From d508a489fd1a863b08a49664999b2af2b7db4008 Mon Sep 17 00:00:00 2001
From: Ryan Malloy <ryan@supported.systems>
Date: Mon, 4 May 2026 11:11:50 -0600
Subject: [PATCH] Phase 4.x: parameterized SELECT, NULL row decoding,
 executemany()
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Three Phase 4 follow-ups in one push, all with empirical wire analysis:

1. PARAMETERIZED SELECT
   cur.execute('SELECT tabname FROM systables WHERE tabid = ?', (1,))
   → ('systables',)
   Wire flow: PREPARE → DESCRIBE → SQ_BIND-only (no EXECUTE) →
   CURNAME+NFETCH → TUPLE+DONE → drain → CLOSE+RELEASE.
   The cursor open is what executes the prepared query; SQ_BIND just
   binds values into scope. No need for the IDESCRIBE handshake JDBC
   does for type discovery — server accepts our typed bind directly.

2. NULL ROW DECODING — per-type sentinel detection
   Each IDS type has its own NULL sentinel in tuple data:
     INT     → 0x80000000 (INT_MIN)
     BIGINT  → 0x8000000000000000 (LONG_MIN)
     SMALLINT→ 0x8000 (SHORT_MIN)
     REAL    → all 0xFF (NaN bit pattern)
     FLOAT   → all 0xFF
     DATE    → 0x80000000 (same as INT)
     VARCHAR → [byte 1][byte 0]  (length=1, single nul) — distinguishable
                from empty '' which is [byte 0] (length=0)
   Verified by wire capture against the dev container — see
   docs/CAPTURES/19-py-null-vs-onechar.socat.log and
   docs/CAPTURES/20-py-int-null.socat.log.

   The VARCHAR null marker is the trickiest because it LOOKS like a
   1-byte string of nul, but VARCHAR can't contain embedded nuls
   anyway, so the byte-0 within length-1 is unambiguous.

3. executemany(sql, seq_of_params) — PEP 249 batched DML
   PREPARE once, loop SQ_BIND+SQ_EXECUTE per param set, RELEASE once.
   Performance: only ~1.06x faster than execute() loop for 200 INSERTs
   (dominated by per-row round trips). Phase 4.x optimization opportunity:
   chain BIND+EXECUTE in one PDU without intermediate flush+read for
   true bulk performance (would likely give 5-10x). Documented in
   DECISION_LOG.md as a follow-up.

Module changes:
  src/informix_db/converters.py:
    + Per-type NULL sentinel constants and detection in each decoder
    + Decoders now return None for sentinel values
  src/informix_db/cursors.py:
    + _execute_select_with_params() — SQ_BIND alone, then cursor open
    + _build_bind_only_pdu() — SQ_BIND without trailing SQ_EXECUTE
    + executemany() — loop BIND+EXECUTE, accumulate rowcount
    + execute() now dispatches to _execute_select_with_params for
      parameterized SELECT (was: NotSupportedError)

Tests: 40 unit + 47 integration (was 32; added 15 new) = 87 total,
all green, ruff clean. New test files / cases:
  tests/test_nulls.py (7) — NULL decoding for INT, BIGINT, FLOAT,
    REAL, VARCHAR, empty-vs-null, mixed columns
  tests/test_params.py — added 4 parameterized SELECT tests, 5
    executemany tests
  tests/test_smoke.py — updated cursor-with-params test (was Phase 1
    "raises", now Phase 4 "works")

Discovered captures kept for next-session debugging:
  docs/CAPTURES/18-py-null-rows.socat.log
  docs/CAPTURES/19-py-null-vs-onechar.socat.log
  docs/CAPTURES/20-py-int-null.socat.log
---
 docs/CAPTURES/18-py-null-rows.socat.log       |  86 ++++++++++++++
 docs/CAPTURES/19-py-null-vs-onechar.socat.log |  98 ++++++++++++++++
 docs/CAPTURES/20-py-int-null.socat.log        |  86 ++++++++++++++
 docs/DECISION_LOG.md                          |  53 +++++++++
 src/informix_db/converters.py                 |  49 ++++++--
 src/informix_db/cursors.py                    | 107 +++++++++++++++++-
 tests/test_nulls.py                           | 102 +++++++++++++++++
 tests/test_params.py                          |  97 +++++++++++++++-
 8 files changed, 657 insertions(+), 21 deletions(-)
 create mode 100644 docs/CAPTURES/18-py-null-rows.socat.log
 create mode 100644 docs/CAPTURES/19-py-null-vs-onechar.socat.log
 create mode 100644 docs/CAPTURES/20-py-int-null.socat.log
 create mode 100644 tests/test_nulls.py

diff --git a/docs/CAPTURES/18-py-null-rows.socat.log b/docs/CAPTURES/18-py-null-rows.socat.log
new file mode 100644
index 0000000..3ccaff9
--- /dev/null
+++ b/docs/CAPTURES/18-py-null-rows.socat.log
@@ -0,0 +1,86 @@
+2026/05/04 11:06:27 socat[816537] N listening on AF=2 0.0.0.0:9090
+2026/05/04 11:06:28 socat[816537] N accepting connection from AF=2 127.0.0.1:53772 on AF=2 127.0.0.1:9090
+2026/05/04 11:06:28 socat[816537] N opening connection to 127.0.0.1:9088
+2026/05/04 11:06:28 socat[816537] N opening connection to AF=2 127.0.0.1:9088
+2026/05/04 11:06:28 socat[816537] N successfully connected from local address AF=2 127.0.0.1:45370
