DuckDB 1.5.1 (JDBC via DBeaver) sets temp_directory=\\.tmp on Windows causing “IO Error: Failed to create directory '.tmp'”

[aromlibero]

What happens?
When connecting to DuckDB 1.5.1 through JDBC using DBeaver on Windows, DuckDB assigns the temporary directory:
temp_directory = .tmp

This path is invalid on Windows, resulting in:
IO Error: Failed to create directory ".tmp": Invalid path

The same setup with DuckDB 1.4.4 correctly sets:
temp_directory = \mydatabase.duckdb.tmp

so no error occurs.

Expected behavior
DuckDB should assign a valid writable temporary directory on Windows, similar to:
temp_directory = <db_file>.tmp

as done in version 1.4.4, or fall back to a correct system temp folder (e.g. %TEMP%).

Observed behavior
In version 1.5.1:

• temp_directory becomes \.tmp
• Windows interprets \.tmp as an invalid UNC path
• Any operation requiring temp spill fails with an IO error
  This behavior matches known issues where DuckDB attempts to create a .tmp directory in read‑only or invalid locations.

Steps to reproduce
Windows 11
DBeaver

• Create a connection using DuckDB JDBC driver 1.5.1
• Open SQL editor and run:
  SELECT * FROM duckdb_settings() WHERE name='temp_directory';

Observe the value \.tmp
Run any query requiring temporary storage (e.g. large intermediate sort)
DuckDB fails with:
IO Error: Failed to create directory ".tmp"

DuckDB versions tested
1.4.4 → works correctly
1.5.1 → temp_directory=.tmp (invalid)

Environment
OS: Windows 11
Client: DBeaver + JDBC
Database: Single-file DuckDB
Filesystem: Local, fully writable

Workaround
Manually setting:
SET temp_directory='C:/Users//AppData/Local/Temp';
fixes the issue, confirming the problem is in DuckDB's automatic temp directory resolution.

Request
Please restore the previous behavior or implement a Windows‑safe fallback when computing temp_directory, so JDBC users on Windows do not receive invalid paths like \.tmp.

[staticlibs]

Hi, thanks for the report! Seems to be an unintended behaviour on Windows, I will take a look.

[staticlibs]

The problem appeared to be Windows-specific and Java-specific (CLI is unaffected).

When connection to C:\path\to\my.db is opened, the intended temp_directory is C:\path\to\my.db.tmp.

In JDBC 1.5.1.0 the directory is instead set to \\.tmp that is not a valid path. This was already fixed for 1.5.2 (can be checked with JDBC builds from this run). But the path now is returned with a long-path prefix: \\?\C:\path\to\my.db.tmp. This is not desired so I filed duckdb/duckdb#21652 to fix this.

Until 1.5.2 update is available the workaround is to set temp_directory explicitly, for example in the connection URL:

jdbc:duckdb:C:\path\to\my.db.db;temp_directory=C:\Users\vboxuser\AppData\Local\Temp\my.db.tmp