fix(dwh): align adapter sampling contract

docs/superpowers/plans/2026-07-11-adapter-foundations.md

@@ -26,7 +26,7 @@
 - Test: `harness/tests/test_dwh_port_contract.py`
 
 **Interfaces:**
-- Produces: `DwhCapabilities`, `DwhAdapter`, `DwhHealth`, and `UnsupportedCapability`.
+- Produces: `DwhCapabilities`, `DwhAdapter`, `DwhHealth`, `DistinctValues`, and `UnsupportedCapability`.
 - Consumes: existing catalog models from `tht.db.introspect` and execution result types from `tht.db.execute`.
 
 - [ ] **Step 1: Write the failing protocol-shape test**
@@ -54,16 +54,21 @@ class DwhCapabilities:
     sampling: bool = True
     distinct_values: bool = True
 
+@dataclass(frozen=True)
+class DistinctValues:
+    values: list[object]
+    truncated: bool
+
 @runtime_checkable
 class DwhAdapter(Protocol):
     @property
     def capabilities(self) -> DwhCapabilities: ...
     def health(self) -> DwhHealth: ...
-    def introspect(self) -> DatabaseCatalog: ...
-    def run_query(self, sql: str, *, limit: int | None = None) -> QueryResult: ...
+    def introspect(self) -> PhysicalSchema: ...
+    def run_query(self, sql: str, *, limit: int) -> ExecResult: ...
     def explain(self, sql: str) -> PlanSummary: ...
     def sample_column(self, table: str, column: str, *, limit: int) -> list[object]: ...
-    def distinct_values(self, table: str, column: str) -> list[object]: ...
+    def distinct_values(self, table: str, column: str) -> DistinctValues: ...
 ```
 
 - [ ] **Step 4: Run contract test and type-oriented import smoke test**
@@ -97,8 +102,8 @@ git commit -m "refactor(dwh): define adapter contract"
 ```python
 @pytest.mark.parametrize("factory", [postgres_factory, rest_factory])
 def test_adapter_rejects_write_sql(factory):
-    with pytest.raises(ReadOnlyViolation):
-        factory().run_query("delete from fact_sales")
+    with pytest.raises(ExecutionError):
+        factory().run_query("delete from fact_sales", limit=10)
 ```
 
 - [ ] **Step 2: Verify failure**
@@ -111,12 +116,17 @@ Expected: FAIL because the adapter classes are absent.
 ```python
 class PostgresDwhAdapter:
     capabilities = DwhCapabilities()
-    def __init__(self, config: DatabaseConfig): self._config = config
-    def run_query(self, sql: str, *, limit: int | None = None) -> QueryResult:
-        return run_query(self._config, sql, limit=limit)
+    def __init__(self, config: DatabaseConfig):
+        self._config = config
+        self._engine = make_engine(config)
+    def run_query(self, sql: str, *, limit: int) -> ExecResult:
+        return run_query(self._engine, sql, limit=limit)
 ```
 
-Implement the analogous REST wrapper by delegating to `tht.rest.*`; translate transport-specific errors only at the adapter boundary.
+Implement the analogous REST wrapper by delegating to `tht.rest.*`; translate transport-specific
+errors only at the adapter boundary. Both wrappers delegate frequency-ranked, distinct sampling to
+the paired implementations in `tht.db.sampling`. A non-positive query limit is rejected, and
+`distinct_values` reports any cap through `DistinctValues.truncated`.
