sqlglot.optimizer.pushdown_predicates API documentation

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# sqlglot.optimizer.pushdown_predicates

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

def pushdown_predicates(expression): View Source

     8def pushdown_predicates(expression):
     9    """
    10    Rewrite sqlglot AST to pushdown predicates in FROMS and JOINS
    11
    12    Example:
    13        >>> import sqlglot
    14        >>> sql = "SELECT * FROM (SELECT * FROM x AS x) AS y WHERE y.a = 1"
    15        >>> expression = sqlglot.parse_one(sql)
    16        >>> pushdown_predicates(expression).sql()
    17        'SELECT * FROM (SELECT * FROM x AS x WHERE y.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    root = build_scope(expression)
    25    scope_ref_count = root.ref_count()
    26
    27    for scope in reversed(list(root.traverse())):
    28        select = scope.expression
    29        where = select.args.get("where")
    30        if where:
    31            selected_sources = scope.selected_sources
    32            # a right join can only push down to itself and not the source FROM table
    33            for k, (node, source) in selected_sources.items():
    34                parent = node.find_ancestor(exp.Join, exp.From)
    35                if isinstance(parent, exp.Join) and parent.side == "RIGHT":
    36                    selected_sources = {k: (node, source)}
    37                    break
    38            pushdown(where.this, selected_sources, scope_ref_count)
    39
    40        # joins should only pushdown into itself, not to other joins
    41        # so we limit the selected sources to only itself
    42        for join in select.args.get("joins") or []:
    43            name = join.this.alias_or_name
    44            pushdown(join.args.get("on"), {name: scope.selected_sources[name]}, scope_ref_count)
    45
    46    return expression

Rewrite sqlglot AST to pushdown predicates in FROMS and JOINS

###### Example:

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

    49def pushdown(condition, sources, scope_ref_count):
    50    if not condition:
    51        return
    52
    53    condition = condition.replace(simplify(condition))
    54    cnf_like = normalized(condition) or not normalized(condition, dnf=True)
    55
    56    predicates = list(
    57        condition.flatten()
    58        if isinstance(condition, exp.And if cnf_like else exp.Or)
    59        else [condition]
    60    )
    61
    62    if cnf_like:
    63        pushdown_cnf(predicates, sources, scope_ref_count)
    64    else:
    65        pushdown_dnf(predicates, sources, scope_ref_count)

def pushdown_cnf(predicates, scope, scope_ref_count): View Source

    68def pushdown_cnf(predicates, scope, scope_ref_count):
    69    """
    70    If the predicates are in CNF like form, we can simply replace each block in the parent.
    71    """
    72    for predicate in predicates:
    73        for node in nodes_for_predicate(predicate, scope, scope_ref_count).values():
    74            if isinstance(node, exp.Join):
    75                predicate.replace(exp.true())
    76                node.on(predicate, copy=False)
    77                break
    78            if isinstance(node, exp.Select):
    79                predicate.replace(exp.true())
    80                node.where(replace_aliases(node, predicate), copy=False)

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

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

     83def pushdown_dnf(predicates, scope, scope_ref_count):
     84    """
     85    If the predicates are in DNF form, we can only push down conditions that are in all blocks.
     86    Additionally, we can't remove predicates from their original form.
     87    """
     88    # find all the tables that can be pushdown too
     89    # these are tables that are referenced in all blocks of a DNF
     90    # (a.x AND b.x) OR (a.y AND c.y)
     91    # only table a can be push down
     92    pushdown_tables = set()
     93
     94    for a in predicates:
     95        a_tables = set(exp.column_table_names(a))
     96
     97        for b in predicates:
     98            a_tables &= set(exp.column_table_names(b))
     99
    100        pushdown_tables.update(a_tables)
    101
    102    conditions = {}
    103
    104    # for every pushdown table, find all related conditions in all predicates
    105    # combine them with ORS
    106    # (a.x AND and a.y AND b.x) OR (a.z AND c.y) -> (a.x AND a.y) OR (a.z)
    107    for table in sorted(pushdown_tables):
    108        for predicate in predicates:
    109            nodes = nodes_for_predicate(predicate, scope, scope_ref_count)
    110
    111            if table not in nodes:
    112                continue
    113
    114            predicate_condition = None
    115
    116            for column in predicate.find_all(exp.Column):
    117                if column.table == table:
    118                    condition = column.find_ancestor(exp.Condition)
    119                    predicate_condition = (
    120                        exp.and_(predicate_condition, condition)
    121                        if predicate_condition
    122                        else condition
    123                    )
    124
    125            if predicate_condition:
    126                conditions[table] = (
    127                    exp.or_(conditions[table], predicate_condition)
    128                    if table in conditions
    129                    else predicate_condition
    130                )
    131
    132        for name, node in nodes.items():
    133            if name not in conditions:
    134                continue
    135
    136            predicate = conditions[name]
    137
    138            if isinstance(node, exp.Join):
    139                node.on(predicate, copy=False)
    140            elif isinstance(node, exp.Select):
    141                node.where(replace_aliases(node, 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

    144def nodes_for_predicate(predicate, sources, scope_ref_count):
    145    nodes = {}
    146    tables = exp.column_table_names(predicate)
    147    where_condition = isinstance(predicate.find_ancestor(exp.Join, exp.Where), exp.Where)
    148
    149    for table in tables:
    150        node, source = sources.get(table) or (None, None)
    151
    152        # if the predicate is in a where statement we can try to push it down
    153        # we want to find the root join or from statement
    154        if node and where_condition:
    155            node = node.find_ancestor(exp.Join, exp.From)
    156
    157        # a node can reference a CTE which should be pushed down
    158        if isinstance(node, exp.From) and not isinstance(source, exp.Table):
    159            with_ = source.parent.expression.args.get("with")
    160            if with_ and with_.recursive:
    161                return {}
    162            node = source.expression
    163
    164        if isinstance(node, exp.Join):
    165            if node.side and node.side != "RIGHT":
    166                return {}
    167            nodes[table] = node
    168        elif isinstance(node, exp.Select) and len(tables) == 1:
    169            # We can't push down window expressions
    170            has_window_expression = any(
    171                select for select in node.selects if select.find(exp.Window)
    172            )
    173            # we can't push down predicates to select statements if they are referenced in
    174            # multiple places.
    175            if (
    176                not node.args.get("group")
    177                and scope_ref_count[id(source)] < 2
    178                and not has_window_expression
    179            ):
    180                nodes[table] = node
    181    return nodes

def replace_aliases(source, predicate): View Source

    184def replace_aliases(source, predicate):
    185    aliases = {}
    186
    187    for select in source.selects:
    188        if isinstance(select, exp.Alias):
    189            aliases[select.alias] = select.this
    190        else:
    191            aliases[select.name] = select
    192
    193    def _replace_alias(column):
    194        if isinstance(column, exp.Column) and column.name in aliases:
    195            return aliases[column.name].copy()
    196        return column
    197
    198    return predicate.transform(_replace_alias)