sqlglot.optimizer.pushdown_predicates API documentation

Edit on GitHub

# sqlglot.optimizer.pushdown_predicates

View Source

      1from sqlglot import exp
      2from sqlglot.optimizer.normalize import normalized
      3from sqlglot.optimizer.scope import build_scope, find_in_scope
      4from sqlglot.optimizer.simplify import simplify
      5from sqlglot import Dialect
      6
      7
      8def pushdown_predicates(expression, dialect=None):
      9    """
     10    Rewrite sqlglot AST to pushdown predicates in FROMS and JOINS
     11
     12    Example:
     13        >>> import sqlglot
     14        >>> sql = "SELECT y.a AS a FROM (SELECT x.a AS a FROM x AS x) AS y WHERE y.a = 1"
     15        >>> expression = sqlglot.parse_one(sql)
     16        >>> pushdown_predicates(expression).sql()
     17        'SELECT y.a AS a FROM (SELECT x.a AS a FROM x AS x WHERE x.a = 1) AS y WHERE TRUE'
     18
     19    Args:
     20        expression (sqlglot.Expression): expression to optimize
     21    Returns:
     22        sqlglot.Expression: optimized expression
     23    """
     24    from sqlglot.dialects.presto import Presto
     25
     26    root = build_scope(expression)
     27
     28    dialect = Dialect.get_or_raise(dialect)
     29    unnest_requires_cross_join = isinstance(dialect, Presto)
     30
     31    if root:
     32        scope_ref_count = root.ref_count()
     33
     34        for scope in reversed(list(root.traverse())):
     35            select = scope.expression
     36            where = select.args.get("where")
     37            if where:
     38                selected_sources = scope.selected_sources
     39                join_index = {
     40                    join.alias_or_name: i for i, join in enumerate(select.args.get("joins") or [])
     41                }
     42
     43                # a right join can only push down to itself and not the source FROM table
     44                # presto, trino and athena don't support inner joins where the RHS is an UNNEST expression
     45                pushdown_allowed = True
     46                for k, (node, source) in selected_sources.items():
     47                    parent = node.find_ancestor(exp.Join, exp.From)
     48                    if isinstance(parent, exp.Join):
     49                        if parent.side == "RIGHT":
     50                            selected_sources = {k: (node, source)}
     51                            break
     52                        if isinstance(node, exp.Unnest) and unnest_requires_cross_join:
     53                            pushdown_allowed = False
     54                            break
     55
     56                if pushdown_allowed:
     57                    pushdown(where.this, selected_sources, scope_ref_count, dialect, join_index)
     58
     59            # joins should only pushdown into itself, not to other joins
     60            # so we limit the selected sources to only itself
     61            for join in select.args.get("joins") or []:
     62                name = join.alias_or_name
     63                if name in scope.selected_sources:
     64                    pushdown(
     65                        join.args.get("on"),
     66                        {name: scope.selected_sources[name]},
     67                        scope_ref_count,
     68                        dialect,
     69                    )
     70
     71    return expression
     72
     73
     74def pushdown(condition, sources, scope_ref_count, dialect, join_index=None):
     75    if not condition:
     76        return
     77
     78    condition = condition.replace(simplify(condition, dialect=dialect))
     79    cnf_like = normalized(condition) or not normalized(condition, dnf=True)
     80
     81    predicates = list(
     82        condition.flatten()
     83        if isinstance(condition, exp.And if cnf_like else exp.Or)
     84        else [condition]
     85    )
     86
     87    if cnf_like:
     88        pushdown_cnf(predicates, sources, scope_ref_count, join_index=join_index)
     89    else:
     90        pushdown_dnf(predicates, sources, scope_ref_count)
     91
     92
     93def pushdown_cnf(predicates, sources, scope_ref_count, join_index=None):
     94    """
     95    If the predicates are in CNF like form, we can simply replace each block in the parent.
     96    """
     97    join_index = join_index or {}
     98    for predicate in predicates:
     99        for node in nodes_for_predicate(predicate, sources, scope_ref_count).values():
    100            if isinstance(node, exp.Join):
    101                name = node.alias_or_name
    102                predicate_tables = exp.column_table_names(predicate, name)
    103
    104                # Don't push the predicate if it references tables that appear in later joins
    105                this_index = join_index[name]
    106                if all(join_index.get(table, -1) < this_index for table in predicate_tables):
