Add basic predicate-pushdown optimization

dask_sql/physical/rel/logical/filter.py

@@ -1,11 +1,13 @@
 import logging
 from typing import TYPE_CHECKING, Union
 
+import dask.config as dask_config
 import dask.dataframe as dd
 import numpy as np
 
 from dask_sql.datacontainer import DataContainer
 from dask_sql.physical.rel.base import BaseRelPlugin
+from dask_sql.physical.rel.logical.optimize import attempt_predicate_pushdown
 from dask_sql.physical.rex import RexConverter
 
 if TYPE_CHECKING:
@@ -17,7 +19,7 @@
 
 def filter_or_scalar(df: dd.DataFrame, filter_condition: Union[np.bool_, dd.Series]):
     """
-    Some (complex) SQL queries can lead to a strange condition which is always true or false.
+    Some (complex) SQL queries calogger = logging.getLogger(__name__)n lead to a strange condition which is always true or false.
     We do not need to filter in this case.
     See https://github.com/dask-contrib/dask-sql/issues/87.
     """
@@ -31,7 +33,11 @@ def filter_or_scalar(df: dd.DataFrame, filter_condition: Union[np.bool_, dd.Seri
 
     # In SQL, a NULL in a boolean is False on filtering
     filter_condition = filter_condition.fillna(False)
-    return df[filter_condition]
+    out = df[filter_condition]
+    if dask_config.get("sql.predicate_pushdown"):
+        return attempt_predicate_pushdown(out)
+    else:
+        return out
 
 
 class DaskFilterPlugin(BaseRelPlugin):

