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 y.a AS a FROM (SELECT x.a AS a FROM x AS x) AS y WHERE y.a = 1"
     14        >>> expression = sqlglot.parse_one(sql)
     15        >>> pushdown_predicates(expression).sql()
     16        'SELECT y.a AS a FROM (SELECT x.a AS a FROM x AS x WHERE x.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
     25    if root:
     26        scope_ref_count = root.ref_count()
     27
     28        for scope in reversed(list(root.traverse())):
     29            select = scope.expression
     30            where = select.args.get("where")
     31            if where:
     32                selected_sources = scope.selected_sources
     33                # a right join can only push down to itself and not the source FROM table
     34                for k, (node, source) in selected_sources.items():
     35                    parent = node.find_ancestor(exp.Join, exp.From)
     36                    if isinstance(parent, exp.Join) and parent.side == "RIGHT":
     37                        selected_sources = {k: (node, source)}
     38                        break
     39                pushdown(where.this, selected_sources, scope_ref_count)
     40
     41            # joins should only pushdown into itself, not to other joins
     42            # so we limit the selected sources to only itself
     43            for join in select.args.get("joins") or []:
     44                name = join.this.alias_or_name
     45                pushdown(join.args.get("on"), {name: scope.selected_sources[name]}, scope_ref_count)
     46
     47    return expression
     48
     49
     50def pushdown(condition, sources, scope_ref_count):
     51    if not condition:
     52        return
     53
     54    condition = condition.replace(simplify(condition))
     55    cnf_like = normalized(condition) or not normalized(condition, dnf=True)
     56
     57    predicates = list(
     58        condition.flatten()
     59        if isinstance(condition, exp.And if cnf_like else exp.Or)
     60        else [condition]
     61    )
     62
     63    if cnf_like:
     64        pushdown_cnf(predicates, sources, scope_ref_count)
     65    else:
     66        pushdown_dnf(predicates, sources, scope_ref_count)
     67
     68
     69def pushdown_cnf(predicates, scope, scope_ref_count):
     70    """
     71    If the predicates are in CNF like form, we can simply replace each block in the parent.
     72    """
     73    for predicate in predicates:
     74        for node in nodes_for_predicate(predicate, scope, scope_ref_count).values():
     75            if isinstance(node, exp.Join):
     76                predicate.replace(exp.true())
     77                node.on(predicate, copy=False)
     78                break
     79            if isinstance(node, exp.Select):
     80                predicate.replace(exp.true())
     81                node.where(replace_aliases(node, predicate), copy=False)
     82
     83
     84def pushdown_dnf(predicates, scope, scope_ref_count):
     85    """
     86    If the predicates are in DNF form, we can only push down conditions that are in all blocks.
     87    Additionally, we can't remove predicates from their original form.
     88    """
     89    # find all the tables that can be pushdown too
     90    # these are tables that are referenced in all blocks of a DNF
     91    # (a.x AND b.x) OR (a.y AND c.y)
     92    # only table a can be push down
     93    pushdown_tables = set()
     94
     95    for a in predicates:
     96        a_tables = set(exp.column_table_names(a))
     97
     98        for b in predicates:
     99            a_tables &= set(exp.column_table_names(b))
    100
    101        pushdown_tables.update(a_tables)
    102
    103    conditions = {}
    104
    105    # for every pushdown table, find all related conditions in all predicates
    106    # combine them with ORS
    107    # (a.x AND and a.y AND b.x) OR (a.z AND c.y) -> (a.x AND a.y) OR (a.z)
    108    for table in sorted(pushdown_tables):
    109        for predicate in predicates:
    110            nodes = nodes_for_predicate(predicate, scope, scope_ref_count)
    111
    112            if table not in nodes:
    113                continue
    114
    115            predicate_condition = None
    116
    117            for column in predicate.find_all(exp.Column):
    118                if column.table == table:
    119                    condition = column.find_ancestor(exp.Condition)
    120                    predicate_condition = (
    121                        exp.and_(predicate_condition, condition)
    122                        if predicate_condition
    123                        else condition
    124                    )
    125
    126            if predicate_condition:
    127                conditions[table] = (
    128                    exp.or_(conditions[table], predicate_condition)
    129                    if table in conditions
    130                    else predicate_condition
    131                )
    132
    133        for name, node in nodes.items():
    134            if name not in conditions:
    135                continue
    136
    137            predicate = conditions[name]
    138
    139            if isinstance(node, exp.Join):
    140                node.on(predicate, copy=False)
    141            elif isinstance(node, exp.Select):
    142                node.where(replace_aliases(node, predicate), copy=False)
    143
    144
    145def nodes_for_predicate(predicate, sources, scope_ref_count):
    146    nodes = {}
    147    tables = exp.column_table_names(predicate)
    148    where_condition = isinstance(predicate.find_ancestor(exp.Join, exp.Where), exp.Where)
    149
    150    for table in tables:
    151        node, source = sources.get(table) or (None, None)
    152
    153        # if the predicate is in a where statement we can try to push it down
    154        # we want to find the root join or from statement
    155        if node and where_condition:
    156            node = node.find_ancestor(exp.Join, exp.From)
    157
    158        # a node can reference a CTE which should be pushed down
    159        if isinstance(node, exp.From) and not isinstance(source, exp.Table):
    160            with_ = source.parent.expression.args.get("with")
    161            if with_ and with_.recursive:
    162                return {}
    163            node = source.expression
    164
    165        if isinstance(node, exp.Join):
    166            if node.side and node.side != "RIGHT":
    167                return {}
    168            nodes[table] = node
    169        elif isinstance(node, exp.Select) and len(tables) == 1:
    170            # We can't push down window expressions
    171            has_window_expression = any(
    172                select for select in node.selects if select.find(exp.Window)
    173            )
    174            # we can't push down predicates to select statements if they are referenced in
    175            # multiple places.
    176            if (
    177                not node.args.get("group")
    178                and scope_ref_count[id(source)] < 2
    179                and not has_window_expression
    180            ):
    181                nodes[table] = node
    182    return nodes
    183
    184
    185def replace_aliases(source, predicate):
    186    aliases = {}
    187
    188    for select in source.selects:
    189        if isinstance(select, exp.Alias):
    190            aliases[select.alias] = select.this
    191        else:
    192            aliases[select.name] = select
    193
    194    def _replace_alias(column):
    195        if isinstance(column, exp.Column) and column.name in aliases:
    196            return aliases[column.name].copy()
    197        return column
    198
    199    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 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    root = build_scope(expression)
    25
    26    if root:
    27        scope_ref_count = root.ref_count()
    28
    29        for scope in reversed(list(root.traverse())):
    30            select = scope.expression
    31            where = select.args.get("where")
    32            if where:
    33                selected_sources = scope.selected_sources
    34                # a right join can only push down to itself and not the source FROM table
    35                for k, (node, source) in selected_sources.items():
    36                    parent = node.find_ancestor(exp.Join, exp.From)
    37                    if isinstance(parent, exp.Join) and parent.side == "RIGHT":
    38                        selected_sources = {k: (node, source)}
    39                        break
    40                pushdown(where.this, selected_sources, scope_ref_count)
    41
    42            # joins should only pushdown into itself, not to other joins
    43            # so we limit the selected sources to only itself
    44            for join in select.args.get("joins") or []:
    45                name = join.this.alias_or_name
    46                pushdown(join.args.get("on"), {name: scope.selected_sources[name]}, scope_ref_count)
    47
    48    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): View Source