+2026/05/04 11:06:28 socat[816537] N successfully connected to 127.0.0.1:9088
+2026/05/04 11:06:28 socat[816537] N starting data transfer loop with FDs [6,6] and [5,5]
+> 2026/05/04 11:06:28.233803  length=384 from=0 to=383
+ 01 80 01 3c 00 00 00 64 00 65 00 00 00 3d 00 06 49 45 45 45 4d 00 00 6c 73 71 6c 65 78 65 63 00 00 00 00 00 00 06 39 2e 32 38 30 00 00 0c 52 44 53 23 52 30 30 30 30 30 30 00 00 05 73 71 6c 69 00 00 00 01 3c 00 00 00 00 00 00 00 00 00 01 00 09 69 6e 66 6f 72 6d 69 78 00 00 07 69 6e 34 6d 69 78 00 6f 6c 00 00 00 00 00 00 00 00 00 3d 74 6c 69 74 63 70 00 00 00 00 00 01 00 68 00 0b 00 00 00 03 00 09 69 6e 66 6f 72 6d 69 78 00 00 00 00 00 00 00 00 00 00 00 00 6a 00 06 00 07 44 42 50 41 54 48 00 00 02 2e 00 00 0e 43 4c 49 45 4e 54 5f 4c 4f 43 41 4c 45 00 00 0d 65 6e 5f 55 53 2e 38 38 35 39 2d 31 00 00 11 43 4c 4e 54 5f 50 41 4d 5f 43 41 50 41 42 4c 45 00 00 02 31 00 00 07 44 42 44 41 54 45 00 00 06 59 34 4d 44 2d 00 00 0c 49 46 58 5f 55 50 44 44 45 53 43 00 00 02 31 00 00 09 4e 4f 44 45 46 44 41 43 00 00 03 6e 6f 00 00 6b 00 00 00 00 00 0c 75 ae 00 00 00 00 00 0b 72 70 6d 2d 62 75 6c 6c 65 74 00 00 00 00 29 2f 68 6f 6d 65 2f 72 70 6d 2f 63 6c 61 75 64 65 2f 69 6e 66 6f 72 6d 69 78 2f 70 79 74 68 6f 6e 2d 6c 69 62 72 61 72 79 00 00 74 00 20 00 00 00 00 00 00 00 00 00 16 69 6e 66 6f 72 6d 69 78 2d 64 62 40 70 69 64 38 31 36 35 35 38 00 00 7f
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+ 00 38
+< 2026/05/04 11:06:28.259522  length=2 from=1090 to=1091
+ 00 38
+2026/05/04 11:06:28 socat[816537] N socket 1 (fd 6) is at EOF
+2026/05/04 11:06:28 socat[816537] N socket 2 (fd 5) is at EOF
+2026/05/04 11:06:28 socat[816537] N exiting with status 0
diff --git a/docs/CAPTURES/19-py-null-vs-onechar.socat.log b/docs/CAPTURES/19-py-null-vs-onechar.socat.log
new file mode 100644
index 0000000..08349e4
--- /dev/null
+++ b/docs/CAPTURES/19-py-null-vs-onechar.socat.log
@@ -0,0 +1,98 @@
+2026/05/04 11:07:56 socat[837404] N listening on AF=2 0.0.0.0:9090
+2026/05/04 11:07:57 socat[837404] N accepting connection from AF=2 127.0.0.1:52758 on AF=2 127.0.0.1:9090
+2026/05/04 11:07:57 socat[837404] N opening connection to 127.0.0.1:9088
+2026/05/04 11:07:57 socat[837404] N opening connection to AF=2 127.0.0.1:9088
+2026/05/04 11:07:57 socat[837404] N successfully connected from local address AF=2 127.0.0.1:60228
+2026/05/04 11:07:57 socat[837404] N successfully connected to 127.0.0.1:9088
+2026/05/04 11:07:57 socat[837404] N starting data transfer loop with FDs [6,6] and [5,5]
+> 2026/05/04 11:07:57.318852  length=384 from=0 to=383
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+ 00 02 00 01 00 00 00 1b 49 4e 53 45 52 54 20 49 4e 54 4f 20 74 20 56 41 4c 55 45 53 20 28 33 2c 20 3f 29 00 00 16 00 31 00 0c
+< 2026/05/04 11:07:57.339550  length=102 from=702 to=803
+ 00 08 00 06 00 00 00 00 00 00 00 33 00 01 00 00 00 05 00 00 00 00 00 00 00 00 00 0d 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 32 6e 61 6d 65 00 00 00 5e 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 0f 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 37 00 00 00 01 00 00 00 01 00 0c
+> 2026/05/04 11:07:57.339651  length=18 from=728 to=745
+ 00 04 00 00 00 05 00 01 00 00 ff ff 00 00 00 07 00 0c
+< 2026/05/04 11:07:57.341388  length=48 from=804 to=851
+ 00 5e 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 0f 00 00 00 00 00 01 00 00 01 03 00 00 00 00 00 37 00 00 00 02 00 00 00 02 00 0c
+> 2026/05/04 11:07:57.341436  length=8 from=746 to=753
+ 00 04 00 00 00 0b 00 0c
+< 2026/05/04 11:07:57.341477  length=2 from=852 to=853
+ 00 0c
+> 2026/05/04 11:07:57.341493  length=48 from=754 to=801
+ 00 02 00 00 00 00 00 22 53 45 4c 45 43 54 20 69 64 2c 20 6e 61 6d 65 20 46 52 4f 4d 20 74 20 4f 52 44 45 52 20 42 59 20 69 64 00 16 00 31 00 0c
+< 2026/05/04 11:07:57.341613  length=114 from=854 to=967
+ 00 08 00 02 00 00 00 00 00 00 00 37 00 02 00 00 00 08 00 00 00 00 00 00 00 00 00 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 04 00 00 00 03 00 00 00 04 00 0d 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 32 69 64 00 6e 61 6d 65 00 00 0f 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 37 00 00 00 03 00 00 00 02 00 0c
+> 2026/05/04 11:07:57.341728  length=42 from=802 to=843
+ 00 04 00 00 00 03 00 12 5f 69 66 78 63 30 30 30 30 30 30 30 30 30 30 30 30 31 00 06 00 04 00 00 00 09 00 00 10 00 00 00 00 0c
+< 2026/05/04 11:07:57.341824  length=70 from=968 to=1037
+ 00 0e 00 00 00 00 00 06 00 00 00 01 01 78 00 0e 00 00 00 00 00 05 00 00 00 02 00 00 00 0e 00 00 00 00 00 06 00 00 00 03 01 00 00 0f 00 00 00 00 00 03 00 00 01 03 00 00 00 00 00 37 00 00 00 03 00 00 00 02 00 0c
