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# sqlglot.optimizer.optimize_joins

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     1from __future__ import annotations
     2
     3import typing as t
     4
     5from sqlglot import exp
     6from sqlglot.helper import tsort
     7
     8JOIN_ATTRS = ("on", "side", "kind", "using", "method")
     9
    10
    11def optimize_joins(expression):
    12    """
    13    Removes cross joins if possible and reorder joins based on predicate dependencies.
    14
    15    Example:
    16        >>> from sqlglot import parse_one
    17        >>> optimize_joins(parse_one("SELECT * FROM x CROSS JOIN y JOIN z ON x.a = z.a AND y.a = z.a")).sql()
    18        'SELECT * FROM x JOIN z ON x.a = z.a AND TRUE JOIN y ON y.a = z.a'
    19    """
    20
    21    for select in expression.find_all(exp.Select):
    22        references = {}
    23        cross_joins = []
    24
    25        for join in select.args.get("joins", []):
    26            tables = other_table_names(join)
    27
    28            if tables:
    29                for table in tables:
    30                    references[table] = references.get(table, []) + [join]
    31            else:
    32                cross_joins.append((join.alias_or_name, join))
    33
    34        for name, join in cross_joins:
    35            for dep in references.get(name, []):
    36                on = dep.args["on"]
    37
    38                if isinstance(on, exp.Connector):
    39                    if len(other_table_names(dep)) < 2:
    40                        continue
    41
    42                    for predicate in on.flatten():
    43                        if name in exp.column_table_names(predicate):
    44                            predicate.replace(exp.true())
    45                            join.on(predicate, copy=False)
    46
    47    expression = reorder_joins(expression)
    48    expression = normalize(expression)
    49    return expression
    50
    51
    52def reorder_joins(expression):
    53    """
    54    Reorder joins by topological sort order based on predicate references.
    55    """
    56    for from_ in expression.find_all(exp.From):
    57        parent = from_.parent
    58        joins = {join.alias_or_name: join for join in parent.args.get("joins", [])}
    59        dag = {name: other_table_names(join) for name, join in joins.items()}
    60        parent.set(
    61            "joins",
    62            [joins[name] for name in tsort(dag) if name != from_.alias_or_name and name in joins],
    63        )
    64    return expression
    65
    66
    67def normalize(expression):
    68    """
    69    Remove INNER and OUTER from joins as they are optional.
    70    """
    71    for join in expression.find_all(exp.Join):
    72        if not any(join.args.get(k) for k in JOIN_ATTRS):
    73            join.set("kind", "CROSS")
    74
    75        if join.kind != "CROSS":
    76            join.set("kind", None)
    77    return expression
    78
    79
    80def other_table_names(join: exp.Join) -> t.Set[str]:
    81    on = join.args.get("on")
    82    return exp.column_table_names(on, join.alias_or_name) if on else set()

JOIN_ATTRS = ('on', 'side', 'kind', 'using', 'method')
def optimize_joins(expression): View Source

    12def optimize_joins(expression):
    13    """
    14    Removes cross joins if possible and reorder joins based on predicate dependencies.
    15
    16    Example:
    17        >>> from sqlglot import parse_one
    18        >>> optimize_joins(parse_one("SELECT * FROM x CROSS JOIN y JOIN z ON x.a = z.a AND y.a = z.a")).sql()
    19        'SELECT * FROM x JOIN z ON x.a = z.a AND TRUE JOIN y ON y.a = z.a'
    20    """
    21
    22    for select in expression.find_all(exp.Select):
    23        references = {}
    24        cross_joins = []
    25
    26        for join in select.args.get("joins", []):
    27            tables = other_table_names(join)
    28
    29            if tables:
    30                for table in tables:
    31                    references[table] = references.get(table, []) + [join]
    32            else:
    33                cross_joins.append((join.alias_or_name, join))
    34
    35        for name, join in cross_joins:
    36            for dep in references.get(name, []):
    37                on = dep.args["on"]
    38
    39                if isinstance(on, exp.Connector):
    40                    if len(other_table_names(dep)) < 2:
    41                        continue
    42
    43                    for predicate in on.flatten():
    44                        if name in exp.column_table_names(predicate):
    45                            predicate.replace(exp.true())
    46                            join.on(predicate, copy=False)
    47
    48    expression = reorder_joins(expression)
    49    expression = normalize(expression)
    50    return expression

Removes cross joins if possible and reorder joins based on predicate dependencies.

###### Example:

>     >>> from sqlglot import parse_one
>     >>> optimize_joins(parse_one("SELECT * FROM x CROSS JOIN y JOIN z ON x.a = z.a AND y.a = z.a")).sql()
>     'SELECT * FROM x JOIN z ON x.a = z.a AND TRUE JOIN y ON y.a = z.a'

def reorder_joins(expression): View Source

    53def reorder_joins(expression):
    54    """
    55    Reorder joins by topological sort order based on predicate references.
    56    """
    57    for from_ in expression.find_all(exp.From):
    58        parent = from_.parent
    59        joins = {join.alias_or_name: join for join in parent.args.get("joins", [])}
    60        dag = {name: other_table_names(join) for name, join in joins.items()}
    61        parent.set(
    62            "joins",
    63            [joins[name] for name in tsort(dag) if name != from_.alias_or_name and name in joins],
    64        )
    65    return expression

Reorder joins by topological sort order based on predicate references.

def normalize(expression): View Source

    68def normalize(expression):
    69    """
    70    Remove INNER and OUTER from joins as they are optional.
    71    """
    72    for join in expression.find_all(exp.Join):
    73        if not any(join.args.get(k) for k in JOIN_ATTRS):
    74            join.set("kind", "CROSS")
    75
    76        if join.kind != "CROSS":
    77            join.set("kind", None)
    78    return expression

Remove INNER and OUTER from joins as they are optional.

def other_table_names(join: sqlglot.expressions.Join) -> Set[str]: View Source

    81def other_table_names(join: exp.Join) -> t.Set[str]:
    82    on = join.args.get("on")
    83    return exp.column_table_names(on, join.alias_or_name) if on else set()