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szarnyasg merged 1 commit into from
Apr 23, 2026
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Document merge_adjacent_files x schema evolution #341

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4 changes: 4 additions & 0 deletions docs/stable/duckdb/maintenance/merge_adjacent_files.md
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Expand Up @@ -117,5 +117,9 @@ GROUP BY schema_name, table_name;

If a table has a [sort order defined]({% link docs/stable/duckdb/advanced_features/sorted_tables.md %}), `ducklake_merge_adjacent_files` sorts the merged output by those keys before writing the resulting Parquet file. The sort order applied is the one currently active on the table at the time compaction runs — not the order that was active when the original files were written.

## Schema Evolution

`ducklake_merge_adjacent_files` only merges files that share the same schema version. Schema-altering DDL statements such as `ADD COLUMN`, `DROP COLUMN`, `RENAME COLUMN`, or `ALTER COLUMN` create a new schema version, so files written before and after the change end up in separate compaction groups and are never merged together. Cosmetic alterations such as `COMMENT ON` or changes to the sort key do not bump the schema version.

> Calling this function does not immediately delete the old files.
> See the [cleanup old files]({% link docs/stable/duckdb/maintenance/cleanup_of_files.md %}) section on how to trigger a cleanup of these files.
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