FIX: [PERF] Handle utf16 strings using simdutf and std::u16string by ffelixg · Pull Request #526 · microsoft/mssql-python

ffelixg · 2026-04-15T23:50:16Z

Work Item / Issue Reference

GitHub Issue: #514

Summary

Use external dependency simdutf to decode utf16 when decoding in c++
Use pybind11 support for std::u16string to decode utf16 when interacting with python
Remove SQLWCHARToWString, WideToUTF8 and Utf8ToWString, refactor call sites

ffelixg · 2026-04-15T23:56:35Z

I have been evaluating the external library simdutf as a high performance replacement for utf16 -> utf8 conversions, i.e. the functions SQLWCHARToWString, WideToUTF8 and Utf8ToWString. Rather than only using it for the arrow fetch path, I have been trying to make the switch for every location where one of these three functions is used, as the applications follow similar patterns. I didn't use simdutf in every case, another way to eliminate these function calls was to use std::u16string instead of std::wstring when passing strings to/from python. I think this avoids the whole issue where wchars are defined as 32 bit on some OSes but SQLWCHARs are always 16 bit.

This brings arrow performance on linux for nvarchars in line with what it should be.

If the std::u16string type works as well as I hope it does (will have to see what CI says about mac), there are some more spots where it could be used, for example to replace WStringToSQLWCHAR

Copilot

Pull request overview

This PR introduces a higher-performance and more platform-consistent UTF-16LE → UTF-8 conversion path in the pybind ODBC layer by adopting simdutf and shifting several wide-string interfaces from std::wstring to std::u16string (to avoid wchar_t width differences across OSes).

Changes:

Add simdutf (via find_package or FetchContent) and use it for UTF-16LE → UTF-8 conversions in diagnostics and data fetch paths.
Replace a number of std::wstring usages with std::u16string for connection strings, queries, and SQL_C_WCHAR parameter buffers.
Remove legacy SQLWCHAR→wstring conversion utilities that are no longer used.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
`mssql_python/pybind/CMakeLists.txt`	Adds `simdutf` dependency resolution and links it into the `ddbc_bindings` module.
`mssql_python/pybind/ddbc_bindings.h`	Introduces UTF-16 helpers (`dupeSqlWCharAsUtf16Le`, `utf16LeToUtf8Alloc`) and changes `ErrorInfo` to store UTF-8 `std::string`.
`mssql_python/pybind/ddbc_bindings.cpp`	Switches parameter binding, diagnostics, query execution, and fetch conversions to UTF-16 + `simdutf`.
`mssql_python/pybind/connection/connection.h`	Updates connection APIs/state to store connection strings as `std::u16string`.
`mssql_python/pybind/connection/connection.cpp`	Uses UTF-16 connection string/query handling and returns UTF-8 error messages directly.
`mssql_python/pybind/connection/connection_pool.h`	Updates pooling key types and APIs to use `std::u16string`.
`mssql_python/pybind/connection/connection_pool.cpp`	Implements pooling with `std::u16string` connection string keys.
`mssql_python/pybind/unix_utils.h`	Removes the `SQLWCHARToWString` declaration.
`mssql_python/pybind/unix_utils.cpp`	Removes the `SQLWCHARToWString` implementation.

Comments suppressed due to low confidence (1)

mssql_python/pybind/ddbc_bindings.cpp:583

In the SQL_C_WCHAR non-DAE path, the bound buffer is a std::u16string but the bind call uses data() + SQL_NTS and sets bufferLength to size() * sizeof(SQLWCHAR). For SQL_NTS, BufferLength should include the null terminator, and the pointer should be a null-terminated buffer (prefer c_str()). As written, drivers that validate BufferLength may treat this as truncated or read past the provided length.

