FEAT: Streaming support in fetchone, fetchmany, fetchall for varbinarymax data type.

[gargsaumya]

Work Item / Issue Reference

AB#33395

GitHub Issue: #<ISSUE_NUMBER>

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Summary

This pull request significantly improves support for streaming and fetching large binary (VARBINARY(MAX)) and large text columns in the MSSQL Python driver. The main changes include robust chunked retrieval of large objects (LOBs), correct handling of edge cases (such as empty or null values), and enhanced test coverage for these scenarios.

LOB Streaming and Fetching Enhancements:

• Added a new helper function FetchLobColumnData to efficiently stream and assemble large binary/text columns (LOBs) from the database, handling chunking, null/empty values, and correct type conversion for both binary and (wide/narrow) string columns.
• Updated the logic in SQLGetData_wrap, FetchBatchData, FetchMany_wrap, and FetchAll_wrap to detect LOB columns and use the new streaming path for fetching them, including proper fallback to row-by-row fetching when LOBs are present. [1] [2] [3] [4] [5] [6]
• Modified the batch fetch function signatures and logic to propagate LOB column information and ensure correct handling during bulk fetches.

Testing Improvements:

• Replaced and expanded the test for large binary data with a new, comprehensive test (test_varbinarymax_insert_fetch) that verifies insertion and retrieval of empty, small, and large VARBINARY(MAX) values (including edge cases around the 8000-byte threshold) using fetchone, fetchall, and fetchmany.

These changes ensure that the driver can reliably handle large binary and text columns in all fetch scenarios, improving correctness and robustness for users working with LOB data.

[bewithgaurav]

avoiding repeated comments from other PRs, just added some minor changes here
I believe this PR will merge at last since other changes will merge here

The base branch was changed.

[sumitmsft]

Minor fixes are required.

[Copilot]

Pull Request Overview

This PR adds streaming support for VARBINARY(MAX) data types in the fetchone, fetchmany, and fetchall operations, significantly improving the driver's ability to handle large binary data. The changes enable proper insertion and retrieval of binary data larger than 8000 bytes, which previously had limitations.

• Implemented chunked streaming retrieval for large binary columns (LOBs) using a new FetchLobColumnData helper function
• Enhanced parameter binding to support Data-At-Execution (DAE) for large binary parameters
• Updated fetch operations to detect LOB columns and use appropriate streaming vs buffered approaches

Reviewed Changes

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

File	Description
mssql_python/pybind/ddbc_bindings.cpp	Core implementation of LOB streaming support, DAE parameter binding, and enhanced fetch operations
mssql_python/cursor.py	Updated parameter mapping to use VARBINARY(MAX) with DAE for large binary data
tests/test_004_cursor.py	Replaced limited test with comprehensive VARBINARY(MAX) tests covering various data sizes and fetch methods

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[sumitmsft]

All looks good.