Return a single element while iterating over segments

This makes the usage of the segment cursor more natural:

```
cur = conn.cursor('segment')
cur.execute(sql)
spooled_result = cur.fetchall()
for spooled_data in spooled_result:
  row_mapper = RowMapperFactory().create(columns=cur._query.columns, legacy_primitive_types=False)
  rows = list(SegmentIterator(spooled_data, row_mapper))
```

Author: wendigo

tests/integration/test_dbapi_integration.py

@@ -1876,12 +1876,18 @@ def test_segments_cursor(trino_connection):
         start => 1,
         stop => 5,
         step => 1)) n""")
-    rows = cur.fetchall()
-    assert len(rows) > 0
-    for spooled_data, spooled_segment in rows:
+    segments = cur.fetchall()
+    assert len(segments) > 0
+    row_mapper = trino.mapper.RowMapperFactory().create(columns=cur._query.columns, legacy_primitive_types=False)
+    total = 0
+    for spooled_data in segments:
+        assert len(spooled_data.segments) == 1, "Expected SpooledData to contain a single segment"
+        segment = spooled_data.segments[0]
         assert spooled_data.encoding == trino_connection._client_session.encoding
-        assert isinstance(spooled_segment.uri, str), f"Expected string for uri, got {spooled_segment.uri}"
-        assert isinstance(spooled_segment.ack_uri, str), f"Expected string for ack_uri, got {spooled_segment.ack_uri}"
+        assert isinstance(segment[0].uri, str), f"Expected string for uri, got {segment[0].uri}"
+        assert isinstance(segment[0].ack_uri, str), f"Expected string for ack_uri, got {segment[0].ack_uri}"
+        total += len(list(trino.SegmentIterator(spooled_data, row_mapper)))
+    assert total == 300875, f"Expected total rows 300875, got {total}"
 
 
 def get_cursor(legacy_prepared_statements, run_trino):

trino/client.py

@@ -1187,7 +1187,8 @@ def __iter__(self) -> Iterator[Tuple["SpooledData", "Segment"]]:
         return self
 
     def __next__(self) -> Tuple["SpooledData", "Segment"]:
-        return self, next(self._segments_iterator)
+        segment = next(self._segments_iterator)
+        return SpooledData(self._encoding, [segment]), segment
 
     def __repr__(self):
         return (f"SpooledData(encoding={self._encoding}, segments={list(self._segments)})")

trino/dbapi.py

@@ -742,7 +742,7 @@ def execute(self, operation, params=None):
         self._query = trino.client.TrinoQuery(self._request, query=operation,
                                               legacy_primitive_types=self._legacy_primitive_types,
                                               fetch_mode="segments")
-        self._iterator = iter(self._query.execute())
+        self._iterator = map(lambda tuple: tuple[0], iter(self._query.execute()))
         return self