Fetch 'arraysize' records per roundtrip in named cursors iteration

Closes ticket #33.

NEWS-2.3

@@ -5,6 +5,7 @@ What's new in psycopg 2.3.3
 
   - Added `register_composite()` function to cast PostgreSQL composite types
     into Python tuples/namedtuples.
+  - More efficient iteration on named cursors.
   - The build script refuses to guess values if pg_config is not found.
   - Connections and cursors are weakly referenceable.
 

doc/src/cursor.rst

@@ -208,6 +208,11 @@ The ``cursor`` class
             (2, None, 'dada')
             (3, 42, 'bar')
 
+        .. versionchanged:: 2.3.3
+            iterating over a :ref:`named cursor <server-side-cursors>`
+            fetches `~cursor.arraysize` records at time from the backend.
+            Previously only one record was fetched per roundtrip, resulting
+             in unefficient iteration.
 
     .. method:: fetchone()
 
@@ -301,6 +306,18 @@ The ``cursor`` class
         time with `~cursor.fetchmany()`. It defaults to 1 meaning to fetch
         a single row at a time.
 
+        The attribute is also used when iterating a :ref:`named cursor
+        <server-side-cursors>`: when syntax such as ``for i in cursor:`` is
+        used, in order to avoid an excessive number of network roundtrips, the
+        cursor will actually fetch `!arraysize` records at time from the
+        backend. For this task the default value of 1 is a poor value: if
+        `!arraysize` is 1, a default value of 2000 will be used instead. If
+        you really want to retrieve one record at time from the backend use
+        `fetchone()` in a loop.
+
+        .. versionchanged:: 2.3.3
+            `!arraysize` used in named cursor iteration.
+        
 
     .. attribute:: rowcount 
           

psycopg/cursor_type.c

@@ -821,8 +821,13 @@ psyco_curs_next_named(cursorObject *self)
     Dprintf("psyco_curs_next_named: rowcount = %ld", self->rowcount);
     if (self->row >= self->rowcount) {
         char buffer[128];
-        /* FIXME: use a cursor member for the size */
-        PyOS_snprintf(buffer, 127, "FETCH FORWARD 10 FROM %s", self->name);
+
+        /* fetch 'arraysize' records, but shun the default value of 1 */
+        long int size = self->arraysize;
+        if (size == 1) { size = 2000L; }
+
+        PyOS_snprintf(buffer, 127, "FETCH FORWARD %ld FROM %s",
+            size, self->name);
         if (pq_execute(self, buffer, 0) == -1) return NULL;
         if (_psyco_curs_prefetch(self) < 0) return NULL;
     }

tests/test_cursor.py

@@ -142,6 +142,21 @@ class CursorTests(unittest.TestCase):
             "named cursor records fetched in 2 roundtrips (delta: %s)"
             % (t2 - t1))
 
+    def test_iter_named_cursor_default_arraysize(self):
+        curs = self.conn.cursor('tmp')
+        curs.execute('select generate_series(1,50)')
+        rv = [ (r[0], curs.rownumber) for r in curs ]
+        # everything swallowed in one gulp
+        self.assertEqual(rv, [(i,i) for i in range(1,51)])
+
+    def test_iter_named_cursor_arraysize(self):
+        curs = self.conn.cursor('tmp')
+        curs.arraysize = 30
+        curs.execute('select generate_series(1,50)')
+        rv = [ (r[0], curs.rownumber) for r in curs ]
+        # everything swallowed in two gulps
+        self.assertEqual(rv, [(i,((i - 1) % 30) + 1) for i in range(1,51)])
+
 
 def test_suite():
     return unittest.TestLoader().loadTestsFromName(__name__)