dask_sql/physical/rel/logical/optimize.py

@@ -0,0 +1,349 @@
+import logging
+import operator
+
+import dask.dataframe as dd
+import numpy as np
+from dask.blockwise import Blockwise
+from dask.highlevelgraph import HighLevelGraph
+from dask.layers import DataFrameIOLayer
+from dask.utils import M, apply, is_arraylike
+
+logger = logging.getLogger(__name__)
+
+
+def attempt_predicate_pushdown(ddf: dd.DataFrame) -> dd.DataFrame:
+    """Use graph information to update IO-level filters
+
+    The original `ddf` will be returned if/when the
+    predicate-pushdown optimization fails.
+
+    This is a special optimization that must be called
+    eagerly on a DataFrame collection when filters are
+    applied. The "eager" requirement for this optimization
+    is due to the fact that `npartitions` and `divisions`
+    may change when this optimization is applied (invalidating
+    npartition/divisions-specific logic in following Layers).
+    """
+
+    # Check that we have a supported `ddf` object
+    if not isinstance(ddf, dd.DataFrame):
+        raise ValueError(
+            f"Predicate pushdown optimization skipped. Type {type(ddf)} "
+            f"does not support predicate pushdown."
+        )
+    elif not isinstance(ddf.dask, HighLevelGraph):
+        logger.warning(
+            f"Predicate pushdown optimization skipped. Graph must be "
+            f"a HighLevelGraph object (got {type(ddf.dask)})."
+        )
+        return ddf
+
+    # We were able to extract a DNF filter expression.
+    # Check that we have a single IO layer with `filters` support
+    io_layer = []
+    for k, v in ddf.dask.layers.items():
+        if isinstance(v, DataFrameIOLayer):
+            io_layer.append(k)
+            if (
+                "filters" not in v.creation_info.get("kwargs", {})
+                or v.creation_info["kwargs"]["filters"] is not None
+            ):
+                # No filters support, or filters is already set
+                logger.warning(
+                    "Predicate pushdown optimization skipped. The IO "
+                    "layer does not support a `filters` argument, or "
+                    "`filters` was already populated."
+                )
+                return ddf
+    if len(io_layer) != 1:
+        # Not a single IO layer
+        logger.warning(
+            f"Predicate pushdown optimization skipped. {len(io_layer)} "
+            f"IO layers detected, but only one IO layer is allowed."
+        )
+        return ddf
+    io_layer = io_layer.pop()
+
+    # Start by converting the HLG to a `RegenerableGraph`.
+    # Succeeding here means that all layers in the graph
+    # are regenerable.
+    try:
+        dsk = RegenerableGraph.from_hlg(ddf.dask)
+    except (ValueError, TypeError):
+        logger.warning(
+            "Predicate pushdown optimization skipped. One or more "
+            "layers in the HighLevelGraph was not 'regenerable'."
+        )
+        return ddf
+
+    # Extract a DNF-formatted filter expression
+    name = ddf._name
+    try:
+        filters = dsk.layers[name]._dnf_filter_expression(dsk)
+        if filters:
+            if isinstance(filters[0], (list, tuple)):
+                filters = list(filters)
+            else:
+                filters = [filters]
+        else:
+            return ddf
+        if not isinstance(filters, list):
+            filters = [filters]
+    except ValueError:
+        # DNF dispatching failed for 1+ layers
+        logger.warning(
+            "Predicate pushdown optimization skipped. One or more "
+            "layers has an unknown filter expression."
+        )
+        return ddf
+
+    # Regenerate collection with filtered IO layer
+    try:
+        return dsk.layers[name]._regenerate_collection(
+            dsk, new_kwargs={io_layer: {"filters": filters}},
+        )
+    except ValueError as err:
+        # Most-likely failed to apply filters in read_parquet.
+        # We can just bail on predicate pushdown, but we also
+        # raise a warning to encourage the user to file an issue.
+        logger.warning(
+            f"Predicate pushdown failed to apply filters: {filters}. "
+            f"Please open a bug report at "
+            f"https://github.com/dask-contrib/dask-sql/issues/new/choose "
+            f"and include the following error message: {err}"
+        )
+
+        return ddf
+
+
+# Define all supported comparison functions
+# (and their mapping to a string expression)
+_comparison_symbols = {
+    operator.eq: "==",
+    operator.ne: "!=",
+    operator.lt: "<",
+    operator.le: "<=",
+    operator.gt: ">",
+    operator.ge: ">=",
+    np.greater: ">",
+    np.greater_equal: ">=",
+    np.less: "<",
+    np.less_equal: "<=",
+    np.equal: "==",
+    np.not_equal: "!=",
+}
+
+# Define set of all "regenerable" operations.
+# Predicate pushdown is supported for graphs
+# comprised of `Blockwise` layers based on these
+# operations
+_regenerable_ops = set(_comparison_symbols.keys()) | {
+    operator.and_,
+    operator.or_,
+    operator.getitem,
+    M.fillna,
+}
+
+# Specify functions that must be generated with
+# a different API at the dataframe-collection level
+_special_op_mappings = {M.fillna: dd._Frame.fillna}
+
+
+class RegenerableLayer:
+    """Regenerable Layer
+
+    Wraps ``dask.highlevelgraph.Blockwise`` to ensure that a
+    ``creation_info`` attribute  is defined. This class
+    also defines the necessary methods for recursive
+    layer regeneration and filter-expression generation.
+    """
+
+    def __init__(self, layer, creation_info):
+        self.layer = layer  # Original Blockwise layer reference
+        self.creation_info = creation_info
+
+    def _regenerate_collection(
+        self, dsk, new_kwargs: dict = None, _regen_cache: dict = None,
+    ):
+        """Regenerate a Dask collection for this layer using the
+        provided inputs and key-word arguments
+        """
+
+        # Return regenerated layer if the work was
+        # already done
+        _regen_cache = _regen_cache or {}
+        if self.layer.output in _regen_cache:
+            return _regen_cache[self.layer.output]
+
+        # Recursively generate necessary inputs to
+        # this layer to generate the collection
+        inputs = []
+        for key, ind in self.layer.indices:
+            if ind is None:
+                if isinstance(key, (str, tuple)) and key in dsk.layers:
+                    continue
+                inputs.append(key)
+            elif key in self.layer.io_deps:
+                continue
+            else:
+                inputs.append(