 
 - [ ] **Step 4: Run adapter, read-only, sampling, and REST tests**
 

harness/tests/l0/test_db_sampling.py

@@ -6,7 +6,7 @@ import pytest
 
 from tht.config import LshConfig
 from tht.db.introspect import introspect
-from tht.db.sampling import is_text_type, unique_values_for_lsh
+from tht.db.sampling import distinct_values, is_text_type, sample_column, unique_values_for_lsh
 
 pytestmark = [pytest.mark.l0]
 
@@ -52,3 +52,16 @@ def test_unique_values_for_lsh_truncation_reported(admin_engine):
     truncated_cols = {(t.table, t.column) for t in truncated}
     # fct_ricoveri has several eligible text columns with distinct values
     assert any(t[0] == "fct_ricoveri" for t in truncated_cols)
+
+
+def test_adapter_sampling_is_distinct_and_frequency_ranked(admin_engine):
+    values = sample_column(admin_engine, "dw", "fct_ricoveri", "reparto", limit=2)
+    assert values == ["cardiologia", "pronto soccorso"]
+
+
+def test_adapter_distinct_values_reports_truncation(admin_engine):
+    result = distinct_values(
+        admin_engine, "dw", "fct_ricoveri", "reparto", max_values=1
+    )
+    assert result.values == ["cardiologia"]
+    assert result.truncated is True

harness/tests/test_dwh_adapters.py

@@ -1,8 +1,10 @@
 import pytest
+from sqlalchemy.exc import OperationalError
 
 from tht.config import DatabaseConfig, RestConfig
 from tht.execute import ExecutionError
-from tht.ports import DwhAdapter
+from tht.ports import DistinctValues, DwhAdapter
+from tht.rest.client import RestError
 
 
 def postgres_factory():
@@ -32,7 +34,7 @@ def rest_factory():
 @pytest.mark.parametrize("factory", [postgres_factory, rest_factory])
 def test_adapter_rejects_write_sql_without_using_transport(factory):
     with pytest.raises(ExecutionError, match="read-only enforcement"):
-        factory().run_query("delete from fact_sales")
+        factory().run_query("delete from fact_sales", limit=10)
 
 
 @pytest.mark.parametrize("factory", [postgres_factory, rest_factory])
@@ -40,3 +42,85 @@ def test_adapter_satisfies_dwh_protocol(factory):
     adapter = factory()
     assert isinstance(adapter, DwhAdapter)
     assert adapter.capabilities.introspection is True
+
+
+@pytest.mark.parametrize("factory", [postgres_factory, rest_factory])
+def test_run_query_requires_explicit_positive_limit(factory):
+    adapter = factory()
+    with pytest.raises(TypeError):
+        adapter.run_query("select 1")
+    with pytest.raises(ValueError, match="positive"):
+        adapter.run_query("select 1", limit=0)
+
+
+def test_postgres_sampling_delegates_to_paired_sampling_functions(monkeypatch):
+    adapter = postgres_factory()
+    calls = []
+    expected = DistinctValues(values=["A"], truncated=True)
+
+    monkeypatch.setattr(
+        "tht.adapters.dwh.postgres.sampling.sample_column",
+        lambda engine, schema, table, column, *, limit: calls.append(
+            (engine, schema, table, column, limit)
+        )
+        or ["A", "B"],
+    )
+    monkeypatch.setattr(
+        "tht.adapters.dwh.postgres.sampling.distinct_values",
+        lambda engine, schema, table, column: expected,
+    )
+
+    assert adapter.sample_column("sales", "region", limit=2) == ["A", "B"]
+    assert calls == [(adapter._engine, "dw", "sales", "region", 2)]
+    assert adapter.distinct_values("sales", "region") is expected
+
+
+def test_rest_sampling_delegates_and_translates_transport_errors(monkeypatch):
+    adapter = rest_factory()
+    expected = DistinctValues(values=["A", "B"], truncated=False)
+    monkeypatch.setattr(
+        "tht.adapters.dwh.thoth_rest.sampling.sample_column_rest",
+        lambda client, schema, table, column, *, limit: ["A", "B"],
+    )
+    monkeypatch.setattr(
+        "tht.adapters.dwh.thoth_rest.sampling.distinct_values_rest",
+        lambda client, schema, table, column: expected,
+    )
+    assert adapter.sample_column("sales", "region", limit=2) == ["A", "B"]
+    assert adapter.distinct_values("sales", "region") is expected
+
+    def fail(*args, **kwargs):
+        raise RestError("transport failed")
+
+    monkeypatch.setattr("tht.adapters.dwh.thoth_rest.sampling.sample_column_rest", fail)
+    monkeypatch.setattr("tht.adapters.dwh.thoth_rest.sampling.distinct_values_rest", fail)
+    with pytest.raises(ExecutionError, match="transport failed"):
+        adapter.sample_column("sales", "region", limit=2)
+    with pytest.raises(ExecutionError, match="transport failed"):
