diff --git a/src/executors/postgres_retention_executor.py b/src/executors/postgres_retention_executor.py
new file mode 100644
index 0000000..2ede15e
--- /dev/null
+++ b/src/executors/postgres_retention_executor.py
@@ -0,0 +1,109 @@
+"""
+Postgres Retention Executor
+
+Deletes rows older than a retention window from a table on the shared Postgres
+server. Written for sysmon's `check_history`, which grows with every monitoring
+check and had no retention at all, but the executor is table-agnostic.
+
+Connects with the Scheduler's own Postgres credentials and only overrides the
+database name. That keeps a second set of credentials out of the stack; the
+target database grants `scheduler_user` exactly SELECT and DELETE on the table,
+so a bug here can drop old rows but cannot corrupt or forge history.
+
+Config schema:
+    {
+        "database": "sysmon",           # defaults to the Scheduler's own database
+        "table": "check_history",       # required
+        "timestamp_column": "ts",       # required
+        "retention_days": 30,           # required, must be >= 1
+        "dry_run": false                # count what would go, delete nothing
+    }
+
+Table and column names cannot be passed as query parameters, so both are
+validated against a strict identifier pattern before being interpolated.
+
+Autovacuum reclaims the space afterwards; this deliberately does not VACUUM,
+which would need table ownership the Scheduler intentionally does not have.
+"""
+import asyncio
+import logging
+import re
+from typing import Any, Dict
+
+import psycopg2
+
+from src.config import Settings
+
+logger = logging.getLogger(__name__)
+
+# Deliberately strict: unquoted lowercase identifiers only. Anything needing
+# quoting is out of scope and would be a hole in the interpolation below.
+IDENTIFIER_RE = re.compile(r"^[a-z_][a-z0-9_]*$")
+
+MAX_RETENTION_DAYS = 3650
+
+
+def _validate_identifier(value: str, label: str) -> str:
+    if not isinstance(value, str) or not IDENTIFIER_RE.match(value):
+        raise ValueError(
+            f"invalid {label}: {value!r} (expected an unquoted lowercase identifier)"
+        )
+    return value
+
+
+def _prune(config: Dict[str, Any], settings: Settings) -> str:
+    table = _validate_identifier(config.get("table", ""), "table")
+    column = _validate_identifier(config.get("timestamp_column", ""), "timestamp_column")
+    database = config.get("database") or settings.postgres_db
+    _validate_identifier(database, "database")
+
+    retention_days = config.get("retention_days")
+    if not isinstance(retention_days, int) or isinstance(retention_days, bool):
+        raise ValueError(f"retention_days must be an integer, got {retention_days!r}")
+    # A zero or negative window would delete everything, including the row the
+    # check just wrote. Refuse rather than quietly wipe the table.
+    if retention_days < 1 or retention_days > MAX_RETENTION_DAYS:
+        raise ValueError(
+            f"retention_days must be between 1 and {MAX_RETENTION_DAYS}, got {retention_days}"
+        )
+
+    dry_run = bool(config.get("dry_run", False))
+    cutoff_sql = f"{column} < now() - make_interval(days => %s)"
+
+    conn = psycopg2.connect(
+        host=settings.postgres_host,
+        port=settings.postgres_port,
+        database=database,
+        user=settings.postgres_user,
+        password=settings.postgres_password,
+        connect_timeout=10,
+    )
+    try:
+        with conn:
+            with conn.cursor() as cur:
+                cur.execute(f"SELECT count(*) FROM {table} WHERE {cutoff_sql}", (retention_days,))
+                stale = cur.fetchone()[0]
+
+                if dry_run:
+                    logger.info("dry run: %s rows in %s.%s exceed %sd", stale, database, table, retention_days)
+                    return f"dry run: {stale} rows older than {retention_days}d in {database}.{table}"
+
+                if stale == 0:
+                    return f"nothing to prune in {database}.{table} (retention {retention_days}d)"
+
+                cur.execute(f"DELETE FROM {table} WHERE {cutoff_sql}", (retention_days,))
+                deleted = cur.rowcount
+
+                cur.execute(f"SELECT count(*) FROM {table}")
+                remaining = cur.fetchone()[0]
+    finally:
+        conn.close()
+
+    logger.info("pruned %s rows from %s.%s, %s remain", deleted, database, table, remaining)
+    return f"pruned {deleted} rows older than {retention_days}d from {database}.{table}, {remaining} remain"
+
+
+async def execute(config: dict, settings: Settings) -> str:
+    """Delete rows past the retention window. Returns a one-line summary."""
+    # psycopg2 is synchronous; keep it off the scheduler's event loop.
+    return await asyncio.to_thread(_prune, config, settings)
diff --git a/tests/test_postgres_retention_executor.py b/tests/test_postgres_retention_executor.py
new file mode 100644
index 0000000..18539a3
--- /dev/null
+++ b/tests/test_postgres_retention_executor.py
@@ -0,0 +1,151 @@
+"""
+Tests for the postgres retention executor.
+
+Focus is on the guards. The executor interpolates a table and column name
+straight into SQL (they cannot be bound as parameters), and it issues DELETEs
+against a live table, so the validation in front of both is what keeps a
+malformed config from becoming data loss.
