Avoid per-value stack expansion when decoding RowBinary Tuple

[ruslandoga]

Problem

Tuple decoding currently expands the tuple schema onto the decode stack for every tuple value:

{:tuple, tuple_types} ->
  decode_rows(tuple_types ++ [{:tuple_over, row} | types_rest], bin, [], rows, types)

That allocates proportional to tuple width per decoded row. This is the same general shape as the array/map stack expansion work, except tuple width is fixed and known from the type.

Benchmark

Environment:

commit: 5c9244a
macOS, Apple M2, 8 GB RAM
Elixir 1.19.5, Erlang/OTP 28.3, JIT enabled

Benchmark code:

alias Ch.RowBinary

base = DateTime.from_naive!(~N[2026-01-01 00:00:00.000], "Etc/UTC")

flat_rows =
  Enum.map(1..100_000, fn i ->
    [i, "event", DateTime.add(base, i, :second)]
  end)

tuple_rows =
  Enum.map(flat_rows, fn row -> [List.to_tuple(row)] end)

flat_bin =
  flat_rows
  |> RowBinary.encode_rows(["UInt64", "String", "DateTime"])
  |> IO.iodata_to_binary()

tuple_bin =
  tuple_rows
  |> RowBinary.encode_rows(["Tuple(UInt64, String, DateTime)"])
  |> IO.iodata_to_binary()

Benchee.run(
  %{
    "decode flat UInt64/String/DateTime" => fn ->
      RowBinary.decode_rows(flat_bin, ["UInt64", "String", "DateTime"])
    end,
    "decode Tuple(UInt64,String,DateTime)" => fn ->
      RowBinary.decode_rows(tuple_bin, ["Tuple(UInt64, String, DateTime)"])
    end
  },
  warmup: 1,
  time: 2
)

Results:

Name                                           ips        average
decode flat UInt64/String/DateTime            6.99      143.10 ms
decode Tuple(UInt64,String,DateTime)          4.24      235.69 ms

Tuple decoding: 1.65x slower (+92.59 ms for 100k rows)

The comparison is not claiming tuples should be as cheap as flat rows, but it gives a baseline for the per-value tuple stack allocation.

Suggested direction

Avoid rebuilding tuple_types ++ marker for each tuple value. Possible approaches:

• represent tuple decoding as a small state frame with current tuple types and accumulated tuple row;
• precompute a reusable tuple decode frame during decoding_type/1;
• align this with the array/map non-stack improvement so nested containers use one consistent mechanism.

Tests to add:

• tuple decode round trips for scalar and nested types;
• incomplete tuple payload still returns the same continuation/error behavior;
• benchmark coverage for tuple-heavy rows.