docs: document scalar indexing on nested struct fields

docs/indexing/scalar-index.mdx

@@ -140,3 +140,65 @@ LanceDB supports scalar indexes on UUID columns (stored as `FixedSizeBinary(16)`
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+## Index nested fields
+
+You can build a scalar index on a field inside a struct column by passing the
+canonical dot-separated path to `create_index`. This is useful when filters
+target attributes nested under a `metadata`-style column, for example
+`metadata.user_id` or `metadata.event.type`.
+
+If a literal segment of the path itself contains a dot (for example a column
+named `user.id` nested inside `metadata`), wrap that segment in backticks so
+LanceDB can tell the dot apart from the path separator: `` metadata.`user.id` ``.
+
+`list_indices()` echoes the same canonical path back, so the column you pass in
+round-trips through index metadata regardless of nesting depth or escaping.
+
+```python Python icon="python"
+import pyarrow as pa
+from lancedb.index import BTree
+
+metadata_type = pa.struct(
+    [
+        pa.field("user_id", pa.int32()),
+        pa.field("user.id", pa.int32()),
+    ]
+)
+data = pa.Table.from_arrays(
+    [
+        pa.array([1, 2, 3], type=pa.int32()),
+        pa.array(
+            [
+                {"user_id": 10, "user.id": 100},
+                {"user_id": 20, "user.id": 200},
+                {"user_id": 30, "user.id": 300},
+            ],
+            type=metadata_type,
+        ),
+    ],
+    names=["user_id", "metadata"],
+)
+table = await db.create_table("nested_scalar_index", data)
+
+# Index a nested struct field.
+await table.create_index(
+    "metadata.user_id", config=BTree(), name="nested_user_id_idx"
+)
+
+# Escape literal dots inside a segment with backticks.
+await table.create_index(
+    "metadata.`user.id`", config=BTree(), name="escaped_user_id_idx"
+)
+
+# `columns` is returned as the canonical path you passed in.
+for index in await table.list_indices():
+    print(index.name, index.columns)
+# nested_user_id_idx  ['metadata.user_id']
+# escaped_user_id_idx ['metadata.`user.id`']
+```
+
+<Note>
+Composite indexes that cover multiple columns aren't supported yet. Each
+`create_index` call must target a single (possibly nested) field path.
+</Note>
+