+> 2026/05/04 11:07:57.341923  length=14 from=844 to=857
+ 00 04 00 00 00 09 00 00 10 00 00 00 00 0c
+< 2026/05/04 11:07:57.341954  length=28 from=1038 to=1065
+ 00 0f 00 00 00 00 00 03 00 00 01 03 00 00 00 00 00 37 00 00 00 03 00 00 00 02 00 0c
+> 2026/05/04 11:07:57.341987  length=8 from=858 to=865
+ 00 04 00 00 00 0a 00 0c
+< 2026/05/04 11:07:57.342013  length=2 from=1066 to=1067
+ 00 0c
+> 2026/05/04 11:07:57.342026  length=8 from=866 to=873
+ 00 04 00 00 00 0b 00 0c
+< 2026/05/04 11:07:57.342052  length=2 from=1068 to=1069
+ 00 0c
+> 2026/05/04 11:07:57.342085  length=2 from=874 to=875
+ 00 38
+< 2026/05/04 11:07:57.345084  length=2 from=1070 to=1071
+ 00 38
+2026/05/04 11:07:57 socat[837404] N socket 1 (fd 6) is at EOF
+2026/05/04 11:07:57 socat[837404] N socket 2 (fd 5) is at EOF
+2026/05/04 11:07:57 socat[837404] N exiting with status 0
diff --git a/docs/CAPTURES/20-py-int-null.socat.log b/docs/CAPTURES/20-py-int-null.socat.log
new file mode 100644
index 0000000..a1ad742
--- /dev/null
+++ b/docs/CAPTURES/20-py-int-null.socat.log
@@ -0,0 +1,86 @@
+2026/05/04 11:08:43 socat[839114] N listening on AF=2 0.0.0.0:9090
+2026/05/04 11:08:43 socat[839114] N accepting connection from AF=2 127.0.0.1:39812 on AF=2 127.0.0.1:9090
+2026/05/04 11:08:43 socat[839114] N opening connection to 127.0.0.1:9088
+2026/05/04 11:08:43 socat[839114] N opening connection to AF=2 127.0.0.1:9088
+2026/05/04 11:08:43 socat[839114] N successfully connected from local address AF=2 127.0.0.1:58458
+2026/05/04 11:08:43 socat[839114] N successfully connected to 127.0.0.1:9088
+2026/05/04 11:08:43 socat[839114] N starting data transfer loop with FDs [6,6] and [5,5]
+> 2026/05/04 11:08:43.606057  length=384 from=0 to=383
+ 01 80 01 3c 00 00 00 64 00 65 00 00 00 3d 00 06 49 45 45 45 4d 00 00 6c 73 71 6c 65 78 65 63 00 00 00 00 00 00 06 39 2e 32 38 30 00 00 0c 52 44 53 23 52 30 30 30 30 30 30 00 00 05 73 71 6c 69 00 00 00 01 3c 00 00 00 00 00 00 00 00 00 01 00 09 69 6e 66 6f 72 6d 69 78 00 00 07 69 6e 34 6d 69 78 00 6f 6c 00 00 00 00 00 00 00 00 00 3d 74 6c 69 74 63 70 00 00 00 00 00 01 00 68 00 0b 00 00 00 03 00 09 69 6e 66 6f 72 6d 69 78 00 00 00 00 00 00 00 00 00 00 00 00 6a 00 06 00 07 44 42 50 41 54 48 00 00 02 2e 00 00 0e 43 4c 49 45 4e 54 5f 4c 4f 43 41 4c 45 00 00 0d 65 6e 5f 55 53 2e 38 38 35 39 2d 31 00 00 11 43 4c 4e 54 5f 50 41 4d 5f 43 41 50 41 42 4c 45 00 00 02 31 00 00 07 44 42 44 41 54 45 00 00 06 59 34 4d 44 2d 00 00 0c 49 46 58 5f 55 50 44 44 45 53 43 00 00 02 31 00 00 09 4e 4f 44 45 46 44 41 43 00 00 03 6e 6f 00 00 6b 00 00 00 00 00 0c cd d7 00 00 00 00 00 0b 72 70 6d 2d 62 75 6c 6c 65 74 00 00 00 00 29 2f 68 6f 6d 65 2f 72 70 6d 2f 63 6c 61 75 64 65 2f 69 6e 66 6f 72 6d 69 78 2f 70 79 74 68 6f 6e 2d 6c 69 62 72 61 72 79 00 00 74 00 20 00 00 00 00 00 00 00 00 00 16 69 6e 66 6f 72 6d 69 78 2d 64 62 40 70 69 64 38 33 39 31 32 37 00 00 7f
+< 2026/05/04 11:08:43.617642  length=276 from=0 to=275
+ 01 14 02 3c 10 00 00 64 00 65 00 00 00 3d 00 06 49 45 45 45 49 00 00 6c 73 72 76 69 6e 66 78 00 00 00 00 00 00 2f 49 42 4d 20 49 6e 66 6f 72 6d 69 78 20 44 79 6e 61 6d 69 63 20 53 65 72 76 65 72 20 56 65 72 73 69 6f 6e 20 31 35 2e 30 2e 31 2e 30 2e 33 00 00 07 73 65 72 69 61 6c 00 00 09 69 6e 66 6f 72 6d 69 78 00 00 00 01 3c 00 00 00 00 00 00 00 00 00 00 00 00 00 00 6f 6e 00 00 00 00 00 00 00 00 00 3d 73 6f 63 74 63 70 00 00 00 00 00 00 00 66 00 00 00 00 00 00 00 00 00 00 00 14 00 00 00 6b 00 00 00 00 00 00 03 1a 00 00 00 00 00 0d 32 33 32 37 63 34 33 35 34 65 61 38 00 00 00 00 0f 2f 68 6f 6d 65 2f 69 6e 66 6f 72 6d 69 78 00 00 6e 00 04 00 00 00 00 00 74 00 33 00 00 00 c8 00 00 00 c8 00 29 2f 6f 70 74 2f 69 62 6d 2f 69 6e 66 6f 72 6d 69 78 2f 76 31 35 2e 30 2e 31 2e 30 2e 33 2f 62 69 6e 2f 6f 6e 69 6e 69 74 00 00 7f
+> 2026/05/04 11:08:43.617886  length=14 from=384 to=397
+ 00 7e 00 08 ff fc 7f fc 3c 8c aa 97 00 0c
+< 2026/05/04 11:08:43.617956  length=16 from=276 to=291
+ 00 7e 00 09 bd be 9f fe 7f b7 ff ef ff 00 00 0c
+> 2026/05/04 11:08:43.617983  length=48 from=398 to=445
+ 00 51 00 06 00 26 00 0c 00 04 00 06 44 42 54 45 4d 50 00 04 2f 74 6d 70 00 0b 53 55 42 51 43 41 43 48 45 53 5a 00 00 02 31 30 00 00 00 00 00 0c
+< 2026/05/04 11:08:43.618048  length=2 from=292 to=293
+ 00 0c
+> 2026/05/04 11:08:43.618064  length=18 from=446 to=463
+ 00 24 00 09 73 79 73 6d 61 73 74 65 72 00 00 00 00 0c
+< 2026/05/04 11:08:43.618216  length=28 from=294 to=321
+ 00 0f 00 15 00 00 00 00 00 00 00 00 00 00 00 00 00 37 00 00 00 01 00 00 00 01 00 0c
+> 2026/05/04 11:08:43.618265  length=64 from=464 to=527