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

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

    70def pushdown_cnf(predicates, scope, scope_ref_count):
    71    """
    72    If the predicates are in CNF like form, we can simply replace each block in the parent.
    73    """
    74    for predicate in predicates:
    75        for node in nodes_for_predicate(predicate, scope, scope_ref_count).values():
    76            if isinstance(node, exp.Join):
    77                predicate.replace(exp.true())
    78                node.on(predicate, copy=False)
    79                break
    80            if isinstance(node, exp.Select):
    81                predicate.replace(exp.true())
    82                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

     85def pushdown_dnf(predicates, scope, scope_ref_count):
     86    """
     87    If the predicates are in DNF form, we can only push down conditions that are in all blocks.
     88    Additionally, we can't remove predicates from their original form.
     89    """
     90    # find all the tables that can be pushdown too
     91    # these are tables that are referenced in all blocks of a DNF
     92    # (a.x AND b.x) OR (a.y AND c.y)
     93    # only table a can be push down
     94    pushdown_tables = set()
     95
     96    for a in predicates:
     97        a_tables = set(exp.column_table_names(a))
     98
     99        for b in predicates:
    100            a_tables &= set(exp.column_table_names(b))
    101
    102        pushdown_tables.update(a_tables)
    103
    104    conditions = {}
    105
    106    # for every pushdown table, find all related conditions in all predicates
    107    # combine them with ORS
    108    # (a.x AND and a.y AND b.x) OR (a.z AND c.y) -> (a.x AND a.y) OR (a.z)
    109    for table in sorted(pushdown_tables):
    110        for predicate in predicates:
    111            nodes = nodes_for_predicate(predicate, scope, scope_ref_count)
    112
    113            if table not in nodes:
    114                continue
    115
    116            predicate_condition = None
    117
    118            for column in predicate.find_all(exp.Column):
    119                if column.table == table:
    120                    condition = column.find_ancestor(exp.Condition)
    121                    predicate_condition = (
    122                        exp.and_(predicate_condition, condition)
    123                        if predicate_condition
    124                        else condition
    125                    )
    126
    127            if predicate_condition:
    128                conditions[table] = (
    129                    exp.or_(conditions[table], predicate_condition)
    130                    if table in conditions
    131                    else predicate_condition
    132                )
    133
    134        for name, node in nodes.items():
    135            if name not in conditions:
    136                continue
    137
    138            predicate = conditions[name]
    139
    140            if isinstance(node, exp.Join):
    141                node.on(predicate, copy=False)
    142            elif isinstance(node, exp.Select):
    143                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

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

def replace_aliases(source, predicate): View Source

    186def replace_aliases(source, predicate):
    187    aliases = {}
    188
    189    for select in source.selects:
    190        if isinstance(select, exp.Alias):
    191            aliases[select.alias] = select.this
    192        else:
    193            aliases[select.name] = select
    194
    195    def _replace_alias(column):
    196        if isinstance(column, exp.Column) and column.name in aliases:
    197            return aliases[column.name].copy()
    198        return column
    199
    200    return predicate.transform(_replace_alias)