feat: add DataFrame.peek() as an efficient alternative to head() …

…results preview (#318)

* feat: add efficient peek dataframe preview

* add force parameter to peek to cache full dataframe

* add df.peek docstring

* set peek to default force=False

* update peek docstring and error type

---------

Co-authored-by: Tim Swast <[email protected]>

bigframes/core/__init__.py

@@ -106,10 +106,10 @@ def get_column_type(self, key: str) -> bigframes.dtypes.Dtype:
         return self._compile_ordered().get_column_type(key)
 
     def _compile_ordered(self) -> compiling.OrderedIR:
-        return compiling.compile_ordered(self.node)
+        return compiling.compile_ordered_ir(self.node)
 
     def _compile_unordered(self) -> compiling.UnorderedIR:
-        return compiling.compile_unordered(self.node)
+        return compiling.compile_unordered_ir(self.node)
 
     def row_count(self) -> ArrayValue:
         """Get number of rows in ArrayValue as a single-entry ArrayValue."""

bigframes/core/blocks.py

@@ -432,8 +432,18 @@ def to_pandas(
                 downsampling=sampling, ordered=ordered
             )
         )
+        df.set_axis(self.column_labels, axis=1, copy=False)
         return df, query_job
 
+    def try_peek(self, n: int = 20) -> typing.Optional[pd.DataFrame]:
+        if self.expr.node.peekable:
+            iterator, _ = self.session._peek(self.expr, n)
+            df = self._to_dataframe(iterator)
+            self._copy_index_to_pandas(df)
+            return df
+        else:
+            return None
+
     def to_pandas_batches(self):
         """Download results one message at a time."""
         dtypes = dict(zip(self.index_columns, self.index_dtypes))

bigframes/core/compile/__init__.py

@@ -13,11 +13,11 @@
 # limitations under the License.
 
 from bigframes.core.compile.compiled import OrderedIR, UnorderedIR
-from bigframes.core.compile.compiler import compile_ordered, compile_unordered
+from bigframes.core.compile.compiler import compile_ordered_ir, compile_unordered_ir
 
 __all__ = [
-    "compile_ordered",
-    "compile_unordered",
+    "compile_ordered_ir",
+    "compile_unordered_ir",
     "OrderedIR",
     "UnorderedIR",
 ]

bigframes/core/compile/compiled.py

@@ -208,6 +208,13 @@ def builder(self):
             predicates=self._predicates,
         )
 
+    def peek_sql(self, n: int):
+        # Peek currently implemented as top level LIMIT op.
+        # Execution engine handles limit pushdown.
+        # In future, may push down limit/filters in compilation.
+        sql = ibis_bigquery.Backend().compile(self._to_ibis_expr().limit(n))
+        return typing.cast(str, sql)
+
     def to_sql(
         self,
         offset_column: typing.Optional[str] = None,

bigframes/core/compile/compiler.py

@@ -29,14 +29,18 @@
     import bigframes.session
 
 
-def compile_ordered(node: nodes.BigFrameNode) -> compiled.OrderedIR:
+def compile_ordered_ir(node: nodes.BigFrameNode) -> compiled.OrderedIR:
     return typing.cast(compiled.OrderedIR, compile_node(node, True))
 
 
-def compile_unordered(node: nodes.BigFrameNode) -> compiled.UnorderedIR:
+def compile_unordered_ir(node: nodes.BigFrameNode) -> compiled.UnorderedIR:
     return typing.cast(compiled.UnorderedIR, compile_node(node, False))
 
 
+def compile_peak_sql(node: nodes.BigFrameNode, n_rows: int) -> typing.Optional[str]:
+    return compile_unordered_ir(node).peek_sql(n_rows)
+
+
 @functools.cache
 def compile_node(
     node: nodes.BigFrameNode, ordered: bool = True
@@ -56,17 +60,17 @@ def _compile_node(
 @_compile_node.register
 def compile_join(node: nodes.JoinNode, ordered: bool = True):
     if ordered:
-        left_ordered = compile_ordered(node.left_child)
-        right_ordered = compile_ordered(node.right_child)
+        left_ordered = compile_ordered_ir(node.left_child)
+        right_ordered = compile_ordered_ir(node.right_child)
         return bigframes.core.compile.single_column.join_by_column_ordered(
             left=left_ordered,
             right=right_ordered,
             join=node.join,
             allow_row_identity_join=node.allow_row_identity_join,
         )
     else:
-        left_unordered = compile_unordered(node.left_child)
-        right_unordered = compile_unordered(node.right_child)
+        left_unordered = compile_unordered_ir(node.left_child)
+        right_unordered = compile_unordered_ir(node.right_child)
         return bigframes.core.compile.single_column.join_by_column_unordered(
             left=left_unordered,
             right=right_unordered,
@@ -103,7 +107,7 @@ def compile_readgbq(node: nodes.ReadGbqNode, ordered: bool = True):
 