+        adapter.distinct_values("sales", "region")
+
+
+def test_rest_distinct_values_reports_transport_truncation():
+    from tht.db.sampling import distinct_values_rest
+
+    class Client:
+        def top_values(self, schema, table, column, limit):
+            assert (schema, table, column, limit) == ("dw", "sales", "region", 3)
+            return [{"value": "A"}, {"value": "B"}, {"value": "C"}]
+
+    result = distinct_values_rest(Client(), "dw", "sales", "region", max_values=2)
+    assert result == DistinctValues(values=["A", "B"], truncated=True)
+
+
+def test_postgres_health_only_normalizes_database_errors(monkeypatch):
+    adapter = postgres_factory()
+    database_error = OperationalError("select 1", {}, Exception("offline"))
+    monkeypatch.setattr("tht.adapters.dwh.postgres.ping", lambda engine: (_ for _ in ()).throw(database_error))
+    assert adapter.health().ok is False
+
+    monkeypatch.setattr(
+        "tht.adapters.dwh.postgres.ping",
+        lambda engine: (_ for _ in ()).throw(ValueError("programming bug")),
+    )
+    with pytest.raises(ValueError, match="programming bug"):
+        adapter.health()

harness/tests/test_dwh_port_contract.py

@@ -1,9 +1,14 @@
+from dataclasses import FrozenInstanceError
+
+import pytest
+
 from tht.execute import ExecResult, PlanSummary
 from tht.mschema.models import PhysicalSchema
 from tht.ports.dwh import (
     DwhAdapter,
     DwhCapabilities,
     DwhHealth,
+    DistinctValues,
     UnsupportedCapability,
 )
 
@@ -17,7 +22,7 @@ class FakeDwhAdapter:
     def introspect(self) -> PhysicalSchema:
         raise NotImplementedError
 
-    def run_query(self, sql: str, *, limit: int | None = None) -> ExecResult:
+    def run_query(self, sql: str, *, limit: int) -> ExecResult:
         raise NotImplementedError
 
     def explain(self, sql: str) -> PlanSummary:
@@ -26,7 +31,7 @@ class FakeDwhAdapter:
     def sample_column(self, table: str, column: str, *, limit: int) -> list[object]:
         raise NotImplementedError
 
-    def distinct_values(self, table: str, column: str) -> list[object]:
+    def distinct_values(self, table: str, column: str) -> DistinctValues:
         raise NotImplementedError
 
 
@@ -45,3 +50,13 @@ def test_contract_types_are_public_and_capabilities_are_immutable():
     assert capabilities.sampling is True
     assert capabilities.distinct_values is True
     assert issubclass(UnsupportedCapability, Exception)
+    with pytest.raises(FrozenInstanceError):
+        capabilities.explain = False
+
+
+def test_all_contract_types_are_exported_from_public_package():
+    from tht.ports import DistinctValues as PublicDistinctValues
+
+    result = PublicDistinctValues(values=["a"], truncated=True)
+    assert result.values == ["a"]
+    assert result.truncated is True

harness/tht/adapters/dwh/postgres.py

@@ -1,12 +1,14 @@
 """Direct PostgreSQL implementation of the DWH port."""
 
 from tht.config import DatabaseConfig
-from tht.db import execute
+from sqlalchemy.exc import SQLAlchemyError
+
+from tht.db import execute, sampling
 from tht.db.connection import make_engine, ping
 from tht.db.introspect import introspect
 from tht.execute import ExecResult, PlanSummary
 from tht.mschema.models import PhysicalSchema
-from tht.ports.dwh import DwhCapabilities, DwhHealth
+from tht.ports.dwh import DistinctValues, DwhCapabilities, DwhHealth
 
 
 class PostgresDwhAdapter:
@@ -19,23 +21,25 @@ class PostgresDwhAdapter:
     def health(self) -> DwhHealth:
         try:
             ping(self._engine)
-        except Exception as exc:
+        except SQLAlchemyError as exc:
             return DwhHealth(ok=False, detail=str(exc))
         return DwhHealth(ok=True)
 
     def introspect(self) -> PhysicalSchema:
         return introspect(self._engine, self._config.database, self._config.db_schema)
 
-    def run_query(self, sql: str, *, limit: int | None = None) -> ExecResult:
+    def run_query(self, sql: str, *, limit: int) -> ExecResult:
         return execute.run_query(self._engine, sql, limit=limit)
 
     def explain(self, sql: str) -> PlanSummary:
         return execute.explain(self._engine, sql)
 
     def sample_column(self, table: str, column: str, *, limit: int) -> list[object]:
-        return execute.sample_column(
+        return sampling.sample_column(
             self._engine, self._config.db_schema, table, column, limit=limit