Suggested fix: use sqlwcharBuffer->c_str() and set bufferLength to (sqlwcharBuffer->size() + 1) * sizeof(SQLWCHAR), or alternatively set *strLenOrIndPtr to the explicit byte length (excluding terminator) and keep BufferLength consistent.

                    std::u16string* sqlwcharBuffer = AllocateParamBuffer<std::u16string>(
                        paramBuffers, param.cast<std::u16string>());
                    LOG("BindParameters: param[%d] SQL_C_WCHAR - String "
                        "length=%zu characters, buffer=%zu bytes",
                        paramIndex, sqlwcharBuffer->size(), sqlwcharBuffer->size() * sizeof(SQLWCHAR));
                    dataPtr = sqlwcharBuffer->data();
                    bufferLength = sqlwcharBuffer->size() * sizeof(SQLWCHAR);
                    strLenOrIndPtr = AllocateParamBuffer<SQLLEN>(paramBuffers);
                    *strLenOrIndPtr = SQL_NTS;

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

gargsaumya · 2026-04-16T02:19:28Z

/azp run

azure-pipelines · 2026-04-16T02:19:38Z

Azure Pipelines successfully started running 1 pipeline(s).

github-actions · 2026-04-16T02:25:58Z

📊 Code Coverage Report

🔥 Diff Coverage 94%	🎯 Overall Coverage 25%	📈 Total Lines Covered: `6902` out of `26791` 📁 Project: `mssql-python`

Diff Coverage

Diff: main...HEAD, staged and unstaged changes

mssql_python/pybind/connection/connection.cpp (100%)
mssql_python/pybind/ddbc_bindings.cpp (94.3%): Missing lines 1491,1566,1583,3628,3637,3965,4067
mssql_python/pybind/ddbc_bindings.h (95.5%): Missing lines 47

Summary

Total: 150 lines
Missing: 8 lines
Coverage: 94%

mssql_python/pybind/ddbc_bindings.cpp

Lines 1487-1495

  1487     LOG("SQLCheckError: Checking ODBC errors - handleType=%d, retcode=%d", handleType, retcode);
  1488     ErrorInfo errorInfo;
  1489     if (retcode == SQL_INVALID_HANDLE) {
  1490         LOG("SQLCheckError: SQL_INVALID_HANDLE detected - handle is invalid");
! 1491         errorInfo.ddbcErrorMsg = "Invalid handle!";
  1492         return errorInfo;
  1493     }
  1494     assert(handle != 0);
  1495     SQLHANDLE rawHandle = handle->get();

Lines 1562-1570

  1562     return records;
  1563 }
  1564 
  1565 // Wrap SQLExecDirect
! 1566 SQLRETURN SQLExecDirect_wrap(SqlHandlePtr StatementHandle, const std::u16string& Query) {
  1567     LOG("SQLExecDirect: Executing query directly - statement_handle=%p, "
  1568         "query_length=%zu chars",
  1569         (void*)StatementHandle->get(), Query.length());
  1570     if (!SQLExecDirect_ptr) {

Lines 1579-1587

  1579         SQLSetStmtAttr_ptr(StatementHandle->get(), SQL_ATTR_CONCURRENCY,
  1580                            (SQLPOINTER)SQL_CONCUR_READ_ONLY, 0);
  1581     }
  1582 
! 1583     SQLWCHAR* queryPtr = reinterpretU16stringAsSqlWChar(Query);
  1584     SQLRETURN ret = SQLExecDirect_ptr(StatementHandle->get(), queryPtr, SQL_NTS);
  1585     if (!SQL_SUCCEEDED(ret)) {
  1586         LOG("SQLExecDirect: Query execution failed - SQLRETURN=%d", ret);
  1587     }

Lines 3624-3632

  3624                                      sizeof(DateTimeOffset) * fetchSize,
  3625                                      buffers.indicators[col - 1].data());
  3626                 break;
  3627             default:
! 3628                 std::string columnName = columnMeta["ColumnName"].cast<std::string>();
  3629                 std::ostringstream errorString;
  3630                 errorString << "Unsupported data type for column - " << columnName.c_str()
  3631                             << ", Type - " << dataType << ", column ID - " << col;
  3632                 LOG("SQLBindColums: %s", errorString.str().c_str());

