Research: Duckdb 1.5 compatibility

[anuunchin]

Caveats uncovered:

• Motherduck: Your MotherDuck extension is running on DuckDB v1.5.0 which is not yet supported. Please downgrade your DuckDB. We recommend DuckDB v1.4.4. For the installation guide, please refer to [https://duckdb.org/docs/installation/.](https://duckdb.org/docs/installation/) (you are using version v1.5.0-2026-03-258)" -> This is okay.

• Filesystem: register_filesystem is no longer idempotent: Validate filesystem registration duckdb/duckdb#19168 -> This uncovered the following issue:

FilesystemSqlClient.open_connection gates one-time setup (secrets, filesystem registration) behind conn_pool.never_borrowed. Despite the name, this property returns True whenever there are zero active borrows, not just on the first borrow ever. For FilesystemSqlClient, the underlying connection is externally owned and never destroyed between borrow/return cycles. So after the first query returns its borrow (count goes to 0), the second query sees never_borrowed = True and tries to register_filesystem again on the same connection, which already has that filesystem registered. Before DuckDB 1.5 this was silently accepted because register_filesystem was idempotent, so now it raises. -> This is okay in most cases, but may need attention. Fix will be minimal.

• Ducklake merge jobs issue opened and fixed: INSERT ... SELECT on ALTER-added column not persisted in duckdb 1.5.0 duckdb/ducklake#838

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	docs	f6516e1	Commit Preview URL Branch Preview URL	Mar 16 2026, 10:57 AM