Support for SAMPLE BY in schema creation, SAMPLE clause, and toStartOfDay datetime function

clickhouse_backend/backend/schema.py

@@ -254,13 +254,16 @@ def _model_extra_sql(self, model, engine):
             order_by = engine.order_by
             partition_by = engine.partition_by
             primary_key = engine.primary_key
+            sample_by = engine.sample_by
 
             if order_by is not None:
                 yield "ORDER BY (%s)" % self._get_expression(model, *order_by)
             if partition_by:
                 yield "PARTITION BY (%s)" % self._get_expression(model, *partition_by)
             if primary_key is not None:
                 yield "PRIMARY KEY (%s)" % self._get_expression(model, *primary_key)
+            if sample_by is not None:
+                yield "SAMPLE BY (%s)" % self._get_expression(model, *sample_by)
         if engine.settings:
             result = []
             for setting, value in engine.settings.items():

clickhouse_backend/models/engines.py

@@ -128,6 +128,7 @@ def __init__(
         order_by=None,
         partition_by=None,
         primary_key=None,
+        sample_by=None,
         **settings,
     ):
         assert (
@@ -136,8 +137,9 @@ def __init__(
         self.order_by = order_by
         self.primary_key = primary_key
         self.partition_by = partition_by
+        self.sample_by = sample_by
 
-        for key in ["order_by", "primary_key", "partition_by"]:
+        for key in ["order_by", "primary_key", "partition_by", "sample_by"]:
             value = getattr(self, key)
             if value is not None:
                 if isinstance(value, str) or not isinstance(value, Iterable):

clickhouse_backend/models/functions/datetime.py

@@ -11,6 +11,7 @@
     "toStartOfTenMinutes",
     "toStartOfFifteenMinutes",
     "toStartOfHour",
+    "toStartOfDay",
     "toYYYYMM",
     "toYYYYMMDD",
     "toYYYYMMDDhhmmss",
@@ -80,6 +81,10 @@ class toStartOfHour(toStartOfMinute):
     pass
 
 
+class toStartOfDay(toStartOfMinute):
+    pass
+
+
 class toYearWeek(Func):
     output_field = fields.UInt32Field()
 

clickhouse_backend/models/query.py

@@ -31,6 +31,11 @@ def prewhere(self, *args, **kwargs):
         clone._query.add_prewhere(Q(*args, **kwargs))
         return clone
 
+    def sample(self, sample_fraction, sample_offset=None):
+        clone = self._chain()
+        clone._query.add_sample(sample_fraction, sample_offset)
+        return clone
+
     def datetimes(self, field_name, kind, order="ASC", tzinfo=None):
         """
         Return a list of datetime objects representing all available

clickhouse_backend/models/sql/compiler.py

@@ -122,6 +122,8 @@ def as_sql(self, with_limits=True, with_col_aliases=False):
         refcounts_before = self.query.alias_refcount.copy()
         try:
             combinator = self.query.combinator
+            sample_fraction = self.query.sample_fraction
+            sample_offset = self.query.sample_offset
             if compat.dj_ge42:
                 extra_select, order_by, group_by = self.pre_sql_setup(
                     with_col_aliases=with_col_aliases or bool(combinator),
@@ -203,6 +205,13 @@ def as_sql(self, with_limits=True, with_col_aliases=False):
                     result += ["FROM", *from_]
                 params.extend(f_params)
 
+                if sample_fraction:
+                    if sample_offset:
+                        sample_sql = "SAMPLE %s OFFSET %s" % (sample_fraction, sample_offset)
+                    else:
+                        sample_sql = "SAMPLE %s" % sample_fraction
+                    result.append(sample_sql)
+
                 if prewhere:
                     result.append("PREWHERE %s" % prewhere)
                     params.extend(p_params)

clickhouse_backend/models/sql/query.py

@@ -19,6 +19,8 @@ def __init__(self, model, where=query.WhereNode, alias_cols=True):
             super().__init__(model, where, alias_cols)
         self.setting_info = {}
         self.prewhere = query.WhereNode()
+        self.sample_fraction = None
+        self.sample_offset = None
 
     def sql_with_params(self):
         """Choose the right db when database router is used."""
@@ -28,6 +30,8 @@ def clone(self):
         obj = super().clone()
         obj.setting_info = self.setting_info.copy()
         obj.prewhere = self.prewhere.clone()
+        obj.sample_fraction = self.sample_fraction
+        obj.sample_offset = self.sample_offset
         return obj
 
     def explain(self, using, format=None, type=None, **settings):
@@ -36,6 +40,10 @@ def explain(self, using, format=None, type=None, **settings):
         compiler = q.get_compiler(using=using)
         return "\n".join(compiler.explain_query())
 
+    def add_sample(self, sample_fraction, sample_offset):
+        self.sample_fraction = sample_fraction
+        self.sample_offset = sample_offset
+
     def add_prewhere(self, q_object):
         """
         refer add_q