Lines 3633-3641

  3633                 ThrowStdException(errorString.str());
  3634                 break;
  3635         }
  3636         if (!SQL_SUCCEEDED(ret)) {
! 3637             std::string columnName = columnMeta["ColumnName"].cast<std::string>();
  3638             std::ostringstream errorString;
  3639             errorString << "Failed to bind column - " << columnName.c_str() << ", Type - "
  3640                         << dataType << ", column ID - " << col;
  3641             LOG("SQLBindColums: %s", errorString.str().c_str());

Lines 3961-3969

  3961                     break;
  3962                 }
  3963                 default: {
  3964                     const auto& columnMeta = columnNames[col - 1].cast<py::dict>();
! 3965                     std::string columnName = columnMeta["ColumnName"].cast<std::string>();
  3966                     std::ostringstream errorString;
  3967                     errorString << "Unsupported data type for column - " << columnName.c_str()
  3968                                 << ", Type - " << dataType << ", column ID - " << col;
  3969                     LOG("FetchBatchData: %s", errorString.str().c_str());

Lines 4063-4071

  4063             case SQL_SS_TIMESTAMPOFFSET:
  4064                 rowSize += sizeof(DateTimeOffset);
  4065                 break;
  4066             default:
! 4067                 std::string columnName = columnMeta["ColumnName"].cast<std::string>();
  4068                 std::ostringstream errorString;
  4069                 errorString << "Unsupported data type for column - " << columnName.c_str()
  4070                             << ", Type - " << dataType << ", column ID - " << col;
  4071                 LOG("calculateRowSize: %s", errorString.str().c_str());

mssql_python/pybind/ddbc_bindings.h

  43 #endif
  44 
  45 inline std::string utf16LeToUtf8Alloc(std::u16string utf16) {
  46     if (utf16.empty()) {
! 47         return {};
  48     }
  49 
  50     simdutf::result utf8Length =
  51         simdutf::utf8_length_from_utf16le_with_replacement(utf16.data(), utf16.size());

📋 Files Needing Attention

📉 Files with overall lowest coverage (click to expand)

mssql_python.pybind.build._deps.simdutf-src.src.haswell.implementation.cpp: 0.8%
mssql_python.pybind.build._deps.simdutf-src.include.simdutf.error.h: 2.1%
mssql_python.pybind.build._deps.simdutf-src.src.implementation.cpp: 6.9%
mssql_python.pybind.build._deps.simdutf-src.include.simdutf.implementation.h: 10.4%
mssql_python.pybind.build._deps.simdutf-src.include.simdutf.scalar.utf16.h: 18.2%
mssql_python.pybind.build._deps.simdutf-src.include.simdutf.scalar.utf16_to_utf8.utf16_to_utf8.h: 28.7%
mssql_python.pybind.build._deps.simdutf-src.src.simdutf.haswell.simd16-inl.h: 33.3%
mssql_python.pybind.logger_bridge.cpp: 59.2%
mssql_python.pybind.build._deps.simdutf-src.src.generic.utf16.utf8_length_from_utf16_bytemask.h: 61.5%
mssql_python.pybind.ddbc_bindings.h: 64.6%

🔗 Quick Links

⚙️ Build Summary	📋 Coverage Details
View Azure DevOps Build	Browse Full Coverage Report

ffelixg · 2026-04-16T23:44:55Z

Only issue seems to be that some Linux CI containers don't have git. I'm trying to fetch simdutf via url instead.

gargsaumya · 2026-04-17T03:58:12Z

/azp run

azure-pipelines · 2026-04-17T03:58:23Z

Azure Pipelines successfully started running 1 pipeline(s).

ffelixg · 2026-04-19T22:44:42Z

@gargsaumya The second CI error was due to the ubuntu image using an older version of cmake. Should be fine now I hope.

