so a self-consistent layout error round-trips perfectly while putting bytes on the wire the server reads as a different vector. All the QBit(Float32, 9) and QBit(Float32, 17) corpus cases passed against a real server with the bug present.

What catches it is asymmetry — one side done by the client, the other by the server:

• QBitIntegrationTests inserts through the client and has the server render the value with toString(), then inserts through the server and has the client decode it.
• A unit fixture holds real server bytes for a 16-element vector, the narrowest that spans two bytes per row.

Reverting the fix fails exactly those three and none of the 65 symmetric cases.

The mapping was pinned with QBit(Float32, 72) (9 bytes per row) by making one element at a time negative and reading the sign plane: element 0 → byte 8 bit 0, element 8 → byte 7 bit 0, element 64 → byte 0 bit 0, element 71 → byte 0 bit 7. native-format.md is corrected to match.

Tests

• Corpus (InsertRoundTripCase, gated on TcpFeature.QBit): Float32 at dimensions 4, 9 and 17; Float64 at 3 and 17; BFloat16 at 4; and Nullable(QBit(Float32, 4)), which is the only thing that reads the codec's all-zero placeholder. Dimensions 17 cross two whole 8-element groups plus a tail, so they reach the vector path; anything narrower than 9 is handled entirely by the scalar tail. Signed zero, infinity and NaN pin the sign and exponent planes.
• Unit (QBitColumnCodecTests): server-captured bytes for dimensions 4 and 16, the significance ordering of GetPlane, plane bounds, the row-slice write, the pooled Values cache, zero rows, BFloat16 narrowing, CanWrite rejections, and the resolution error paths.
• Asymmetric integration (QBitIntegrationTests): the two directions above, plus GetPlane against the server's own value.

3187 tests pass against a local 26.6.

Notes for review

• No changelog fragment, per the convention for this epic stack.
• ClickHouse.Driver.Tcp has no PublicAPI/*.txt, so IQBitColumn is not analyzer-tracked yet; that lands with R1.
• The vector path's stores are scattered across the plane-major scratch and it manages only ~0.1 GB/s of plane output even so — dimension 1024 costs more per row than 768, which looks like a power-of-two stride conflict. Filed as a follow-up; blocking the rows should recover most of it.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds native/TCP QBit codec support with transposed plane access and SIMD-accelerated writes.

Changes:

• Implements QBit decoding, encoding, and dense reinsertion.
• Adds public IQBitColumn plane access.
• Adds unit, round-trip, and asymmetric integration coverage.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
QBitColumn.cs	Implements decoded QBit columns.
IQBitColumn.cs	Defines public plane-access API.
ColumnCodecRegistry.cs	Registers the QBit codec.
QBitColumnCodec.cs	Implements parsing and wire serialization.
InsertRoundTripCase.cs	Adds QBit corpus cases.
QBitColumnCodecTests.cs	Tests codec layout and errors.
QBitIntegrationTests.cs	Verifies client/server interoperability.

Suppressed comments (2)

ClickHouse.Driver.Tcp/Types/Codecs/QBitColumnCodec.cs:75

• Released ClickHouse v26.6.3.62 now accepts QBit(T, dimension, stride) and reports that three-argument type in block metadata. This check therefore makes a SELECT of a valid strided QBit column fail during codec resolution. Because supported server versions include newer 26.6 patches, the codec needs to parse the stride and implement the group-major strided wire layout rather than treating this form as permanently invalid.

        if (node.Arguments.Count != 2)
        {
            throw new FormatException(
                $"QBit type '{node}' must have exactly two arguments: the element type and the vector length.");

ClickHouse.Driver.Tcp/Types/Codecs/QBitColumnCodec.cs:94

• The current released ClickHouse v26.6.3.62 also permits QBit(Int8, N). Rejecting it here means any query returning that valid server type fails before reading the block. Add the 8-plane codec/column surface (normally IColumn<sbyte[]>) and corresponding native round-trip coverage.

        return element switch
        {
            "BFloat16" => new QBitFloatColumnCodec(typeName, dimension, bitWidth: 16),
            "Float32" => new QBitFloatColumnCodec(typeName, dimension, bitWidth: 32),
            "Float64" => new QBitDoubleColumnCodec(typeName, dimension),

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