         )
 
-    def distinct_values(self, table: str, column: str) -> list[object]:
-        return execute.distinct_values(self._engine, self._config.db_schema, table, column)
+    def distinct_values(self, table: str, column: str) -> DistinctValues:
+        return sampling.distinct_values(
+            self._engine, self._config.db_schema, table, column
+        )

harness/tht/adapters/dwh/thoth_rest.py

@@ -2,16 +2,12 @@
 
 from tht.config import DatabaseConfig, RestConfig
 from tht.db.introspect import introspect_rest
-from tht.execute import ExecResult, PlanSummary
+from tht.db import sampling
+from tht.execute import ExecResult, ExecutionError, PlanSummary
 from tht.mschema.models import PhysicalSchema
-from tht.ports.dwh import DwhCapabilities, DwhHealth
+from tht.ports.dwh import DistinctValues, DwhCapabilities, DwhHealth
 from tht.rest.client import RestClient, RestError
-from tht.rest.execute import (
-    distinct_values_rest,
-    explain_rest,
-    run_controlled_rest,
-    sample_column_rest,
-)
+from tht.rest.execute import explain_rest, run_controlled_rest
 
 
 class ThothRestDwhAdapter:
@@ -34,18 +30,24 @@ class ThothRestDwhAdapter:
             self._client, self._database.database, self._database.db_schema
         )
 
-    def run_query(self, sql: str, *, limit: int | None = None) -> ExecResult:
-        return run_controlled_rest(self._client, sql, limit=10 if limit is None else limit)
+    def run_query(self, sql: str, *, limit: int) -> ExecResult:
+        return run_controlled_rest(self._client, sql, limit=limit)
 
     def explain(self, sql: str) -> PlanSummary:
         return explain_rest(self._client, sql)
 
     def sample_column(self, table: str, column: str, *, limit: int) -> list[object]:
-        return sample_column_rest(
-            self._client, self._database.db_schema, table, column, limit=limit
-        )
+        try:
+            return sampling.sample_column_rest(
+                self._client, self._database.db_schema, table, column, limit=limit
+            )
+        except RestError as exc:
+            raise ExecutionError(str(exc)) from exc
 
-    def distinct_values(self, table: str, column: str) -> list[object]:
-        return distinct_values_rest(
-            self._client, self._database.db_schema, table, column
-        )
+    def distinct_values(self, table: str, column: str) -> DistinctValues:
+        try:
+            return sampling.distinct_values_rest(
+                self._client, self._database.db_schema, table, column
+            )
+        except RestError as exc:
+            raise ExecutionError(str(exc)) from exc

harness/tht/db/execute.py

@@ -4,39 +4,19 @@ from sqlalchemy import Engine
 
 from tht.execute import ExecResult, PlanSummary, explain as _explain, run_controlled
 
-DEFAULT_LIMIT = 10
 DEFAULT_TIMEOUT_MS = 30_000
 
 
-def run_query(engine: Engine, sql: str, *, limit: int | None = None) -> ExecResult:
+def run_query(engine: Engine, sql: str, *, limit: int) -> ExecResult:
+    if limit <= 0:
+        raise ValueError("limit must be a positive integer")
     return run_controlled(
         engine,
         sql,
-        limit=DEFAULT_LIMIT if limit is None else limit,
+        limit=limit,
         timeout_ms=DEFAULT_TIMEOUT_MS,
     )
 
 
 def explain(engine: Engine, sql: str) -> PlanSummary:
     return _explain(engine, sql, timeout_ms=DEFAULT_TIMEOUT_MS)
-
-
-def sample_column(
-    engine: Engine, schema: str, table: str, column: str, *, limit: int
-) -> list[object]:
-    quote = engine.dialect.identifier_preparer.quote
-    sql = (
-        f"SELECT {quote(column)} FROM {quote(schema)}.{quote(table)} "
-        f"WHERE {quote(column)} IS NOT NULL"
-    )
-    return [row[0] for row in run_query(engine, sql, limit=limit).rows]
-
-
-def distinct_values(engine: Engine, schema: str, table: str, column: str) -> list[object]:
-    quote = engine.dialect.identifier_preparer.quote
-    sql = (
-        f"SELECT DISTINCT {quote(column)} FROM {quote(schema)}.{quote(table)} "
-        f"WHERE {quote(column)} IS NOT NULL ORDER BY {quote(column)}"
-    )
-    result = run_controlled(engine, sql, limit=1000, timeout_ms=DEFAULT_TIMEOUT_MS)
-    return [row[0] for row in result.rows]

harness/tht/db/sampling.py

@@ -5,10 +5,66 @@ from sqlalchemy import Engine, text
 
 from tht.config import ExamplesConfig, LshConfig
 from tht.mschema.models import Annotations, PhysicalSchema
