fix(dwh): align adapter sampling contract

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: