Phase 35: CRITICAL fix - NFETCH loop for large result sets (2026.05.05.7)

DATA-LOSS BUG: cursor.fetchall() on result sets larger than ~200 rows
was silently truncating to the first ~200 rows. The exact cap depended
on row width and the server's per-NFETCH buffer (4096 bytes default).

The bug:

_execute_select sent NFETCH twice and stopped:
  self._conn._send_pdu(self._build_curname_nfetch_pdu(cursor_name))
  self._read_fetch_response()
  self._conn._send_pdu(self._build_nfetch_pdu())  # comment: "DONE only"
  self._read_fetch_response()
  # then CLOSE+RELEASE — discarding remaining queued rows

The "second fetch returns DONE only" comment was wrong. For any
result set larger than the server's per-NFETCH batch, the second
fetch returns more tuples AND there are still tuples queued
server-side. The cursor closed and dropped them.

Latent for 30 phases because every existing test used either a small
result set (FIRST 10) or relied on row counts that fit naturally in
1-2 batches. Discovered by Phase 34's scaling benchmark when
SELECT FIRST 100000 from a 100k-row table returned 200 rows.

The fix: loop NFETCH until a response yields zero new tuples.

  self._conn._send_pdu(self._build_curname_nfetch_pdu(cursor_name))
  rows_before = len(self._rows)
  self._read_fetch_response()
  rows_received = len(self._rows) - rows_before
  while rows_received > 0:
      self._conn._send_pdu(self._build_nfetch_pdu())
      rows_before = len(self._rows)
      self._read_fetch_response()
      rows_received = len(self._rows) - rows_before

249 integration tests pass. The scaling benchmark suite (Phase 34,
shipping next) is the regression test going forward.

Workaround for users on older versions: use scrollable cursors
(cursor(scrollable=True)) which use the SQ_SFETCH protocol path
and don't have this bug.

If you've been using this driver for queries returning large result
sets, your queries may have been truncating silently. Re-run them
against 2026.05.05.7+ to verify your data.

CHANGELOG.md

@@ -2,6 +2,56 @@
 
 All notable changes to `informix-db`. Versioning is [CalVer](https://calver.org/) — `YYYY.MM.DD` for date-based releases, `YYYY.MM.DD.N` for same-day post-releases per PEP 440.
 
+## 2026.05.05.7 — CRITICAL: Fix NFETCH loop for large result sets (Phase 35)
+
+**This is a data-loss bug fix.** Anyone running `cursor.fetchall()` (or iterating a non-scrollable cursor) on a result set larger than ~200 rows was silently getting **only the first ~200 rows** and missing the rest. The exact cap depends on row width and the server's NFETCH buffer (4096 bytes default), but the bug affects every result set that doesn't fit in 1-2 server fetch batches.
+
+### The bug
+
+`Cursor._execute_select` sent NFETCH twice and stopped:
+
+```python
+self._conn._send_pdu(self._build_curname_nfetch_pdu(cursor_name))
+self._read_fetch_response()
+# Drain — fetch again to confirm no more rows.
+# (JDBC always does this; the second fetch returns DONE only.)
+self._conn._send_pdu(self._build_nfetch_pdu())
+self._read_fetch_response()
+```
+
+The "second fetch returns DONE only" comment was wrong — for any result set larger than the server's per-NFETCH batch, the second fetch returns more tuples and there are still tuples queued server-side. After the second fetch, the cursor closed and the rest of the rows were discarded.
+
+This bug has been latent for ~30 phases because every existing test used either a small result set (e.g., systables FIRST 10) or relied on row-counts that fit naturally in 1-2 batches. The scaling benchmark (Phase 34) was the first time we tried `SELECT FIRST 100000` and got back 200 rows.
+
+### The fix
+
+`_execute_select` now loops NFETCH until a response yields zero new tuples:
+
+```python
+self._conn._send_pdu(self._build_curname_nfetch_pdu(cursor_name))
+rows_before = len(self._rows)
+self._read_fetch_response()
+rows_received = len(self._rows) - rows_before
+
+while rows_received > 0:
+    self._conn._send_pdu(self._build_nfetch_pdu())
+    rows_before = len(self._rows)
+    self._read_fetch_response()
+    rows_received = len(self._rows) - rows_before
+```
+
+### Tests
+
+All 249 existing integration tests still pass. The scaling benchmark suite (Phase 34) is the regression test that would have caught this earlier — `SELECT FIRST 100000` from a 100k-row table now returns the expected 100,000 rows.
+
+### Impact
+
+- **Severity**: CRITICAL (silent data loss).
+- **Workaround prior to this fix**: use scrollable cursors (`conn.cursor(scrollable=True)`) which use the SQ_SFETCH protocol path and don't have this bug.
+- **Affected versions**: every release before `2026.05.05.7`.
+
+If you've been using this driver for queries that returned large result sets, **you may have been getting truncated results without knowing it.** Re-run those queries against `2026.05.05.7+` to verify your data.
+
 ## 2026.05.05.6 — Pipelined `executemany` (Phase 33) — 2.85× faster on bulk inserts
 
 The previous serial-loop `executemany` paid one wire round-trip per row (~30 µs/row on loopback × N rows = the dominant cost for any sizeable batch). It was the *one* benchmark where IfxPy beat us in the comparison work — 10% slower at `executemany(1000)` in transaction.

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "informix-db"
-version = "2026.05.05.6"
+version = "2026.05.05.7"
 description = "Pure-Python driver for IBM Informix IDS — speaks the SQLI wire protocol over raw sockets. No CSDK, no JVM, no native libraries."
 readme = "README.md"
 license = { text = "MIT" }

src/informix_db/cursors.py

@@ -398,13 +398,22 @@ class Cursor:
             self._finalizer_state[0] = True  # arm the GC-time fallback
             self._scroll_total_rows = None
             return  # don't close; cursor stays live for SQ_SFETCH
+        # Phase 35: NFETCH loop — keep fetching until a response yields
+        # zero new tuples. The previous "two NFETCHes" pattern silently
+        # truncated any result set whose tuples didn't fit in 1-2 server
+        # batches (~200 rows at default 4096-byte buffer × 5-col rows).
+        # This bug was latent for ~30 phases because no test used a
+        # large enough result set to trigger it.
         self._conn._send_pdu(self._build_curname_nfetch_pdu(cursor_name))
+        rows_before = len(self._rows)
         self._read_fetch_response()
+        rows_received = len(self._rows) - rows_before
 
-        # Drain — fetch again to confirm no more rows.
-        # (JDBC always does this; the second fetch returns DONE only.)
-        self._conn._send_pdu(self._build_nfetch_pdu())
-        self._read_fetch_response()
+        while rows_received > 0:
+            self._conn._send_pdu(self._build_nfetch_pdu())
+            rows_before = len(self._rows)
+            self._read_fetch_response()
+            rows_received = len(self._rows) - rows_before
 
         # Dereference BYTE/TEXT blob descriptors BEFORE CLOSE — the
         # locators are only valid while the cursor is open. No-op when

uv.lock

@@ -34,7 +34,7 @@ wheels = [
 
 [[package]]
 name = "informix-db"
-version = "2026.5.5.4"
+version = "2026.5.5.6"
 source = { editable = "." }
 
 [package.optional-dependencies]