+"""
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from src.config import Settings
+from src.executors import postgres_retention_executor as retention
+
+
+@pytest.mark.executor
+@pytest.mark.unit
+class TestIdentifierValidation:
+    """Table/column/database names are interpolated, so they must be rejected early."""
+
+    @pytest.mark.parametrize(
+        "bad",
+        [
+            "check_history; DROP TABLE users",
+            'check_history"',
+            "check history",
+            "Check_History",   # uppercase would need quoting to resolve
+            "1_history",
+            "",
+            "--comment",
+        ],
+    )
+    def test_rejects_unsafe_identifiers(self, bad):
+        with pytest.raises(ValueError):
+            retention._validate_identifier(bad, "table")
+
+    @pytest.mark.parametrize("good", ["check_history", "ts", "_private", "a1"])
+    def test_accepts_plain_identifiers(self, good):
+        assert retention._validate_identifier(good, "table") == good
+
+
+@pytest.mark.executor
+@pytest.mark.unit
+class TestRetentionGuards:
+    """A bad retention window must never reach the database."""
+
+    def _config(self, **overrides):
+        config = {
+            "database": "sysmon",
+            "table": "check_history",
+            "timestamp_column": "ts",
+            "retention_days": 30,
+        }
+        config.update(overrides)
+        return config
+
+    @pytest.mark.parametrize("days", [0, -1, -30, 3651])
+    def test_rejects_out_of_range_retention(self, days, test_settings: Settings):
+        # 0 or negative would delete every row including the one just written.
+        with patch("psycopg2.connect") as connect:
+            with pytest.raises(ValueError):
+                retention._prune(self._config(retention_days=days), test_settings)
+            connect.assert_not_called()
+
+    @pytest.mark.parametrize("days", ["30", None, 1.5, True])
+    def test_rejects_non_integer_retention(self, days, test_settings: Settings):
+        with patch("psycopg2.connect") as connect:
+            with pytest.raises(ValueError):
+                retention._prune(self._config(retention_days=days), test_settings)
+            connect.assert_not_called()
+
+    def test_rejects_injection_in_table_before_connecting(self, test_settings: Settings):
+        with patch("psycopg2.connect") as connect:
+            with pytest.raises(ValueError):
+                retention._prune(
+                    self._config(table="check_history; DELETE FROM check_history --"),
+                    test_settings,
+                )
+            connect.assert_not_called()
+
+
+@pytest.mark.executor
+@pytest.mark.unit
+class TestRetentionBehaviour:
+    """Behaviour against a mocked cursor."""
+
+    def _mock_conn(self, counts):
+        cursor = MagicMock()
+        cursor.fetchone.side_effect = [(c,) for c in counts]
+        cursor.rowcount = counts[0] if counts else 0
+        conn = MagicMock()
+        conn.cursor.return_value.__enter__.return_value = cursor
+        conn.__enter__.return_value = conn
+        return conn, cursor
+
+    def _config(self, **overrides):
+        config = {
+            "database": "sysmon",
+            "table": "check_history",
+            "timestamp_column": "ts",
+            "retention_days": 30,
+        }
+        config.update(overrides)
+        return config
+
+    def test_dry_run_does_not_delete(self, test_settings: Settings):
+        conn, cursor = self._mock_conn([7])
+        with patch("psycopg2.connect", return_value=conn):
+            result = retention._prune(self._config(dry_run=True), test_settings)
+
+        assert "dry run" in result
+        assert "7" in result
+        executed = " ".join(str(c) for c in cursor.execute.call_args_list)
+        assert "DELETE" not in executed.upper()
+
+    def test_no_stale_rows_skips_delete(self, test_settings: Settings):
+        conn, cursor = self._mock_conn([0])
+        with patch("psycopg2.connect", return_value=conn):
+            result = retention._prune(self._config(), test_settings)
+
+        assert "nothing to prune" in result
+        executed = " ".join(str(c) for c in cursor.execute.call_args_list)
+        assert "DELETE" not in executed.upper()
+
+    def test_deletes_and_reports(self, test_settings: Settings):
+        # count(stale) -> 5, then count(remaining) -> 42
+        conn, cursor = self._mock_conn([5, 42])
+        cursor.rowcount = 5
+        with patch("psycopg2.connect", return_value=conn):
+            result = retention._prune(self._config(), test_settings)
+
+        assert "pruned 5 rows" in result
+        assert "42 remain" in result
+        executed = " ".join(str(c) for c in cursor.execute.call_args_list)
+        assert "DELETE" in executed.upper()
+
+    def test_defaults_to_scheduler_database(self, test_settings: Settings):
+        conn, _ = self._mock_conn([0])
+        config = self._config()
+        del config["database"]
+        with patch("psycopg2.connect", return_value=conn) as connect:
+            retention._prune(config, test_settings)
+        assert connect.call_args.kwargs["database"] == test_settings.postgres_db
+
+    @pytest.mark.asyncio
+    async def test_execute_wraps_prune(self, test_settings: Settings):
+        conn, _ = self._mock_conn([0])
+        with patch("psycopg2.connect", return_value=conn):
+            result = await retention.execute(self._config(), test_settings)
+        assert "nothing to prune" in result