Support INSERT OR REPLACE for vec0 virtual tables (fixes #127)

Check sqlite3_vtab_on_conflict() in vec0Update_Insert and delete the
existing row before re-inserting when the conflict mode is SQLITE_REPLACE.
Handles both integer and text primary keys.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/test-insert-delete.py

@@ -537,3 +537,117 @@ def test_wal_concurrent_reader_during_write(tmp_path):
 
     writer.close()
     reader.close()
+
+
+def test_insert_or_replace_integer_pk(db):
+    """INSERT OR REPLACE should update vector when rowid already exists."""
+    db.execute("create virtual table v using vec0(emb float[4], chunk_size=8)")
+
+    db.execute(
+        "insert into v(rowid, emb) values (1, ?)", [_f32([1.0, 2.0, 3.0, 4.0])]
+    )
+    # Replace with new vector
+    db.execute(
+        "insert or replace into v(rowid, emb) values (1, ?)",
+        [_f32([10.0, 20.0, 30.0, 40.0])],
+    )
+
+    # Should still have exactly 1 row
+    count = db.execute("select count(*) from v").fetchone()[0]
+    assert count == 1
+
+    # Vector should be the replaced value
+    row = db.execute("select emb from v where rowid = 1").fetchone()
+    assert row[0] == _f32([10.0, 20.0, 30.0, 40.0])
+
+
+def test_insert_or_replace_new_row(db):
+    """INSERT OR REPLACE with a new rowid should just insert normally."""
+    db.execute("create virtual table v using vec0(emb float[4], chunk_size=8)")
+
+    db.execute(
+        "insert or replace into v(rowid, emb) values (1, ?)",
+        [_f32([1.0, 2.0, 3.0, 4.0])],
+    )
+
+    count = db.execute("select count(*) from v").fetchone()[0]
+    assert count == 1
+
+    row = db.execute("select emb from v where rowid = 1").fetchone()
+    assert row[0] == _f32([1.0, 2.0, 3.0, 4.0])
+
+
+def test_insert_or_replace_text_pk(db):
+    """INSERT OR REPLACE should work with text primary keys."""
+    db.execute(
+        "create virtual table v using vec0("
+        "id text primary key, emb float[4], chunk_size=8"
+        ")"
+    )
+
+    db.execute(
+        "insert into v(id, emb) values ('doc_a', ?)",
+        [_f32([1.0, 2.0, 3.0, 4.0])],
+    )
+    db.execute(
+        "insert or replace into v(id, emb) values ('doc_a', ?)",
+        [_f32([10.0, 20.0, 30.0, 40.0])],
+    )
+
+    count = db.execute("select count(*) from v").fetchone()[0]
+    assert count == 1
+
+    row = db.execute("select emb from v where id = 'doc_a'").fetchone()
+    assert row[0] == _f32([10.0, 20.0, 30.0, 40.0])
+
+
+def test_insert_or_replace_with_auxiliary(db):
+    """INSERT OR REPLACE should also replace auxiliary column values."""
+    db.execute(
+        "create virtual table v using vec0("
+        "emb float[4], +label text, chunk_size=8"
+        ")"
+    )
+
+    db.execute(
+        "insert into v(rowid, emb, label) values (1, ?, 'old')",
+        [_f32([1.0, 2.0, 3.0, 4.0])],
+    )
+    db.execute(
+        "insert or replace into v(rowid, emb, label) values (1, ?, 'new')",
+        [_f32([10.0, 20.0, 30.0, 40.0])],
+    )
+
+    count = db.execute("select count(*) from v").fetchone()[0]
+    assert count == 1
+
+    row = db.execute("select emb, label from v where rowid = 1").fetchone()
+    assert row[0] == _f32([10.0, 20.0, 30.0, 40.0])
+    assert row[1] == "new"
+
+
+def test_insert_or_replace_knn_uses_new_vector(db):
+    """After INSERT OR REPLACE, KNN should find the new vector, not the old one."""
+    db.execute("create virtual table v using vec0(emb float[4], chunk_size=8)")
+
+    db.execute(
+        "insert into v(rowid, emb) values (1, ?)", [_f32([1.0, 0.0, 0.0, 0.0])]
+    )
+    db.execute(
+        "insert into v(rowid, emb) values (2, ?)", [_f32([0.0, 1.0, 0.0, 0.0])]
+    )
+
+    # Replace row 1's vector to be very close to row 2
+    db.execute(
+        "insert or replace into v(rowid, emb) values (1, ?)",
+        [_f32([0.0, 0.9, 0.0, 0.0])],
+    )
+
+    # KNN for [0, 1, 0, 0] should return row 2 first (exact), then row 1 (close)
+    rows = db.execute(
+        "select rowid, distance from v where emb match ? and k = 2",
+        [_f32([0.0, 1.0, 0.0, 0.0])],
+    ).fetchall()
+    assert rows[0][0] == 2
+    assert rows[1][0] == 1
+    assert rows[1][1] < 0.11  # should be close (L2 distance ≈ 0.1)