 @_compile_node.register
 def compile_promote_offsets(node: nodes.PromoteOffsetsNode, ordered: bool = True):
-    result = compile_ordered(node.child).promote_offsets(node.col_id)
+    result = compile_ordered_ir(node.child).promote_offsets(node.col_id)
     return result if ordered else result.to_unordered()
 
 
@@ -115,17 +119,17 @@ def compile_filter(node: nodes.FilterNode, ordered: bool = True):
 @_compile_node.register
 def compile_orderby(node: nodes.OrderByNode, ordered: bool = True):
     if ordered:
-        return compile_ordered(node.child).order_by(node.by)
+        return compile_ordered_ir(node.child).order_by(node.by)
     else:
-        return compile_unordered(node.child)
+        return compile_unordered_ir(node.child)
 
 
 @_compile_node.register
 def compile_reversed(node: nodes.ReversedNode, ordered: bool = True):
     if ordered:
-        return compile_ordered(node.child).reversed()
+        return compile_ordered_ir(node.child).reversed()
     else:
-        return compile_unordered(node.child)
+        return compile_unordered_ir(node.child)
 
 
 @_compile_node.register
@@ -137,36 +141,36 @@ def compile_projection(node: nodes.ProjectionNode, ordered: bool = True):
 @_compile_node.register
 def compile_concat(node: nodes.ConcatNode, ordered: bool = True):
     if ordered:
-        compiled_ordered = [compile_ordered(node) for node in node.children]
+        compiled_ordered = [compile_ordered_ir(node) for node in node.children]
         return concat_impl.concat_ordered(compiled_ordered)
     else:
-        compiled_unordered = [compile_unordered(node) for node in node.children]
+        compiled_unordered = [compile_unordered_ir(node) for node in node.children]
         return concat_impl.concat_unordered(compiled_unordered)
 
 
 @_compile_node.register
 def compile_rowcount(node: nodes.RowCountNode, ordered: bool = True):
-    result = compile_unordered(node.child).row_count()
+    result = compile_unordered_ir(node.child).row_count()
     return result if ordered else result.to_unordered()
 
 
 @_compile_node.register
 def compile_aggregate(node: nodes.AggregateNode, ordered: bool = True):
-    result = compile_unordered(node.child).aggregate(
+    result = compile_unordered_ir(node.child).aggregate(
         node.aggregations, node.by_column_ids, node.dropna
     )
     return result if ordered else result.to_unordered()
 
 
 @_compile_node.register
 def compile_corr(node: nodes.CorrNode, ordered: bool = True):
-    result = compile_unordered(node.child).corr_aggregate(node.corr_aggregations)
+    result = compile_unordered_ir(node.child).corr_aggregate(node.corr_aggregations)
     return result if ordered else result.to_unordered()
 
 
 @_compile_node.register
 def compile_window(node: nodes.WindowOpNode, ordered: bool = True):
-    result = compile_ordered(node.child).project_window_op(
+    result = compile_ordered_ir(node.child).project_window_op(
         node.column_name,
         node.op,
         node.window_spec,

bigframes/core/nodes.py

@@ -16,6 +16,7 @@
 
 from dataclasses import dataclass, field, fields
 import functools
+import itertools
 import typing
 from typing import Tuple
 
@@ -74,6 +75,18 @@ def session(self):
     def _node_hash(self):
         return hash(tuple(hash(getattr(self, field.name)) for field in fields(self)))
 
+    @property
+    def peekable(self) -> bool:
+        """Indicates whether the node can be sampled efficiently"""
+        return all(child.peekable for child in self.child_nodes)
+
+    @property
+    def roots(self) -> typing.Set[BigFrameNode]:
+        roots = itertools.chain.from_iterable(
+            map(lambda child: child.roots, self.child_nodes)
+        )
+        return set(roots)
+
 
 @dataclass(frozen=True)
 class UnaryNode(BigFrameNode):
@@ -98,6 +111,12 @@ def child_nodes(self) -> typing.Sequence[BigFrameNode]:
     def __hash__(self):
         return self._node_hash
 
+    @property
+    def peekable(self) -> bool:
+        children_peekable = all(child.peekable for child in self.child_nodes)
+        single_root = len(self.roots) == 1
+        return children_peekable and single_root
+
 
 @dataclass(frozen=True)
 class ConcatNode(BigFrameNode):
@@ -119,6 +138,14 @@ class ReadLocalNode(BigFrameNode):
     def __hash__(self):
         return self._node_hash
 
+    @property
+    def peekable(self) -> bool:
+        return True
+
+    @property
+    def roots(self) -> typing.Set[BigFrameNode]:
+        return {self}
+
 