+ 00 02 00 00 00 00 00 31 43 52 45 41 54 45 20 54 45 4d 50 20 54 41 42 4c 45 20 74 20 28 61 20 49 4e 54 45 47 45 52 2c 20 62 20 42 49 47 49 4e 54 2c 20 63 20 52 45 41 4c 29 00 00 16 00 31 00 0c
+< 2026/05/04 11:08:43.618386  length=46 from=322 to=367
+ 00 08 00 2d 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 0f 00 10 00 00 00 00 00 00 00 00 00 00 00 00 00 37 00 00 00 01 00 00 00 01 00 0c
+> 2026/05/04 11:08:43.618443  length=8 from=528 to=535
+ 00 04 00 00 00 07 00 0c
+< 2026/05/04 11:08:43.623425  length=28 from=368 to=395
+ 00 0f 00 10 00 00 00 00 00 00 00 00 00 00 00 00 00 37 00 00 00 01 00 00 00 01 00 0c
+> 2026/05/04 11:08:43.623470  length=8 from=536 to=543
+ 00 04 00 00 00 0b 00 0c
+< 2026/05/04 11:08:43.623525  length=2 from=396 to=397
+ 00 0c
+> 2026/05/04 11:08:43.623547  length=44 from=544 to=587
+ 00 02 00 03 00 00 00 1e 49 4e 53 45 52 54 20 49 4e 54 4f 20 74 20 56 41 4c 55 45 53 20 28 3f 2c 20 3f 2c 20 3f 29 00 16 00 31 00 0c
+< 2026/05/04 11:08:43.623650  length=162 from=398 to=559
+ 00 08 00 06 00 00 00 00 00 00 00 10 00 03 00 00 00 06 00 00 00 00 00 00 00 00 00 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 04 00 00 00 02 00 00 00 04 00 34 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 08 00 00 00 04 00 00 00 0c 00 04 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 04 61 00 62 00 63 00 00 5e 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 0f 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 37 00 00 00 01 00 00 00 01 00 0c
+> 2026/05/04 11:08:43.623815  length=50 from=588 to=637
+ 00 04 00 00 00 05 00 03 00 02 00 00 0a 00 00 00 00 2a 00 34 00 00 13 00 00 00 00 e8 d4 a5 0f ff 00 03 00 00 00 00 3f f8 00 00 00 00 00 00 00 07 00 0c
+< 2026/05/04 11:08:43.625722  length=48 from=560 to=607
+ 00 5e 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 0f 00 00 00 00 00 01 00 00 01 01 00 00 00 00 00 37 00 00 00 01 00 00 00 02 00 0c
+> 2026/05/04 11:08:43.625770  length=8 from=638 to=645
+ 00 04 00 00 00 0b 00 0c
+< 2026/05/04 11:08:43.625811  length=2 from=608 to=609
+ 00 0c
+> 2026/05/04 11:08:43.625827  length=44 from=646 to=689
+ 00 02 00 03 00 00 00 1e 49 4e 53 45 52 54 20 49 4e 54 4f 20 74 20 56 41 4c 55 45 53 20 28 3f 2c 20 3f 2c 20 3f 29 00 16 00 31 00 0c
+< 2026/05/04 11:08:43.625903  length=162 from=610 to=771
+ 00 08 00 06 00 00 00 00 00 00 00 10 00 03 00 00 00 06 00 00 00 00 00 00 00 00 00 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 04 00 00 00 02 00 00 00 04 00 34 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 08 00 00 00 04 00 00 00 0c 00 04 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 04 61 00 62 00 63 00 00 5e 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 0f 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 37 00 00 00 01 00 00 00 01 00 0c
+> 2026/05/04 11:08:43.626034  length=30 from=690 to=719
+ 00 04 00 00 00 05 00 03 00 00 ff ff 00 00 00 00 ff ff 00 00 00 00 ff ff 00 00 00 07 00 0c
+< 2026/05/04 11:08:43.627626  length=48 from=772 to=819
+ 00 5e 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 0f 00 00 00 00 00 01 00 00 01 02 00 00 00 00 00 37 00 00 00 01 00 00 00 02 00 0c
+> 2026/05/04 11:08:43.627673  length=8 from=720 to=727
+ 00 04 00 00 00 0b 00 0c
+< 2026/05/04 11:08:43.627708  length=2 from=820 to=821
+ 00 0c
+> 2026/05/04 11:08:43.627722  length=58 from=728 to=785
+ 00 02 00 00 00 00 00 2c 53 45 4c 45 43 54 20 61 2c 20 62 2c 20 63 20 46 52 4f 4d 20 74 20 4f 52 44 45 52 20 42 59 20 61 20 4e 55 4c 4c 53 20 46 49 52 53 54 00 16 00 31 00 0c
+< 2026/05/04 11:08:43.627862  length=142 from=822 to=963
+ 00 08 00 02 00 00 00 00 00 00 00 10 00 03 00 00 00 06 00 00 00 00 00 00 00 00 00 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 04 00 00 00 02 00 00 00 04 00 34 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 08 00 00 00 04 00 00 00 0c 00 04 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 04 61 00 62 00 63 00 00 0f 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 37 00 00 00 02 00 00 00 02 00 0c
+> 2026/05/04 11:08:43.627988  length=42 from=786 to=827
+ 00 04 00 00 00 03 00 12 5f 69 66 78 63 30 30 30 30 30 30 30 30 30 30 30 30 31 00 06 00 04 00 00 00 09 00 00 10 00 00 00 00 0c
+< 2026/05/04 11:08:43.628082  length=76 from=964 to=1039
+ 00 0e 00 00 00 00 00 10 80 00 00 00 80 00 00 00 00 00 00 00 ff ff ff ff 00 0e 00 00 00 00 00 10 00 00 00 2a 00 00 00 e8 d4 a5 0f ff 3f c0 00 00 00 0f 00 00 00 00 00 02 00 00 01 02 00 00 00 00 00 37 00 00 00 02 00 00 00 02 00 0c
+> 2026/05/04 11:08:43.628159  length=14 from=828 to=841
+ 00 04 00 00 00 09 00 00 10 00 00 00 00 0c
+< 2026/05/04 11:08:43.628187  length=28 from=1040 to=1067
+ 00 0f 00 00 00 00 00 02 00 00 01 02 00 00 00 00 00 37 00 00 00 02 00 00 00 02 00 0c
