Phase 21.1: executemany perf - it was the autocommit cliff (2026.05.04.6)

Investigation of the Phase 21 baseline finding that executemany(N) cost
scaled linearly per-row (1.74 ms x N) regardless of batch size.

Root cause: every autocommit=True INSERT forces a server-side
transaction-log flush. Not a wire-protocol bug.

Numbers:
* executemany(1000) autocommit=True: 1.72 s (1.72 ms/row)
* executemany(1000) in single txn:    32 ms (32 us/row)

53x speedup from changing the transaction boundary, not the driver.
Pure protocol overhead is ~32 us/row -> ~31K rows/sec sustained
throughput on a single connection. Comparable to pg8000.

Added test_executemany_1000_rows_in_txn benchmark to make this
visible. Updated README headline numbers and added a "Performance
gotchas" section explaining when autocommit=False matters.

Decision: don't pipeline. The remaining 32 us is already excellent;
the autocommit gotcha is the real user-facing footgun. Docs > code.
If someone reports needing >31K rows/sec single-connection, that
becomes Phase 22.

CHANGELOG.md

@@ -2,6 +2,39 @@
 
 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.
 
+## 2026.05.04.6 — `executemany` perf finding: it was the autocommit cliff
+
+Investigation of the Phase 21 finding that `executemany(N)` cost scaled linearly per-row (1.74 ms × N) regardless of batch size. **Root cause: every autocommit-True INSERT forces a server-side transaction-log flush.** Not a wire-protocol bug.
+
+### Added
+
+- **`test_executemany_1000_rows_in_txn`** benchmark — same workload, but inside a single transaction with one COMMIT at the end. Isolates pure protocol cost from server-storage cost.
+- New module-scoped `txn_conn` fixture in `tests/benchmarks/test_insert_perf.py` for autocommit-False benchmarks.
+
+### Findings
+
+| Mode | Total | Per row |
+|-|-:|-:|
+| `executemany(1000)` autocommit=True | 1.72 s | 1.72 ms |
+| `executemany(1000)` in single txn | 32 ms | **32 µs** |
+
+**53× speedup from changing the transaction boundary, not the driver.** Pure protocol overhead is ~32 µs/row → ~31,000 rows/sec sustained throughput on a single connection. Comparable to mature pure-Python drivers (pg8000).
+
+### Changed
+
+- **`tests/benchmarks/README.md`** — updated headline numbers to show both modes, added a "Performance gotchas" section explaining when to use `autocommit=False` for bulk loads.
+- **`tests/benchmarks/baseline.json`** — refreshed to include the new txn-mode measurement (now 29 entries, was 28).
+
+### Decision: don't pipeline
+
+Pipelining BIND+EXECUTE PDUs (writing N without waiting for responses between them) could potentially halve the 32 µs/row figure on loopback. Decided against:
+
+- The remaining 32 µs is already excellent — single-connection bulk-load performance is not where users hit limits.
+- Pipelining adds complexity around TCP send-buffer management, partial-failure semantics, and error reporting (which row failed when 50 are in flight).
+- The autocommit gotcha is the *real* user-facing footgun. Better docs > more code.
+
+If someone reports needing >31K rows/sec single-connection, this becomes Phase 22 work.
+
 ## 2026.05.04.5 — Performance benchmarks (Phase 21)
 
 Adds `tests/benchmarks/` — a `pytest-benchmark` driven suite covering codec micro-benchmarks (no server required) and end-to-end SELECT/INSERT/pool/async benchmarks. Establishes a committed `baseline.json` so future PRs can be compared against the floor and regressions caught at review.

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "informix-db"
-version = "2026.05.04.5"
+version = "2026.05.04.6"
 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" }

tests/benchmarks/README.md

@@ -21,22 +21,40 @@ Performance baselines for `informix-db`. Two layers:
 | **Cold connect + close** (login handshake) | **11.2 ms** | **89** |
 | 1000-row SELECT * | 1.56 ms | 640 |
 | INSERT (single, prepared) | 1.88 ms | 530 |
-| `executemany(100 rows)` | 181 ms | 5.5 (i.e. ~550 rows/sec) |
+| `executemany(100)` autocommit=True | 181 ms | ~550 rows/sec |
-| `executemany(1000 rows)` | 1.74 s | 0.57 (i.e. ~575 rows/sec) |
+| `executemany(1000)` autocommit=True | 1.72 s | ~580 rows/sec |
+| **`executemany(1000)` in single transaction** | **32 ms** | **~31,000 rows/sec** |
 