 # TODO: Refactor to take raw gbq object reference
 @dataclass(frozen=True)
@@ -136,6 +163,14 @@ def session(self):
     def __hash__(self):
         return self._node_hash
 
+    @property
+    def peekable(self) -> bool:
+        return True
+
+    @property
+    def roots(self) -> typing.Set[BigFrameNode]:
+        return {self}
+
 
 # Unary nodes
 @dataclass(frozen=True)
@@ -145,6 +180,10 @@ class PromoteOffsetsNode(UnaryNode):
     def __hash__(self):
         return self._node_hash
 
+    @property
+    def peekable(self) -> bool:
+        return False
+
 
 @dataclass(frozen=True)
 class FilterNode(UnaryNode):
@@ -194,6 +233,10 @@ class AggregateNode(UnaryNode):
     def __hash__(self):
         return self._node_hash
 
+    @property
+    def peekable(self) -> bool:
+        return False
+
 
 # TODO: Unify into aggregate
 @dataclass(frozen=True)
@@ -203,6 +246,10 @@ class CorrNode(UnaryNode):
     def __hash__(self):
         return self._node_hash
 
+    @property
+    def peekable(self) -> bool:
+        return False
+
 
 @dataclass(frozen=True)
 class WindowOpNode(UnaryNode):
@@ -216,6 +263,10 @@ class WindowOpNode(UnaryNode):
     def __hash__(self):
         return self._node_hash
 
+    @property
+    def peekable(self) -> bool:
+        return False
+
 
 @dataclass(frozen=True)
 class ReprojectOpNode(UnaryNode):
@@ -239,6 +290,10 @@ class UnpivotNode(UnaryNode):
     def __hash__(self):
         return self._node_hash
 
+    @property
+    def peekable(self) -> bool:
+        return False
+
 
 @dataclass(frozen=True)
 class RandomSampleNode(UnaryNode):

bigframes/dataframe.py

@@ -1066,6 +1066,37 @@ def head(self, n: int = 5) -> DataFrame:
     def tail(self, n: int = 5) -> DataFrame:
         return typing.cast(DataFrame, self.iloc[-n:])
 
+    def peek(self, n: int = 5, *, force: bool = False) -> pandas.DataFrame:
+        """
+        Preview n arbitrary rows from the dataframe. No guarantees about row selection or ordering.
+        DataFrame.peek(force=False) will always be very fast, but will not succeed if data requires
+        full data scanning. Using force=True will always succeed, but may be perform expensive
+        computations.
+
+        Args:
+            n (int, default 5):
+                The number of rows to select from the dataframe. Which N rows are returned is non-deterministic.
+            force (bool, default False):
+                If the data cannot be peeked efficiently, the dataframe will instead be fully materialized as part
+                of the operation if force=True. If force=False, the operation will throw a ValueError.
+        Returns:
+            pandas.DataFrame: A pandas DataFrame with n rows.
+
+        Raises:
+            ValueError: If force=False and data cannot be efficiently peeked.
+        """
+        maybe_result = self._block.try_peek(n)
+        if maybe_result is None:
+            if force:
+                self._cached()
+                maybe_result = self._block.try_peek(n)
+                assert maybe_result is not None
+            else:
+                raise ValueError(
+                    "Cannot peek efficiently when data has aggregates, joins or window functions applied. Use force=True to fully compute dataframe."
+                )
+        return maybe_result.set_axis(self._block.column_labels, axis=1, copy=False)
+
     def nlargest(
         self,
         n: int,

bigframes/session/__init__.py

@@ -1508,6 +1508,17 @@ def _execute(
             job_config=job_config,
         )
 
+    def _peek(
+        self, array_value: core.ArrayValue, n_rows: int
+    ) -> tuple[bigquery.table.RowIterator, bigquery.QueryJob]:
+        """A 'peek' efficiently accesses a small number of rows in the dataframe."""
+        if not array_value.node.peekable:
+            raise NotImplementedError("cannot efficient peek this dataframe")
+        sql = self._compile_unordered(array_value).peek_sql(n_rows)
+        return self._start_query(
+            sql=sql,
+        )
+
     def _to_sql(
         self,
         array_value: core.ArrayValue,
@@ -1528,12 +1539,12 @@ def _to_sql(
     def _compile_ordered(
         self, array_value: core.ArrayValue
     ) -> bigframes.core.compile.OrderedIR:
-        return bigframes.core.compile.compile_ordered(array_value.node)
+        return bigframes.core.compile.compile_ordered_ir(array_value.node)
 
     def _compile_unordered(
         self, array_value: core.ArrayValue
     ) -> bigframes.core.compile.UnorderedIR:
-        return bigframes.core.compile.compile_unordered(array_value.node)
+        return bigframes.core.compile.compile_unordered_ir(array_value.node)
 
     def _get_table_size(self, destination_table):
         table = self.bqclient.get_table(destination_table)