[alexcastgonz (pcbeingused333)]

Fixes #14695

Motivation and Context

FaissCollection.delete with several keys leaves stale vectors in the FAISS index: the repro in #14695 ends with ntotal == 1 and an empty key map, so later searches can return vectors for records that no longer exist.

Root cause: _inner_delete removes a vector with remove_ids, which shifts every later vector in a flat index down by one position, but the key map kept the old positions. Every delete after the first then targeted the wrong position.

Description

After removing a key's vector, decrement the stored position of every key that sat after it in the same vector field's map. The change is limited to _inner_delete in python/semantic_kernel/connectors/faiss.py.

Tests added in tests/unit/connectors/memory/test_faiss.py:

• test_delete_multiple_keeps_index_in_sync: the issue's scenario; index and key map are empty after deleting all records.
• test_delete_updates_remaining_positions: positions stay correct after partial deletes.

Both fail on main and pass with the change; the rest of test_faiss.py passes, and ruff format --check / ruff check are clean on the touched files.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

This change was prepared with the help of an AI coding assistant.

[Copilot]

🟡 Changes recommended

Deletion can corrupt key mappings when index removal fails and incorrectly renumber stable-ID indexes, leaving orphaned vectors.

2 open issues

• Preserve ID mapping when index removal fails · New
• Avoid renumbering IDs in indexes that preserve them · New

What changed in this PR

This PR addresses #14695 by updating the Python FAISS connector’s key mappings after flat-index deletions, preventing subsequent deletes from targeting stale positions.

Changes:

• Decrement later mapped positions after each vector removal.
• Add regression tests for deleting all records and maintaining correct positions after partial deletes.

File	Description
python/​tests/​unit/​connectors/​memory/​test_faiss.py	Adds full and partial deletion regression tests.
python/​semantic_kernel/​connectors/​faiss.py	Updates remaining key positions after vector deletion.

_{Review effort: Balanced}

[alexcastgonz (pcbeingused333)]

Thanks, both Copilot points were valid and are addressed in the new commit:

• _inner_delete now calls remove_ids first and drops the key mapping only after it succeeds, so an index that can't remove vectors (e.g. the custom IndexHNSWFlat) no longer loses its mapping. Test: test_delete_keeps_key_map_when_index_cannot_remove.
• Only flat indexes compact ids on removal, and the key map relies on that (search resolves ids by position, upsert starts from the current count). Deleting from any other index (e.g. IVF, HNSW) now raises VectorStoreOperationException before anything is mutated, instead of leaving a broken mapping. Test: test_delete_rejected_for_index_that_preserves_ids.

Both new tests fail against the previous revision; tests/unit/connectors/memory/test_faiss.py passes (25 tests) and ruff format/ruff check are clean.

This comment was drafted with Claude Code.