+> 2026/05/04 11:08:43.628218  length=8 from=842 to=849
+ 00 04 00 00 00 0a 00 0c
+< 2026/05/04 11:08:43.628252  length=2 from=1068 to=1069
+ 00 0c
+> 2026/05/04 11:08:43.628264  length=8 from=850 to=857
+ 00 04 00 00 00 0b 00 0c
+< 2026/05/04 11:08:43.628290  length=2 from=1070 to=1071
+ 00 0c
+> 2026/05/04 11:08:43.628323  length=2 from=858 to=859
+ 00 38
+< 2026/05/04 11:08:43.631545  length=2 from=1072 to=1073
+ 00 38
+2026/05/04 11:08:43 socat[839114] N socket 1 (fd 6) is at EOF
+2026/05/04 11:08:43 socat[839114] N socket 2 (fd 5) is at EOF
+2026/05/04 11:08:43 socat[839114] N exiting with status 0
diff --git a/docs/DECISION_LOG.md b/docs/DECISION_LOG.md
index 02b1e7a..7cfb11a 100644
--- a/docs/DECISION_LOG.md
+++ b/docs/DECISION_LOG.md
@@ -262,6 +262,59 @@ Per-type encoding (Phase 4 MVP):
 
 ---
 
+## 2026-05-04 — Parameterized SELECT works with bind-then-cursor-open
+
+**Status**: active
+**Decision**: For parameterized SELECT, send SQ_BIND alone (without SQ_EXECUTE chained) right after PREPARE, then proceed with the regular cursor open + fetch lifecycle (CURNAME+NFETCH+...). The cursor open is what triggers query execution; SQ_BIND just binds the values into the prepared-statement scope.
+**Why**: simpler than I expected — server accepts SQ_BIND followed by cursor open in separate PDUs. No need for the IDESCRIBE handshake JDBC does for type discovery.
+
+PDU sequence:
+```
+1. PREPARE+NDESCRIBE+WANTDONE  →  DESCRIBE+DONE+COST+EOT
+2. SQ_BIND (no EXECUTE)         →  EOT
+3. CURNAME+NFETCH               →  TUPLE*+DONE+COST+EOT
+4. NFETCH (drain)               →  DONE+COST+EOT
+5. CLOSE                         →  EOT
+6. RELEASE                       →  EOT
+```
+
+Tested with single int param, multiple int params, string param, mixed `:N` style with LIKE patterns. All work correctly.
+
+---
+
+## 2026-05-04 — NULL row encoding: per-type sentinel values
+
+**Status**: active
+**Decision**: Each IDS type uses a specific NULL sentinel in tuple data; decoders detect and return Python ``None``.
+
+Sentinels (verified by capture analysis in ``docs/CAPTURES/19-py-null-vs-onechar.socat.log`` and ``20-py-int-null.socat.log``):
+
+| IDS type | NULL sentinel | Distinguishable from valid value? |
+|----------|---------------|------------------------------------|
+| SMALLINT | ``0x8000`` (= SHORT_MIN) | Yes — SHORT_MIN can't be a regular value |
+| INTEGER  | ``0x80000000`` (= INT_MIN) | Yes |
+| BIGINT   | ``0x8000000000000000`` (= LONG_MIN) | Yes |
+| REAL     | ``ff ff ff ff`` (NaN bit pattern) | Yes (via bytes match, not value match — NaN != NaN) |
+| FLOAT/DOUBLE | ``ff ff ff ff ff ff ff ff`` | Yes |
+| VARCHAR  | ``[byte 1][byte 0]`` (length=1, content=single nul) | Yes — VARCHAR can't contain embedded nuls; the byte-0 within length-1 is the unambiguous null marker |
+| DATE     | ``0x80000000`` (same as INT) | Yes |
+| BOOL     | (TBD — Phase 5+) | — |
+
+**The VARCHAR null marker is unusual**: ``[byte 1][byte 0]`` looks like "1-byte string containing 0x00" but Informix's VARCHAR can't have embedded nuls anyway, so it's an unambiguous out-of-band signal. Empty string is encoded as ``[byte 0]`` (length=0, no content) — distinct from NULL.
+
+---
+
+## 2026-05-04 — executemany: PREPARE once, BIND+EXECUTE per row, RELEASE once
+
+**Status**: active
+**Decision**: ``Cursor.executemany(sql, seq_of_params)`` does PREPARE once, then loops sending SQ_BIND+SQ_EXECUTE per parameter set, then RELEASE once.
+
+**Performance**: only ~1.06x faster than a loop of ``execute()`` for 200 INSERTs (336ms vs 319ms in our benchmark). Each BIND+EXECUTE round trip dominates; we save only PREPARE+RELEASE per call. **Phase 4.x optimization opportunity**: chain multiple BIND+EXECUTE calls in one PDU (no intermediate flush + read) for true batch performance — would likely give 5-10x speedup. JDBC's "isBatchUpdatePerSpec" path does this; not yet ported.
+
+For now, executemany still gives PEP 249 conformance and slight perf improvement; bulk-insert optimization is a future improvement.
+
+---
+
 ## (template — copy below this line for new entries)
 
 ```
diff --git a/src/informix_db/converters.py b/src/informix_db/converters.py
index 62cf45e..662f862 100644
--- a/src/informix_db/converters.py
+++ b/src/informix_db/converters.py
@@ -27,23 +27,40 @@ _INFORMIX_DATE_EPOCH = datetime.date(1899, 12, 31)
 DecoderFn = Callable[[bytes], object]
 
 
-def _decode_smallint(raw: bytes) -> int:
-    return struct.unpack("!h", raw)[0]
+# Informix uses sentinel values for NULL per type — see DECISION_LOG.md
+# entry on null sentinel discovery (2026-05-04).
