POC: reduce per-row cost in the TDS row decode path

[Srinivasan S A (NivasSA)]

Proof of concept, not for merge. Shows five changes and what each is worth.

Measured on 1,499,000 rows of 48 columns. Wall clock 7.45s -> 3.92s. CPU on the reading process 7.35s -> 2.58s. CPU inside mssql-tds 4.69s -> 0.97s. The server stopped blocking on ASYNC_NETWORK_IO, from 4.54s to none.

The five changes:

1. Resolve each column's type, precision, scale and collation once per result set into a DecodeOp plan, instead of rederiving it from ColumnMetadata on every row.

2. Add a synchronous row path that decodes a whole ROW or NBCROW token from one borrow of the reader's buffer. It runs two passes, the first to measure and bounds check and the second to decode without checks. It declines when the row crosses a packet boundary, and the async path handles that case.

3. Remove the two tracing callsites that fired once per row. This was done to measure their cost, not as a proposal.

4. Offer decoded strings and binaries to the RowWriter as borrows, so a consumer that does not need to own them avoids a copy.

5. Let the RowWriter supply the destination for a PLP value whose length is declared up front, so large values are read into consumer storage directly.

Known gaps are listed in POC_README.md. The largest is that NOT NULL columns arrive as fixed length TDS types, which are not in the decode plan, so a table of NOT NULL columns gets none of this.

Description

Related Issues

Checklist

• cargo bfmt passes
• cargo bclippy passes
• cargo btest passes
• New/changed functionality has tests
• Public API changes are documented

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[Copilot]

Pull request overview

Proof of concept for reducing per-row TDS decoding overhead through precomputed decode plans, buffered row decoding, borrowed values, and direct PLP destinations.

Changes:

• Adds a two-pass buffered ROW/NBCROW decoder.
• Extends RowWriter with borrowed-value and destination-buffer APIs.
• Adds non-blocking packet reads and an unboxed row API.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
POC_README.md	Documents goals, benchmarks, and limitations.
mssql-tds/src/token/tokens.rs	Adds cached per-column decode plans.
mssql-tds/src/token/parsers/row_parser.rs	Initializes decode-plan state in tests.
mssql-tds/src/token/parsers/nbcrow_parser.rs	Initializes decode-plan state in tests.
mssql-tds/src/token/parsers/colmetadata_parser.rs	Initializes decode plans during metadata parsing.
mssql-tds/src/io/token_stream.rs	Implements buffered two-pass row decoding.
mssql-tds/src/io/packet_reader.rs	Adds synchronous buffered-read APIs.
mssql-tds/src/datatypes/sql_string.rs	Supports borrowed UTF-8 decoding.
mssql-tds/src/datatypes/row_writer.rs	Adds borrowed writes and PLP destinations.
mssql-tds/src/datatypes/decoder.rs	Integrates buffered reads and direct PLP storage.
mssql-tds/src/connection/transport/network_transport.rs	Implements buffered reads for network transport.
mssql-tds/src/connection/tds_client.rs	Removes row tracing and adds an unboxed API.
mssql-tds/src/connection/metadata_retriever.rs	Initializes decode-plan state in tests.

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