+                    dsk.layers[key]._regenerate_collection(
+                        dsk, new_kwargs=new_kwargs, _regen_cache=_regen_cache,
+                    )
+                )
+
+        # Extract the callable func and key-word args.
+        # Then return a regenerated collection
+        func = self.creation_info.get("func", None)
+        if func is None:
+            raise ValueError(
+                "`_regenerate_collection` failed. "
+                "Not all HLG layers are regenerable."
+            )
+        regen_args = self.creation_info.get("args", [])
+        regen_kwargs = self.creation_info.get("kwargs", {}).copy()
+        regen_kwargs = {k: v for k, v in self.creation_info.get("kwargs", {}).items()}
+        regen_kwargs.update((new_kwargs or {}).get(self.layer.output, {}))
+        result = func(*inputs, *regen_args, **regen_kwargs)
+        _regen_cache[self.layer.output] = result
+        return result
+
+    def _dnf_filter_expression(self, dsk):
+        """Return a DNF-formatted filter expression for the
+        graph terminating at this layer
+        """
+        op = self.creation_info["func"]
+        if op in _comparison_symbols.keys():
+            func = _blockwise_comparison_dnf
+        elif op in (operator.and_, operator.or_):
+            func = _blockwise_logical_dnf
+        elif op == operator.getitem:
+            func = _blockwise_getitem_dnf
+        elif op == dd._Frame.fillna:
+            func = _blockwise_fillna_dnf
+        else:
+            raise ValueError(f"No DNF expression for {op}")
+
+        return func(op, self.layer.indices, dsk)
+
+
+class RegenerableGraph:
+    """Regenerable Graph
+
+    This class is similar to ``dask.highlevelgraph.HighLevelGraph``.
+    However, all layers in a ``RegenerableGraph`` graph must be
+    ``RegenerableLayer`` objects (which wrap ``Blockwise`` layers).
+    """
+
+    def __init__(self, layers: dict):
+        self.layers = layers
+
+    @classmethod
+    def from_hlg(cls, hlg: HighLevelGraph):
+        """Construct a ``RegenerableGraph`` from a ``HighLevelGraph``"""
+
+        if not isinstance(hlg, HighLevelGraph):
+            raise TypeError(f"Expected HighLevelGraph, got {type(hlg)}")
+
+        _layers = {}
+        for key, layer in hlg.layers.items():
+            regenerable_layer = None
+            if isinstance(layer, DataFrameIOLayer):
+                regenerable_layer = RegenerableLayer(layer, layer.creation_info or {})
+            elif isinstance(layer, Blockwise):
+                tasks = list(layer.dsk.values())
+                if len(tasks) == 1 and tasks[0]:
+                    kwargs = {}
+                    if tasks[0][0] == apply:
+                        op = tasks[0][1]
+                        options = tasks[0][3]
+                        if isinstance(options, dict):
+                            kwargs = options
+                        elif (
+                            isinstance(options, tuple)
+                            and options
+                            and callable(options[0])
+                        ):
+                            kwargs = options[0](*options[1:])
+                    else:
+                        op = tasks[0][0]
+                    if op in _regenerable_ops:
+                        regenerable_layer = RegenerableLayer(
+                            layer,
+                            {
+                                "func": _special_op_mappings.get(op, op),
+                                "kwargs": kwargs,
+                            },
+                        )
+
+            if regenerable_layer is None:
+                raise ValueError(f"Graph contains non-regenerable layer: {layer}")
+
+            _layers[key] = regenerable_layer
+
+        return RegenerableGraph(_layers)
+
+
+def _get_blockwise_input(input_index, indices: list, dsk: RegenerableGraph):
+    # Simple utility to get the required input expressions
+    # for a Blockwise layer (using indices)
+    key = indices[input_index][0]
+    if indices[input_index][1] is None:
+        return key
+    return dsk.layers[key]._dnf_filter_expression(dsk)
+
+
+def _blockwise_comparison_dnf(op, indices: list, dsk: RegenerableGraph):
+    # Return DNF expression pattern for a simple comparison
+    left = _get_blockwise_input(0, indices, dsk)
+    right = _get_blockwise_input(1, indices, dsk)
+
+    def _inv(symbol: str):
+        return {">": "<", "<": ">", ">=": "<=", "<=": ">=",}.get(symbol, symbol)
+
+    if is_arraylike(left) and hasattr(left, "item") and left.size == 1:
+        left = left.item()
+        # Need inverse comparison in read_parquet
+        return (right, _inv(_comparison_symbols[op]), left)
+    if is_arraylike(right) and hasattr(right, "item") and right.size == 1:
+        right = right.item()
+    return (left, _comparison_symbols[op], right)
+
+
+def _blockwise_logical_dnf(op, indices: list, dsk: RegenerableGraph):
+    # Return DNF expression pattern for logical "and" or "or"
+    left = _get_blockwise_input(0, indices, dsk)
+    right = _get_blockwise_input(1, indices, dsk)
+
+    def _maybe_list(val):
+        if isinstance(val, tuple) and val and isinstance(val[0], (tuple, list)):
+            return list(val)
+        return [val]
+
+    def _maybe_tuple(val):
+        if isinstance(val, tuple) and val and isinstance(val[0], tuple):
+            return val
+        return (val,)
+
+    if op == operator.or_:
+        # NDF "or" is List[List[Tuple]]
+        return [_maybe_list(left), _maybe_list(right)]
+    elif op == operator.and_:
+        # NDF "and" is List[Tuple]
+        # However, we don't want to add the outer list
+        # until the filter is finished, or this expression
+        # is combined with another in an "or" expression
+        return _maybe_tuple(left) + _maybe_tuple(right)
+    else:
+        raise ValueError
+
+
+def _blockwise_getitem_dnf(op, indices: list, dsk: RegenerableGraph):
+    # Return dnf of key (selected by getitem)
+    key = _get_blockwise_input(1, indices, dsk)
+    return key
+
+
+def _blockwise_fillna_dnf(op, indices: list, dsk: RegenerableGraph):
+    # Return dnf of input collection
+    return _get_blockwise_input(0, indices, dsk)

dask_sql/sql-schema.yaml

@@ -26,3 +26,8 @@ properties:
             type: boolean
             description: |
               Whether sql identifiers are considered case sensitive while parsing.
+
+      predicate_pushdown:
+        type: bool
+        description: |
+          Whether to try pushing down filter predicates into IO (when possible).