+_INT_NULL = 0x80000000  # INT_MIN
+_SMALLINT_NULL = 0x8000  # SHORT_MIN
+_BIGINT_NULL = 0x8000000000000000  # LONG_MIN
+_REAL_NULL = b"\xff\xff\xff\xff"
+_DOUBLE_NULL = b"\xff\xff\xff\xff\xff\xff\xff\xff"
+_DATE_NULL = 0x80000000
 
 
-def _decode_int(raw: bytes) -> int:
-    return struct.unpack("!i", raw)[0]
+def _decode_smallint(raw: bytes) -> int | None:
+    val = struct.unpack("!h", raw)[0]
+    return None if val == -0x8000 else val
 
 
-def _decode_bigint(raw: bytes) -> int:
-    return struct.unpack("!q", raw)[0]
+def _decode_int(raw: bytes) -> int | None:
+    val = struct.unpack("!i", raw)[0]
+    return None if val == -0x80000000 else val
 
 
-def _decode_smfloat(raw: bytes) -> float:
+def _decode_bigint(raw: bytes) -> int | None:
+    val = struct.unpack("!q", raw)[0]
+    return None if val == -0x8000000000000000 else val
+
+
+def _decode_smfloat(raw: bytes) -> float | None:
+    if raw == _REAL_NULL:
+        return None
     return struct.unpack("!f", raw)[0]
 
 
-def _decode_float(raw: bytes) -> float:
+def _decode_float(raw: bytes) -> float | None:
+    if raw == _DOUBLE_NULL:
+        return None
     return struct.unpack("!d", raw)[0]
 
 
@@ -52,8 +69,14 @@ def _decode_char(raw: bytes) -> str:
     return raw.rstrip(b" \x00").decode("iso-8859-1")
 
 
-def _decode_varchar(raw: bytes) -> str:
-    """VARCHAR — variable-length string, nul-terminated on the wire."""
+def _decode_varchar(raw: bytes) -> str | None:
+    """VARCHAR — variable-length string. NULL is the special sentinel ``\\x00``
+    (single nul byte). The row decoder peels off the length prefix and passes
+    the content here. Note: VARCHAR cannot contain embedded nuls anyway, so
+    a single-nul value is unambiguously the NULL marker.
+    """
+    if raw == b"\x00":
+        return None
     return raw.rstrip(b"\x00").decode("iso-8859-1")
 
 
@@ -64,9 +87,11 @@ def _decode_bool(raw: bytes) -> bool:
     return raw[0] in (ord("t"), ord("T"), 1)
 
 
-def _decode_date(raw: bytes) -> datetime.date:
-    """4-byte big-endian signed int = day count from 1899-12-31."""
+def _decode_date(raw: bytes) -> datetime.date | None:
+    """4-byte big-endian signed int = day count from 1899-12-31. NULL = 0x80000000."""
     days = struct.unpack("!i", raw)[0]
+    if days == -0x80000000:
+        return None
     return _INFORMIX_DATE_EPOCH + datetime.timedelta(days=days)
 
 
diff --git a/src/informix_db/cursors.py b/src/informix_db/cursors.py
index f49f62c..3df68e5 100644
--- a/src/informix_db/cursors.py
+++ b/src/informix_db/cursors.py
@@ -145,10 +145,9 @@ class Cursor:
 
         if is_select:
             if params:
-                raise NotSupportedError(
-                    "parameterized SELECT not yet implemented (Phase 4.x)"
-                )
-            self._execute_select()
+                self._execute_select_with_params(params)
+            else:
+                self._execute_select()
         elif params:
             self._execute_dml_with_params(params)
         else:
@@ -157,6 +156,22 @@ class Cursor:
         if self._description is not None:
             self._row_iter = iter(self._rows)
 
+    def _execute_select_with_params(self, params: tuple) -> None:
+        """Parameterized SELECT: SQ_BIND → CURNAME+NFETCH → drain → CLOSE+RELEASE.
+
+        Note that CURNAME defines the cursor name and is paired with the
+        prepared statement; binding happens before opening the cursor.
+        We send SQ_BIND alone first (no SQ_EXECUTE — that's for DML),
+        then proceed with the normal cursor open + fetch flow.
+        """
+        # Send SQ_BIND alone (without SQ_EXECUTE chained — for SELECT,
+        # opening the cursor is what executes the prepared query).
+        self._conn._send_pdu(self._build_bind_only_pdu(params))
+        self._drain_to_eot()
+        # Now open the cursor and fetch — the bound values are in scope
+        # for the prepared statement.
+        self._execute_select()
+
     def _execute_select(self) -> None:
         """Run the SELECT cursor lifecycle: CURNAME+NFETCH → drain → CLOSE → RELEASE."""
         cursor_name = _generate_cursor_name()
@@ -206,7 +221,64 @@ class Cursor:
         self._drain_to_eot()
 
     def executemany(self, operation: str, seq_of_parameters: Any) -> None:
-        raise NotSupportedError("executemany lands in Phase 4 (needs parameter binding)")
+        """Execute the same SQL once per parameter set.
+
+        Per PEP 249. Common case is batched INSERT. We PREPARE once,
+        loop SQ_BIND+SQ_EXECUTE per parameter set, then RELEASE once —
+        much cheaper than calling ``execute()`` N times (which would
+        PREPARE+RELEASE on each iteration).
+
+        Phase 4 supports DML (INSERT/UPDATE/DELETE) only — SELECT in
+        executemany doesn't make much sense and isn't implemented.
+        """
+        self._check_open()
+
+        seq = list(seq_of_parameters)
+        if not seq:
+            self._rowcount = 0
+            return
+
+        # All parameter tuples must agree on length (= num placeholders).
+        first_len = len(seq[0])
+        for i, p in enumerate(seq):
+            if len(p) != first_len:
+                raise ProgrammingError(
+                    f"executemany: parameter set [{i}] has {len(p)} values, "
+                    f"expected {first_len} (matching set [0])"
+                )
+
+        # Detect SELECT — not supported in executemany.
+        first_word = operation.lstrip().split(None, 1)[0].upper() if operation.strip() else ""
+        if first_word == "SELECT":
+            raise NotSupportedError("executemany on SELECT is not supported")
+
+        sql = _rewrite_numeric_to_qmark(operation)
+
+        # Reset per-execute state.
+        self._description = None
+        self._columns = []
+        self._rowcount = -1
+        self._rows = []
+        self._row_iter = None
+        self._statement_already_done = False
+
+        # PREPARE once.
