Don't recalculate loaders when not needed

The case it's not needed is when the new result is guaranteed to have
the same fields as the previous one. This happens querying in single-row
mode and on server-side cursors fetch.

diff --git a/psycopg3/psycopg3/cursor.py b/psycopg3/psycopg3/cursor.py
index 585b11cecd46a0618f229e298bd21cc016366f84..807ee1f422401e642eac16c34612bf0351100fcb 100644 (file)
--- a/psycopg3/psycopg3/cursor.py
+++ b/psycopg3/psycopg3/cursor.py
@@ -126,8 +126,6 @@ class BaseCursor(Generic[ConnectionType]):
     @pgresult.setter
     def pgresult(self, result: Optional["PGresult"]) -> None:
         self._pgresult = result
-        if result and self._tx:
-            self._tx.set_pgresult(result)
 
     @property
     def description(self) -> Optional[List[Column]]:
@@ -172,6 +170,7 @@ class BaseCursor(Generic[ConnectionType]):
         self._iresult += 1
         if self._iresult < len(self._results):
             self.pgresult = self._results[self._iresult]
+            self._tx.set_pgresult(self._results[self._iresult])
             self._pos = 0
             nrows = self.pgresult.command_tuples
             self._rowcount = nrows if nrows is not None else -1
@@ -264,16 +263,15 @@ class BaseCursor(Generic[ConnectionType]):
         self._conn.pgconn.set_single_row_mode()
         self._last_query = query
 
-    def _stream_fetchone_gen(self) -> PQGen[Optional["PGresult"]]:
+    def _stream_fetchone_gen(self, first: bool) -> PQGen[Optional["PGresult"]]:
         yield from generators.send(self._conn.pgconn)
         res = yield from generators.fetch(self._conn.pgconn)
         if res is None:
             return None
 
         elif res.status == ExecStatus.SINGLE_TUPLE:
-            self.pgresult = res  # will set it on the transformer too
-            # TODO: the transformer may do excessive work here: create a
-            # path that doesn't clear the loaders every time.
+            self.pgresult = res
+            self._tx.set_pgresult(res, set_loaders=first)
             return res
 
         elif res.status in (ExecStatus.TUPLES_OK, ExecStatus.COMMAND_OK):
@@ -317,7 +315,8 @@ class BaseCursor(Generic[ConnectionType]):
         self._execute_send(query, no_pqexec=True)
         (result,) = yield from execute(self._conn.pgconn)
         self._check_copy_result(result)
-        self.pgresult = result  # will set it on the transformer too
+        self.pgresult = result
+        self._tx.set_pgresult(result)
 
     def _execute_send(
         self, query: PostgresQuery, no_pqexec: bool = False
@@ -374,6 +373,7 @@ class BaseCursor(Generic[ConnectionType]):
 
         self._results = list(results)
         self.pgresult = results[0]
+        self._tx.set_pgresult(results[0])
         nrows = self.pgresult.command_tuples
         if nrows is not None:
             if self._rowcount < 0:
@@ -513,10 +513,12 @@ class Cursor(BaseCursor["Connection"]):
         """
         with self._conn.lock:
             self._conn.wait(self._stream_send_gen(query, params))
-            while self._conn.wait(self._stream_fetchone_gen()):
+            first = True
+            while self._conn.wait(self._stream_fetchone_gen(first)):
                 rec = self._tx.load_row(0)
                 assert rec is not None
                 yield rec
+                first = False
 
     def fetchone(self) -> Optional[Sequence[Any]]:
         """
@@ -636,10 +638,12 @@ class AsyncCursor(BaseCursor["AsyncConnection"]):
     ) -> AsyncIterator[Sequence[Any]]:
         async with self._conn.lock:
             await self._conn.wait(self._stream_send_gen(query, params))
-            while await self._conn.wait(self._stream_fetchone_gen()):
+            first = True
+            while await self._conn.wait(self._stream_fetchone_gen(first)):
                 rec = self._tx.load_row(0)
                 assert rec is not None
                 yield rec
+                first = False
 
     async def fetchone(self) -> Optional[Sequence[Any]]:
         self._check_result()

diff --git a/psycopg3/psycopg3/server_cursor.py b/psycopg3/psycopg3/server_cursor.py
index 88f7bad0e180a30d21818115df91ea6132eace34..058a0392141fedf620bc3adba89b7d4dd9ba9d52 100644 (file)
--- a/psycopg3/psycopg3/server_cursor.py
+++ b/psycopg3/psycopg3/server_cursor.py
@@ -116,8 +116,8 @@ class ServerCursorHelper(Generic[ConnectionType]):
         )
         res = yield from cur._conn._exec_command(query)
 
-        # TODO: loaders don't need to be refreshed
         cur.pgresult = res
+        cur._tx.set_pgresult(res, set_loaders=False)
         return cur._tx.load_rows(0, res.ntuples)
 
     def _scroll_gen(