 ### What these tell you
 
 - **Pool gives 72× speedup** over cold connect. If your app opens a
   connection per request, fix that first.
+- **Wrap bulk INSERTs in a transaction.** That's a **53× speedup** over
+  the autocommit-True default. With autocommit on, each row forces the
+  server to flush its transaction log; in transaction mode the flush
+  happens once at COMMIT. Per-row cost drops from 1.72 ms (storage-bound)
+  to 32 µs (pure protocol). PEP 249's default `autocommit=False` was
+  designed for this — we just default to `False`.
 - **Codec is not the bottleneck.** Per-row decode (2.9 µs) is 1000× faster
   than wire round-trip (177 µs for `SELECT 1`). Network and server-side
   cost dominate.
 - **UTF-8 carries no measurable cost.** `decode_varchar_utf8` runs at
   216 ns vs `decode_varchar_short` at 170 ns — the 27% delta is the
   multibyte string walk inherent in UTF-8 decoding, not Phase 20 overhead.
-- **`executemany` doesn't scale linearly.** 100 rows in 181 ms = 1.81 ms/row;
+
-  1000 rows in 1.74 s = 1.74 ms/row. Suggests per-row cost dominates over
+### Performance gotchas
-  PREPARE amortization. Worth investigating in Phase 21.x.
+
+- **`autocommit=True` + `executemany` is the slowest reasonable pattern.**
+  Use it only when each row genuinely needs to land independently. For
+  bulk loads, default `autocommit=False` and call `conn.commit()` at the
+  end of the batch.
+- **Single `INSERT` in a tight loop is 1.88 ms each** — strictly worse
+  than `executemany` (which saves PREPARE/RELEASE overhead). If you find
+  yourself looping over `cur.execute("INSERT...")` hundreds of times,
+  switch to `executemany`.
+- **Cold connect is 11 ms.** The login handshake is *expensive* compared
+  to anything you'll do with the connection. Pool everything in
+  long-lived processes.
 
 ## Regression policy
 

tests/benchmarks/test_insert_perf.py

@@ -3,19 +3,47 @@
 The single-row vs. executemany delta is the ``executemany`` win — we
 PREPARE+RELEASE once and BIND+EXECUTE per row, vs PREPARE+RELEASE per
 row. On any decent network this is 10-50x.
+
+The autocommit-True vs. autocommit-False delta is the **transaction-flush
+cost** — every autocommit INSERT forces the server to flush its
+transaction log per row, drowning out everything else. The benchmark
+splits these so we can see protocol overhead independently.
 """
 
 from __future__ import annotations
 
 import contextlib
+from collections.abc import Iterator
 
 import pytest
 
 import informix_db
+from tests.conftest import ConnParams
 
 pytestmark = [pytest.mark.benchmark, pytest.mark.integration]
 
 
+@pytest.fixture(scope="module")
+def txn_conn(conn_params: ConnParams) -> Iterator[informix_db.Connection]:
+    """A separate connection with autocommit=False so we can wrap an
+    executemany call in a single explicit transaction. Uses ``testdb``
+    (the logged user DB) — autocommit-off is meaningless on unlogged DBs.
+    """
+    conn = informix_db.connect(
+        host=conn_params.host,
+        port=conn_params.port,
+        user=conn_params.user,
+        password=conn_params.password,
+        database="testdb",
+        server=conn_params.server,
+        autocommit=False,
+    )
+    try:
+        yield conn
+    finally:
+        conn.close()
+
+
 def _setup_temp_table(conn: informix_db.Connection, name: str) -> None:
     cur = conn.cursor()
     with contextlib.suppress(informix_db.Error):
@@ -82,7 +110,9 @@ def test_executemany_100_rows(
 def test_executemany_1000_rows(
     benchmark, bench_conn: informix_db.Connection
 ) -> None:
-    """1000 INSERTs via executemany — sustained-batch throughput."""
+    """1000 INSERTs via executemany under autocommit=True — every row
+    forces a transaction-log flush. Worst-case protocol *plus* server
+    storage cost."""
     table = "p21_ins_emany_1000"
     _setup_temp_table(bench_conn, table)
     counter = [0]
@@ -104,3 +134,37 @@ def test_executemany_1000_rows(
         benchmark.pedantic(run, rounds=3, iterations=1)
     finally:
         _drop_temp_table(bench_conn, table)
+
+
+def test_executemany_1000_rows_in_txn(
+    benchmark, txn_conn: informix_db.Connection
+) -> None:
+    """1000 INSERTs via executemany inside ONE transaction — single
+    log flush at COMMIT time. Isolates the protocol cost from the
+    autocommit-flush cost. The delta vs the autocommit variant is the
+    server-side log-flush penalty (un-fixable from the client side)."""
+    table = "p21_ins_emany_txn"
+    _setup_temp_table(txn_conn, table)
+    txn_conn.commit()  # Land the CREATE TABLE before timing
+    counter = [0]
+
+    def run() -> None:
+        counter[0] += 1
+        base = counter[0] * 1000
+        rows = [
+            (base + i, f"row_{base + i}", float(base + i)) for i in range(1000)
+        ]
+        cur = txn_conn.cursor()
+        cur.executemany(
+            f"INSERT INTO {table} VALUES (?, ?, ?)",
+            rows,
+        )
+        cur.close()
+        txn_conn.commit()
+
+    try:
+        benchmark.pedantic(run, rounds=3, iterations=1)
+    finally:
+        with contextlib.suppress(informix_db.Error):
+            _drop_temp_table(txn_conn, table)
+            txn_conn.commit()

uv.lock

@@ -34,7 +34,7 @@ wheels = [
 
 [[package]]
 name = "informix-db"
-version = "2026.5.4.4"
+version = "2026.5.4.6"
 source = { editable = "." }
 
 [package.optional-dependencies]