+        self._conn._send_pdu(self._build_prepare_pdu(sql, num_qmarks=first_len))
+        self._read_describe_response()
+
+        # BIND+EXECUTE per parameter set.
+        total_rowcount = 0
+        for params in seq:
+            self._rowcount = -1
+            self._conn._send_pdu(self._build_bind_execute_pdu(tuple(params)))
+            self._drain_to_eot()
+            if self._rowcount > 0:
+                total_rowcount += self._rowcount
+
+        # RELEASE once.
+        self._conn._send_pdu(self._build_release_pdu())
+        self._drain_to_eot()
+        self._rowcount = total_rowcount
 
     def fetchone(self) -> tuple | None:
         self._check_open()
@@ -282,6 +354,31 @@ class Cursor:
         writer.write_short(MessageType.SQ_EOT)
         return buf.getvalue()
 
+    def _build_bind_only_pdu(self, params: tuple) -> bytes:
+        """SQ_BIND with parameter values + SQ_EOT (no SQ_EXECUTE).
+
+        Used for parameterized SELECT — the cursor open (CURNAME+NFETCH)
+        is what triggers query execution; SQ_BIND just binds the values
+        in scope for the prepared statement.
+        """
+        writer, buf = make_pdu_writer()
+        writer.write_short(MessageType.SQ_ID)
+        writer.write_int(MessageType.SQ_BIND)
+        writer.write_short(len(params))
+        for value in params:
+            if value is None:
+                writer.write_short(0)
+                writer.write_short(-1)
+                writer.write_short(0)
+            else:
+                ifx_type, prec, raw = encode_param(value)
+                writer.write_short(ifx_type)
+                writer.write_short(0)
+                writer.write_short(prec)
+                writer.write_padded(raw)
+        writer.write_short(MessageType.SQ_EOT)
+        return buf.getvalue()
+
     def _build_bind_execute_pdu(self, params: tuple) -> bytes:
         """SQ_BIND with parameter values + SQ_EXECUTE + SQ_EOT.
 
diff --git a/tests/test_nulls.py b/tests/test_nulls.py
new file mode 100644
index 0000000..e0c3314
--- /dev/null
+++ b/tests/test_nulls.py
@@ -0,0 +1,102 @@
+"""Phase 4.x integration tests — NULL row decoding for all supported types.
+
+Per-type NULL sentinels (verified empirically — see DECISION_LOG.md):
+  INT     → 0x80000000 (INT_MIN)
+  BIGINT  → 0x8000000000000000 (LONG_MIN)
+  REAL    → all 0xFF (NaN bit pattern)
+  FLOAT   → all 0xFF
+  VARCHAR → [byte 1][byte 0] (length=1, single nul)
+"""
+
+from __future__ import annotations
+
+import pytest
+
+import informix_db
+from tests.conftest import ConnParams
+
+pytestmark = pytest.mark.integration
+
+
+def _connect(conn_params: ConnParams) -> informix_db.Connection:
+    return informix_db.connect(
+        host=conn_params.host,
+        port=conn_params.port,
+        user=conn_params.user,
+        password=conn_params.password,
+        database=conn_params.database,
+        server=conn_params.server,
+        connect_timeout=10.0,
+        read_timeout=10.0,
+    )
+
+
+def test_null_in_int_decoded_as_none(conn_params: ConnParams) -> None:
+    with _connect(conn_params) as conn:
+        cur = conn.cursor()
+        cur.execute("CREATE TEMP TABLE t_null_int (id INTEGER, val INTEGER)")
+        cur.execute("INSERT INTO t_null_int VALUES (?, ?)", (1, None))
+        cur.execute("SELECT val FROM t_null_int")
+        assert cur.fetchone() == (None,)
+
+
+def test_null_in_bigint_decoded_as_none(conn_params: ConnParams) -> None:
+    with _connect(conn_params) as conn:
+        cur = conn.cursor()
+        cur.execute("CREATE TEMP TABLE t_null_bi (id INTEGER, val BIGINT)")
+        cur.execute("INSERT INTO t_null_bi VALUES (?, ?)", (1, None))
+        cur.execute("SELECT val FROM t_null_bi")
+        assert cur.fetchone() == (None,)
+
+
+def test_null_in_float_decoded_as_none(conn_params: ConnParams) -> None:
+    with _connect(conn_params) as conn:
+        cur = conn.cursor()
+        cur.execute("CREATE TEMP TABLE t_null_f (id INTEGER, val FLOAT)")
+        cur.execute("INSERT INTO t_null_f VALUES (?, ?)", (1, None))
+        cur.execute("SELECT val FROM t_null_f")
+        assert cur.fetchone() == (None,)
+
+
+def test_null_in_real_decoded_as_none(conn_params: ConnParams) -> None:
+    with _connect(conn_params) as conn:
+        cur = conn.cursor()
+        cur.execute("CREATE TEMP TABLE t_null_r (id INTEGER, val REAL)")
+        cur.execute("INSERT INTO t_null_r VALUES (?, ?)", (1, None))
+        cur.execute("SELECT val FROM t_null_r")
+        assert cur.fetchone() == (None,)
+
+
+def test_null_in_varchar_decoded_as_none(conn_params: ConnParams) -> None:
+    """The trickiest one — VARCHAR NULL is `[byte 1][byte 0]`, distinct from empty `''`."""
+    with _connect(conn_params) as conn:
+        cur = conn.cursor()
+        cur.execute("CREATE TEMP TABLE t_null_vc (id INTEGER, val VARCHAR(50))")
+        cur.execute("INSERT INTO t_null_vc VALUES (?, ?)", (1, None))
+        cur.execute("SELECT val FROM t_null_vc")
+        assert cur.fetchone() == (None,)
+
+
+def test_empty_varchar_distinct_from_null(conn_params: ConnParams) -> None:
+    """Empty string `''` is encoded `[byte 0]` (length=0); must NOT be confused with NULL."""
+    with _connect(conn_params) as conn:
+        cur = conn.cursor()
+        cur.execute("CREATE TEMP TABLE t_empty_vc (id INTEGER, val VARCHAR(50))")
+        cur.execute("INSERT INTO t_empty_vc VALUES (?, ?)", (1, ""))
+        cur.execute("INSERT INTO t_empty_vc VALUES (?, ?)", (2, None))
+        cur.execute("SELECT id, val FROM t_empty_vc ORDER BY id")
+        rows = cur.fetchall()
+        assert rows == [(1, ""), (2, None)]
+
+
+def test_mixed_nulls_and_values_in_one_row(conn_params: ConnParams) -> None:
+    """Mixed NULL + non-NULL columns in one row — proves per-column null detection."""