+from tht.ports.dwh import DistinctValues
 
 logger = logging.getLogger(__name__)
 
 TEXT_TYPE_PREFIXES = ("text", "varchar", "character", "char")
+DEFAULT_DISTINCT_VALUES_LIMIT = 1000
+
+
+def _quoted_top_values_query(engine: Engine, schema: str, table: str, column: str):
+    quote = engine.dialect.identifier_preparer.quote
+    identifier = quote(column)
+    return text(
+        f"SELECT {identifier} AS value FROM {quote(schema)}.{quote(table)} "
+        f"WHERE {identifier} IS NOT NULL GROUP BY {identifier} "
+        f"ORDER BY count(*) DESC, {identifier} LIMIT :lim"
+    )
+
+
+def sample_column(
+    engine: Engine, schema: str, table: str, column: str, *, limit: int
+) -> list[object]:
+    if limit <= 0:
+        raise ValueError("limit must be a positive integer")
+    query = _quoted_top_values_query(engine, schema, table, column)
+    with engine.connect() as conn:
+        rows = conn.execute(query, {"lim": limit}).fetchall()
+    return [row[0] for row in rows]
+
+
+def sample_column_rest(
+    client, schema: str, table: str, column: str, *, limit: int
+) -> list[object]:
+    if limit <= 0:
+        raise ValueError("limit must be a positive integer")
+    rows = client.top_values(schema, table, column, limit)
+    return [row["value"] for row in rows if row.get("value") is not None]
+
+
+def distinct_values(
+    engine: Engine,
+    schema: str,
+    table: str,
+    column: str,
+    *,
+    max_values: int = DEFAULT_DISTINCT_VALUES_LIMIT,
+) -> DistinctValues:
+    values = sample_column(engine, schema, table, column, limit=max_values + 1)
+    return DistinctValues(values=values[:max_values], truncated=len(values) > max_values)
+
+
+def distinct_values_rest(
+    client,
+    schema: str,
+    table: str,
+    column: str,
+    *,
+    max_values: int = DEFAULT_DISTINCT_VALUES_LIMIT,
+) -> DistinctValues:
+    values = sample_column_rest(client, schema, table, column, limit=max_values + 1)
+    return DistinctValues(values=values[:max_values], truncated=len(values) > max_values)
 
 
 def is_text_type(pg_type: str) -> bool:

harness/tht/ports/__init__.py

@@ -4,6 +4,7 @@ from tht.ports.dwh import (
     DwhAdapter,
     DwhCapabilities,
     DwhHealth,
+    DistinctValues,
     UnsupportedCapability,
 )
 
@@ -11,5 +12,6 @@ __all__ = [
     "DwhAdapter",
     "DwhCapabilities",
     "DwhHealth",
+    "DistinctValues",
     "UnsupportedCapability",
 ]

harness/tht/ports/dwh.py

@@ -21,6 +21,12 @@ class DwhHealth:
     detail: str | None = None
 
 
+@dataclass(frozen=True)
+class DistinctValues:
+    values: list[object]
+    truncated: bool
+
+
 class UnsupportedCapability(Exception):
     """Raised when an adapter cannot provide an optional DWH operation."""
 
@@ -34,10 +40,10 @@ class DwhAdapter(Protocol):
 
     def introspect(self) -> PhysicalSchema: ...
 
-    def run_query(self, sql: str, *, limit: int | None = None) -> ExecResult: ...
+    def run_query(self, sql: str, *, limit: int) -> ExecResult: ...
 
     def explain(self, sql: str) -> PlanSummary: ...
 
     def sample_column(self, table: str, column: str, *, limit: int) -> list[object]: ...
 
-    def distinct_values(self, table: str, column: str) -> list[object]: ...
+    def distinct_values(self, table: str, column: str) -> DistinctValues: ...

harness/tht/rest/execute.py

@@ -17,6 +17,8 @@ from tht.rest.explain import parse_text_plan
 def run_controlled_rest(client, sql: str, *, limit: int) -> ExecResult:
     # Guard read-only client-side anche sul path REST (D7): non delegare l'unica verifica
     # al server. Stesso check strutturale del path diretto.
+    if limit <= 0:
+        raise ValueError("limit must be a positive integer")
     assert_read_only(sql)
     final_sql, injected = _inject_limit(sql, limit)
     start = time.monotonic()
@@ -46,14 +48,3 @@ def explain_rest(client, sql: str) -> PlanSummary:
     except RestError as e:
         raise ExecutionError(str(e)) from e
     return parse_text_plan(lines)
-
-
-def sample_column_rest(
-    client, schema: str, table: str, column: str, *, limit: int
-) -> list[object]:
-    rows = client.top_values(schema, table, column, limit)
-    return [row["value"] for row in rows if row.get("value") is not None]
-
-
-def distinct_values_rest(client, schema: str, table: str, column: str) -> list[object]:
-    return sample_column_rest(client, schema, table, column, limit=1000)