    107                    predicate.replace(exp.true())
    108                    node.on(predicate, copy=False)
    109                    break
    110            if isinstance(node, exp.Select):
    111                predicate.replace(exp.true())
    112                inner_predicate = replace_aliases(node, predicate)
    113                if find_in_scope(inner_predicate, exp.AggFunc):
    114                    node.having(inner_predicate, copy=False)
    115                else:
    116                    node.where(inner_predicate, copy=False)
    117
    118
    119def pushdown_dnf(predicates, sources, scope_ref_count):
    120    """
    121    If the predicates are in DNF form, we can only push down conditions that are in all blocks.
    122    Additionally, we can't remove predicates from their original form.
    123    """
    124    # find all the tables that can be pushdown too
    125    # these are tables that are referenced in all blocks of a DNF
    126    # (a.x AND b.x) OR (a.y AND c.y)
    127    # only table a can be push down
    128    pushdown_tables = set()
    129
    130    for a in predicates:
    131        a_tables = exp.column_table_names(a)
    132
    133        for b in predicates:
    134            a_tables &= exp.column_table_names(b)
    135
    136        pushdown_tables.update(a_tables)
    137
    138    conditions = {}
    139
    140    # pushdown all predicates to their respective nodes
    141    for table in sorted(pushdown_tables):
    142        for predicate in predicates:
    143            nodes = nodes_for_predicate(predicate, sources, scope_ref_count)
    144
    145            if table not in nodes:
    146                continue
    147
    148            conditions[table] = (
    149                exp.or_(conditions[table], predicate) if table in conditions else predicate
    150            )
    151
    152        for name, node in nodes.items():
    153            if name not in conditions:
    154                continue
    155
    156            predicate = conditions[name]
    157
    158            if isinstance(node, exp.Join):
    159                node.on(predicate, copy=False)
    160            elif isinstance(node, exp.Select):
    161                inner_predicate = replace_aliases(node, predicate)
    162                if find_in_scope(inner_predicate, exp.AggFunc):
    163                    node.having(inner_predicate, copy=False)
    164                else:
    165                    node.where(inner_predicate, copy=False)
    166
    167
    168def nodes_for_predicate(predicate, sources, scope_ref_count):
    169    nodes = {}
    170    tables = exp.column_table_names(predicate)
    171    where_condition = isinstance(predicate.find_ancestor(exp.Join, exp.Where), exp.Where)
    172
    173    for table in sorted(tables):
    174        node, source = sources.get(table) or (None, None)
    175
    176        # if the predicate is in a where statement we can try to push it down
    177        # we want to find the root join or from statement
    178        if node and where_condition:
    179            node = node.find_ancestor(exp.Join, exp.From)
    180
    181        # a node can reference a CTE which should be pushed down
    182        if isinstance(node, exp.From) and not isinstance(source, exp.Table):
    183            with_ = source.parent.expression.args.get("with")
    184            if with_ and with_.recursive:
    185                return {}
    186            node = source.expression
    187
    188        if isinstance(node, exp.Join):
    189            if node.side and node.side != "RIGHT":
    190                return {}
    191            nodes[table] = node
    192        elif isinstance(node, exp.Select) and len(tables) == 1:
    193            # We can't push down window expressions
    194            has_window_expression = any(
    195                select for select in node.selects if select.find(exp.Window)
    196            )
    197            # we can't push down predicates to select statements if they are referenced in
    198            # multiple places.
    199            if (
    200                not node.args.get("group")
    201                and scope_ref_count[id(source)] < 2
    202                and not has_window_expression
    203            ):
    204                nodes[table] = node
    205    return nodes
    206
    207
    208def replace_aliases(source, predicate):
    209    aliases = {}
    210
    211    for select in source.selects:
    212        if isinstance(select, exp.Alias):
    213            aliases[select.alias] = select.this
    214        else:
    215            aliases[select.name] = select
    216
    217    def _replace_alias(column):
    218        if isinstance(column, exp.Column) and column.name in aliases:
    219            return aliases[column.name].copy()
    220        return column
    221
    222    return predicate.transform(_replace_alias)