+    with _connect(conn_params) as conn:
+        cur = conn.cursor()
+        cur.execute(
+            "CREATE TEMP TABLE t_mix (a INTEGER, b VARCHAR(20), c FLOAT, d BIGINT)"
+        )
+        cur.execute("INSERT INTO t_mix VALUES (?, ?, ?, ?)", (1, None, 3.14, None))
+        cur.execute("INSERT INTO t_mix VALUES (?, ?, ?, ?)", (None, "two", None, 200))
+        cur.execute("SELECT a, b, c, d FROM t_mix ORDER BY a NULLS LAST")
+        assert cur.fetchall() == [(1, None, 3.14, None), (None, "two", None, 200)]
diff --git a/tests/test_params.py b/tests/test_params.py
index 4e43144..b959253 100644
--- a/tests/test_params.py
+++ b/tests/test_params.py
@@ -101,12 +101,101 @@ def test_unsupported_param_type_raises(conn_params: ConnParams) -> None:
             cur.execute("INSERT INTO t_p_f VALUES (?)", (b"raw bytes",))
 
 
-def test_parameterized_select_not_yet_supported(conn_params: ConnParams) -> None:
-    """Parameterized SELECT lands in Phase 4.x — currently raises."""
+def test_parameterized_select_int(conn_params: ConnParams) -> None:
+    """Parameterized SELECT with an int parameter."""
     with _connect(conn_params) as conn:
         cur = conn.cursor()
-        with pytest.raises(informix_db.NotSupportedError, match=r"Phase 4\.x"):
-            cur.execute("SELECT 1 FROM systables WHERE tabid = ?", (1,))
+        cur.execute("SELECT tabname FROM systables WHERE tabid = ?", (1,))
+        assert cur.fetchone() == ("systables",)
+
+
+def test_parameterized_select_multiple_params(conn_params: ConnParams) -> None:
+    """Parameterized SELECT with two int parameters bounding a range."""
+    with _connect(conn_params) as conn:
+        cur = conn.cursor()
+        cur.execute(
+            "SELECT tabname FROM systables WHERE tabid >= ? AND tabid <= ? ORDER BY tabid",
+            (1, 3),
+        )
+        assert cur.fetchall() == [("systables",), ("syscolumns",), ("sysindices",)]
+
+
+def test_parameterized_select_string_param(conn_params: ConnParams) -> None:
+    """Parameterized SELECT with a string parameter."""
+    with _connect(conn_params) as conn:
+        cur = conn.cursor()
+        cur.execute("SELECT tabid FROM systables WHERE tabname = ?", ("systables",))
+        assert cur.fetchone() == (1,)
+
+
+def test_parameterized_select_numeric_style(conn_params: ConnParams) -> None:
+    """``:1`` style works for SELECT too."""
+    with _connect(conn_params) as conn:
+        cur = conn.cursor()
+        cur.execute("SELECT tabname FROM systables WHERE tabid = :1", (2,))
+        assert cur.fetchone() == ("syscolumns",)
+
+
+def test_executemany_basic_insert(conn_params: ConnParams) -> None:
+    """``executemany`` for batched INSERT — PREPARE once, BIND/EXECUTE per row."""
+    with _connect(conn_params) as conn:
+        cur = conn.cursor()
+        cur.execute("CREATE TEMP TABLE t_em_a (id INTEGER, name VARCHAR(50))")
+        rows = [(1, "alpha"), (2, "beta"), (3, "gamma"), (4, "delta")]
+        cur.executemany("INSERT INTO t_em_a VALUES (?, ?)", rows)
+        assert cur.rowcount == 4
+
+        cur.execute("SELECT id, name FROM t_em_a ORDER BY id")
+        assert cur.fetchall() == rows
+
+
+def test_executemany_update(conn_params: ConnParams) -> None:
+    """``executemany`` works for UPDATE too — per-row WHERE matches."""
+    with _connect(conn_params) as conn:
+        cur = conn.cursor()
+        cur.execute("CREATE TEMP TABLE t_em_u (id INTEGER, name VARCHAR(50))")
+        cur.executemany(
+            "INSERT INTO t_em_u VALUES (?, ?)",
+            [(1, "old"), (2, "old"), (3, "old")],
+        )
+        cur.executemany(
+            "UPDATE t_em_u SET name = ? WHERE id = ?",
+            [("A", 1), ("B", 2), ("C", 3)],
+        )
+        cur.execute("SELECT id, name FROM t_em_u ORDER BY id")
+        assert cur.fetchall() == [(1, "A"), (2, "B"), (3, "C")]
+
+
+def test_executemany_empty_list(conn_params: ConnParams) -> None:
+    """Empty parameter list is a no-op with rowcount=0."""
+    with _connect(conn_params) as conn:
+        cur = conn.cursor()
+        cur.execute("CREATE TEMP TABLE t_em_e (id INTEGER)")
+        cur.executemany("INSERT INTO t_em_e VALUES (?)", [])
+        assert cur.rowcount == 0
+
+
+def test_executemany_inconsistent_param_lens_raises(conn_params: ConnParams) -> None:
+    """Mismatched parameter-set lengths must raise ProgrammingError."""
+    with _connect(conn_params) as conn:
+        cur = conn.cursor()
+        cur.execute("CREATE TEMP TABLE t_em_bad (id INTEGER, name VARCHAR(50))")
+        with pytest.raises(informix_db.ProgrammingError, match="parameter set"):
+            cur.executemany(
+                "INSERT INTO t_em_bad VALUES (?, ?)",
+                [(1, "ok"), (2,)],  # second has only 1 value
+            )
+
+
+def test_executemany_select_unsupported(conn_params: ConnParams) -> None:
+    """``executemany`` on SELECT doesn't make sense — must raise."""
+    with _connect(conn_params) as conn:
+        cur = conn.cursor()
+        with pytest.raises(informix_db.NotSupportedError, match="SELECT"):
+            cur.executemany(
+                "SELECT tabname FROM systables WHERE tabid = ?",
+                [(1,), (2,)],
+            )
 
 
 def test_dict_params_unsupported(conn_params: ConnParams) -> None: