Skip uniqueness constraint validation on Overwrite for DuckDB & SQLite

[lukekim]

Summary

Skip uniqueness-constraint validation when performing InsertOp::Overwrite for the DuckDB and SQLite table providers. By definition, an overwrite replaces the target table's contents, so pre-existing row values cannot produce duplicate-key violations, and values within the incoming batch are resolved by the write path itself.

Motivation

The existing write path always called validate_batch_with_constraints before inserting, which would reject perfectly valid overwrite batches that happened to contain values conflicting with the previous contents of the table (or with themselves, when the caller expected "last write wins" semantics). This made InsertOp::Overwrite unusable for common ETL patterns that re-materialize a PK-constrained table from a source.

Changes

DuckDB (core/src/duckdb/)

• write.rs: Skip validate_batch_with_constraints when self.overwrite == InsertOp::Overwrite.
• creator.rs: Added create_table_without_constraints, used for the staging table during overwrite so intra-batch PK duplicates do not trip DuckDB's own constraint check before the atomic swap.

SQLite (core/src/sqlite/, core/src/sqlite.rs)

• write.rs: Skip validate_batch_with_constraints on InsertOp::Overwrite.
• sqlite.rs: Thread InsertOp through insert_batch / insert_batch_prepared; Overwrite now uses REPLACE INTO so that intra-batch duplicates collapse to the last row written.

SQL generation (core/src/sql/arrow_sql_gen/statement.rs)

• InsertBuilder::build now takes a replace: bool flag.
• New build_sqlite_replace() helper emits REPLACE INTO ... for SQLite overwrite paths.

Utility (core/src/util/constraints.rs)

• Added filter_unique_constraints helper for filtering non-unique constraints out of a Constraints set.

Test coverage

Added four new unit tests in core/src/duckdb/write.rs and core/src/sqlite/write.rs:

• test_overwrite_skips_pk_constraint_validation_with_duplicate_pks (DuckDB & SQLite): writes a batch containing duplicate primary-key values via InsertOp::Overwrite and asserts success.
• test_append_still_enforces_pk_constraint_validation (DuckDB & SQLite): regression guard — the same duplicate-PK batch must still be rejected when the op is Append.

All four pass locally:

test sqlite::write::tests::test_overwrite_skips_pk_constraint_validation_with_duplicate_pks ... ok
test duckdb::write::test::test_overwrite_skips_pk_constraint_validation_with_duplicate_pks ... ok
test sqlite::write::tests::test_append_still_enforces_pk_constraint_validation ... ok
test duckdb::write::test::test_append_still_enforces_pk_constraint_validation ... ok

[phillipleblanc]

constraints should still be applied on the incoming data if the final table will have constraints, otherwise you could end up with a table that has violated its constraints.

[phillipleblanc]

the last write wins should still allow the validation to proceed if that is configured.