def pushdown_predicates(expression, dialect=None): View Source

     9def pushdown_predicates(expression, dialect=None):
    10    """
    11    Rewrite sqlglot AST to pushdown predicates in FROMS and JOINS
    12
    13    Example:
    14        >>> import sqlglot
    15        >>> sql = "SELECT y.a AS a FROM (SELECT x.a AS a FROM x AS x) AS y WHERE y.a = 1"
    16        >>> expression = sqlglot.parse_one(sql)
    17        >>> pushdown_predicates(expression).sql()
    18        'SELECT y.a AS a FROM (SELECT x.a AS a FROM x AS x WHERE x.a = 1) AS y WHERE TRUE'
    19
    20    Args:
    21        expression (sqlglot.Expression): expression to optimize
    22    Returns:
    23        sqlglot.Expression: optimized expression
    24    """
    25    from sqlglot.dialects.presto import Presto
    26
    27    root = build_scope(expression)
    28
    29    dialect = Dialect.get_or_raise(dialect)
    30    unnest_requires_cross_join = isinstance(dialect, Presto)
    31
    32    if root:
    33        scope_ref_count = root.ref_count()
    34
    35        for scope in reversed(list(root.traverse())):
    36            select = scope.expression
    37            where = select.args.get("where")
    38            if where:
    39                selected_sources = scope.selected_sources
    40                join_index = {
    41                    join.alias_or_name: i for i, join in enumerate(select.args.get("joins") or [])
    42                }
    43
    44                # a right join can only push down to itself and not the source FROM table
    45                # presto, trino and athena don't support inner joins where the RHS is an UNNEST expression
    46                pushdown_allowed = True
    47                for k, (node, source) in selected_sources.items():
    48                    parent = node.find_ancestor(exp.Join, exp.From)
    49                    if isinstance(parent, exp.Join):
    50                        if parent.side == "RIGHT":
    51                            selected_sources = {k: (node, source)}
    52                            break
    53                        if isinstance(node, exp.Unnest) and unnest_requires_cross_join:
    54                            pushdown_allowed = False
    55                            break
    56
    57                if pushdown_allowed:
    58                    pushdown(where.this, selected_sources, scope_ref_count, dialect, join_index)
    59
    60            # joins should only pushdown into itself, not to other joins
    61            # so we limit the selected sources to only itself
    62            for join in select.args.get("joins") or []:
    63                name = join.alias_or_name
    64                if name in scope.selected_sources:
    65                    pushdown(
    66                        join.args.get("on"),
    67                        {name: scope.selected_sources[name]},
    68                        scope_ref_count,
    69                        dialect,
    70                    )
    71
    72    return expression

Rewrite sqlglot AST to pushdown predicates in FROMS and JOINS

###### Example:

>     >>> import sqlglot
>     >>> sql = "SELECT y.a AS a FROM (SELECT x.a AS a FROM x AS x) AS y WHERE y.a = 1"
>     >>> expression = sqlglot.parse_one(sql)
>     >>> pushdown_predicates(expression).sql()
>     'SELECT y.a AS a FROM (SELECT x.a AS a FROM x AS x WHERE x.a = 1) AS y WHERE TRUE'

###### Arguments:

- expression (sqlglot.Expression): expression to optimize

###### Returns:

> sqlglot.Expression: optimized expression

def pushdown(condition, sources, scope_ref_count, dialect, join_index=None): View Source

    75def pushdown(condition, sources, scope_ref_count, dialect, join_index=None):
    76    if not condition:
    77        return
    78
    79    condition = condition.replace(simplify(condition, dialect=dialect))
    80    cnf_like = normalized(condition) or not normalized(condition, dnf=True)
    81
    82    predicates = list(
    83        condition.flatten()
    84        if isinstance(condition, exp.And if cnf_like else exp.Or)
    85        else [condition]
    86    )
    87
    88    if cnf_like:
    89        pushdown_cnf(predicates, sources, scope_ref_count, join_index=join_index)
    90    else:
    91        pushdown_dnf(predicates, sources, scope_ref_count)

def pushdown_cnf(predicates, sources, scope_ref_count, join_index=None): View Source

     94def pushdown_cnf(predicates, sources, scope_ref_count, join_index=None):
     95    """
     96    If the predicates are in CNF like form, we can simply replace each block in the parent.
     97    """
     98    join_index = join_index or {}
     99    for predicate in predicates:
    100        for node in nodes_for_predicate(predicate, sources, scope_ref_count).values():
    101            if isinstance(node, exp.Join):
    102                name = node.alias_or_name
    103                predicate_tables = exp.column_table_names(predicate, name)
    104
    105                # Don't push the predicate if it references tables that appear in later joins
    106                this_index = join_index[name]
    107                if all(join_index.get(table, -1) < this_index for table in predicate_tables):
    108                    predicate.replace(exp.true())
    109                    node.on(predicate, copy=False)
    110                    break
    111            if isinstance(node, exp.Select):
    112                predicate.replace(exp.true())
    113                inner_predicate = replace_aliases(node, predicate)
    114                if find_in_scope(inner_predicate, exp.AggFunc):
    115                    node.having(inner_predicate, copy=False)
    116                else:
    117                    node.where(inner_predicate, copy=False)