I have also replaced the remaining std::wstring occurences with std::u16string and eliminated helper functions as well as platform specific code accordingly. Let me know if you are happy with this holistic update to utf16 string handling or if you would prefer to keep it contained to the arrow fetch path.

gargsaumya · 2026-04-20T04:22:31Z

Thanks for the update @ffelixg. I actually prefer this and it LGTM overall. ButI’ll still take a closer look at the changes and get back to you.
I’ve approved the workflow for now so we can run the pipeline and get a passing test run.

gargsaumya · 2026-04-20T04:22:46Z

/azp run

azure-pipelines · 2026-04-20T04:22:56Z

Azure Pipelines successfully started running 1 pipeline(s).

+    static_assert(sizeof(SQLWCHAR) == sizeof(char16_t), "SQLWCHAR must be 16-bit");
+
+    if (length > 0) {
+        std::memcpy(utf16.data(), value, length * sizeof(SQLWCHAR));


PERF: Use simdutf for decoding utf16

3de212c

Copilot AI review requested due to automatic review settings April 15, 2026 23:50

Copilot started reviewing on behalf of ffelixg April 15, 2026 23:52 View session

Copilot AI reviewed Apr 15, 2026

View reviewed changes

Comment thread mssql_python/pybind/ddbc_bindings.h

Include cstring

0e8661f

github-advanced-security AI found potential problems Apr 16, 2026

View reviewed changes

Comment thread mssql_python/pybind/ddbc_bindings.h Fixed

ffelixg added 2 commits April 17, 2026 01:22

Use url instead of git for fetching simdutf

19c46ab

Merge remote-tracking branch 'origin/main' into simdutf

4fbe283

ffelixg added 2 commits April 19, 2026 22:37

Account for older cmake versions

1b9df47

Replace remaining std::wstring uses with std::u16string

12bbb58

ffelixg changed the title ~~PERF: Use simdutf for decoding utf16~~ PERF: Handle utf16 strings using simdutf and std::u16string Apr 19, 2026

github-advanced-security AI found potential problems Apr 20, 2026

View reviewed changes

Comment thread mssql_python/pybind/ddbc_bindings.h

static_assert(sizeof(SQLWCHAR) == sizeof(char16_t), "SQLWCHAR must be 16-bit");

if (length > 0) {

std::memcpy(utf16.data(), value, length * sizeof(SQLWCHAR));

gargsaumya changed the title ~~PERF: Handle utf16 strings using simdutf and std::u16string~~ FIX: [PERF] Handle utf16 strings using simdutf and std::u16string Apr 20, 2026

Conversation

ffelixg commented Apr 15, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Work Item / Issue Reference

Summary

Uh oh!

ffelixg commented Apr 15, 2026

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull request overview

Reviewed changes

Uh oh!

Uh oh!

gargsaumya commented Apr 16, 2026

Uh oh!

azure-pipelines bot commented Apr 16, 2026

Uh oh!

Uh oh!

github-actions bot commented Apr 16, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

📊 Code Coverage Report

🔥 Diff Coverage

94%

🎯 Overall Coverage

25%

Diff Coverage

Diff: main...HEAD, staged and unstaged changes

Summary

mssql_python/pybind/ddbc_bindings.cpp

mssql_python/pybind/ddbc_bindings.h

📋 Files Needing Attention

🔗 Quick Links

Uh oh!

ffelixg commented Apr 16, 2026

Uh oh!

gargsaumya commented Apr 17, 2026

Uh oh!

azure-pipelines bot commented Apr 17, 2026

Uh oh!

ffelixg commented Apr 19, 2026

Uh oh!

gargsaumya commented Apr 20, 2026

Uh oh!

gargsaumya commented Apr 20, 2026

Uh oh!

azure-pipelines bot commented Apr 20, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

ffelixg commented Apr 15, 2026 •

edited

Loading

github-actions bot commented Apr 16, 2026 •

edited

Loading