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# sqlglot.optimizer.optimize_joins

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     1from sqlglot import exp
     2from sqlglot.helper import tsort
     3from sqlglot.optimizer.simplify import simplify
     4
     5
     6def optimize_joins(expression):
     7    """
     8    Removes cross joins if possible and reorder joins based on predicate dependencies.
     9
    10    Example:
    11        >>> from sqlglot import parse_one
    12        >>> optimize_joins(parse_one("SELECT * FROM x CROSS JOIN y JOIN z ON x.a = z.a AND y.a = z.a")).sql()
    13        'SELECT * FROM x JOIN z ON x.a = z.a AND TRUE JOIN y ON y.a = z.a'
    14    """
    15    for select in expression.find_all(exp.Select):
    16        references = {}
    17        cross_joins = []
    18
    19        for join in select.args.get("joins", []):
    20            name = join.this.alias_or_name
    21            tables = other_table_names(join, name)
    22
    23            if tables:
    24                for table in tables:
    25                    references[table] = references.get(table, []) + [join]
    26            else:
    27                cross_joins.append((name, join))
    28
    29        for name, join in cross_joins:
    30            for dep in references.get(name, []):
    31                on = dep.args["on"]
    32                on = on.replace(simplify(on))
    33
    34                if isinstance(on, exp.Connector):
    35                    for predicate in on.flatten():
    36                        if name in exp.column_table_names(predicate):
    37                            predicate.replace(exp.true())
    38                            join.on(predicate, copy=False)
    39
    40    expression = reorder_joins(expression)
    41    expression = normalize(expression)
    42    return expression
    43
    44
    45def reorder_joins(expression):
    46    """
    47    Reorder joins by topological sort order based on predicate references.
    48    """
    49    for from_ in expression.find_all(exp.From):
    50        head = from_.expressions[0]
    51        parent = from_.parent
    52        joins = {join.this.alias_or_name: join for join in parent.args.get("joins", [])}
    53        dag = {head.alias_or_name: []}
    54
    55        for name, join in joins.items():
    56            dag[name] = other_table_names(join, name)
    57
    58        parent.set(
    59            "joins",
    60            [joins[name] for name in tsort(dag) if name != head.alias_or_name],
    61        )
    62    return expression
    63
    64
    65def normalize(expression):
    66    """
    67    Remove INNER and OUTER from joins as they are optional.
    68    """
    69    for join in expression.find_all(exp.Join):
    70        if join.kind != "CROSS":
    71            join.set("kind", None)
    72    return expression
    73
    74
    75def other_table_names(join, exclude):
    76    return [
    77        name
    78        for name in (exp.column_table_names(join.args.get("on") or exp.true()))
    79        if name != exclude
    80    ]

def optimize_joins(expression): View Source

     7def optimize_joins(expression):
     8    """
     9    Removes cross joins if possible and reorder joins based on predicate dependencies.
    10
    11    Example:
    12        >>> from sqlglot import parse_one
    13        >>> optimize_joins(parse_one("SELECT * FROM x CROSS JOIN y JOIN z ON x.a = z.a AND y.a = z.a")).sql()
    14        'SELECT * FROM x JOIN z ON x.a = z.a AND TRUE JOIN y ON y.a = z.a'
    15    """
    16    for select in expression.find_all(exp.Select):
    17        references = {}
    18        cross_joins = []
    19
    20        for join in select.args.get("joins", []):
    21            name = join.this.alias_or_name
    22            tables = other_table_names(join, name)
    23
    24            if tables:
    25                for table in tables:
    26                    references[table] = references.get(table, []) + [join]
    27            else:
    28                cross_joins.append((name, join))
    29
    30        for name, join in cross_joins:
    31            for dep in references.get(name, []):
    32                on = dep.args["on"]
    33                on = on.replace(simplify(on))
    34
    35                if isinstance(on, exp.Connector):
    36                    for predicate in on.flatten():
    37                        if name in exp.column_table_names(predicate):
    38                            predicate.replace(exp.true())
    39                            join.on(predicate, copy=False)
    40
    41    expression = reorder_joins(expression)
    42    expression = normalize(expression)
    43    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

    46def reorder_joins(expression):
    47    """
    48    Reorder joins by topological sort order based on predicate references.
    49    """
    50    for from_ in expression.find_all(exp.From):
    51        head = from_.expressions[0]
    52        parent = from_.parent
    53        joins = {join.this.alias_or_name: join for join in parent.args.get("joins", [])}
    54        dag = {head.alias_or_name: []}
    55
    56        for name, join in joins.items():
    57            dag[name] = other_table_names(join, name)
    58
    59        parent.set(
    60            "joins",
    61            [joins[name] for name in tsort(dag) if name != head.alias_or_name],
    62        )
    63    return expression

Reorder joins by topological sort order based on predicate references.

def normalize(expression): View Source

    66def normalize(expression):
    67    """
    68    Remove INNER and OUTER from joins as they are optional.
    69    """
    70    for join in expression.find_all(exp.Join):
    71        if join.kind != "CROSS":
    72            join.set("kind", None)
    73    return expression

Remove INNER and OUTER from joins as they are optional.

def other_table_names(join, exclude): View Source

    76def other_table_names(join, exclude):
    77    return [
    78        name
    79        for name in (exp.column_table_names(join.args.get("on") or exp.true()))
    80        if name != exclude
    81    ]