If the predicates are in CNF like form, we can simply replace each block in the parent.

def pushdown_dnf(predicates, sources, scope_ref_count): View Source

    120def pushdown_dnf(predicates, sources, scope_ref_count):
    121    """
    122    If the predicates are in DNF form, we can only push down conditions that are in all blocks.
    123    Additionally, we can't remove predicates from their original form.
    124    """
    125    # find all the tables that can be pushdown too
    126    # these are tables that are referenced in all blocks of a DNF
    127    # (a.x AND b.x) OR (a.y AND c.y)
    128    # only table a can be push down
    129    pushdown_tables = set()
    130
    131    for a in predicates:
    132        a_tables = exp.column_table_names(a)
    133
    134        for b in predicates:
    135            a_tables &= exp.column_table_names(b)
    136
    137        pushdown_tables.update(a_tables)
    138
    139    conditions = {}
    140
    141    # pushdown all predicates to their respective nodes
    142    for table in sorted(pushdown_tables):
    143        for predicate in predicates:
    144            nodes = nodes_for_predicate(predicate, sources, scope_ref_count)
    145
    146            if table not in nodes:
    147                continue
    148
    149            conditions[table] = (
    150                exp.or_(conditions[table], predicate) if table in conditions else predicate
    151            )
    152
    153        for name, node in nodes.items():
    154            if name not in conditions:
    155                continue
    156
    157            predicate = conditions[name]
    158
    159            if isinstance(node, exp.Join):
    160                node.on(predicate, copy=False)
    161            elif isinstance(node, exp.Select):
    162                inner_predicate = replace_aliases(node, predicate)
    163                if find_in_scope(inner_predicate, exp.AggFunc):
    164                    node.having(inner_predicate, copy=False)
    165                else:
    166                    node.where(inner_predicate, copy=False)

If the predicates are in DNF form, we can only push down conditions that are in all blocks. Additionally, we can't remove predicates from their original form.

def nodes_for_predicate(predicate, sources, scope_ref_count): View Source

    169def nodes_for_predicate(predicate, sources, scope_ref_count):
    170    nodes = {}
    171    tables = exp.column_table_names(predicate)
    172    where_condition = isinstance(predicate.find_ancestor(exp.Join, exp.Where), exp.Where)
    173
    174    for table in sorted(tables):
    175        node, source = sources.get(table) or (None, None)
    176
    177        # if the predicate is in a where statement we can try to push it down
    178        # we want to find the root join or from statement
    179        if node and where_condition:
    180            node = node.find_ancestor(exp.Join, exp.From)
    181
    182        # a node can reference a CTE which should be pushed down
    183        if isinstance(node, exp.From) and not isinstance(source, exp.Table):
    184            with_ = source.parent.expression.args.get("with")
    185            if with_ and with_.recursive:
    186                return {}
    187            node = source.expression
    188
    189        if isinstance(node, exp.Join):
    190            if node.side and node.side != "RIGHT":
    191                return {}
    192            nodes[table] = node
    193        elif isinstance(node, exp.Select) and len(tables) == 1:
    194            # We can't push down window expressions
    195            has_window_expression = any(
    196                select for select in node.selects if select.find(exp.Window)
    197            )
    198            # we can't push down predicates to select statements if they are referenced in
    199            # multiple places.
    200            if (
    201                not node.args.get("group")
    202                and scope_ref_count[id(source)] < 2
    203                and not has_window_expression
    204            ):
    205                nodes[table] = node
    206    return nodes

def replace_aliases(source, predicate): View Source

    209def replace_aliases(source, predicate):
    210    aliases = {}
    211
    212    for select in source.selects:
    213        if isinstance(select, exp.Alias):
    214            aliases[select.alias] = select.this
    215        else:
    216            aliases[select.name] = select
    217
    218    def _replace_alias(column):
    219        if isinstance(column, exp.Column) and column.name in aliases:
    220            return aliases[column.name].copy()
    221        return column
    222
    223    return predicate.transform(_replace_alias)