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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    ]