83 lines
2.5 KiB
Python
83 lines
2.5 KiB
Python
from sqlglot import exp
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from sqlglot.helper import tsort
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JOIN_ATTRS = ("on", "side", "kind", "using", "method")
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def optimize_joins(expression):
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"""
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Removes cross joins if possible and reorder joins based on predicate dependencies.
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Example:
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>>> from sqlglot import parse_one
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>>> optimize_joins(parse_one("SELECT * FROM x CROSS JOIN y JOIN z ON x.a = z.a AND y.a = z.a")).sql()
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'SELECT * FROM x JOIN z ON x.a = z.a AND TRUE JOIN y ON y.a = z.a'
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"""
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for select in expression.find_all(exp.Select):
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references = {}
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cross_joins = []
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for join in select.args.get("joins", []):
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name = join.this.alias_or_name
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tables = other_table_names(join, name)
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if tables:
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for table in tables:
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references[table] = references.get(table, []) + [join]
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else:
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cross_joins.append((name, join))
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for name, join in cross_joins:
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for dep in references.get(name, []):
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on = dep.args["on"]
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if isinstance(on, exp.Connector):
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for predicate in on.flatten():
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if name in exp.column_table_names(predicate):
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predicate.replace(exp.true())
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join.on(predicate, copy=False)
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expression = reorder_joins(expression)
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expression = normalize(expression)
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return expression
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def reorder_joins(expression):
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"""
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Reorder joins by topological sort order based on predicate references.
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"""
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for from_ in expression.find_all(exp.From):
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head = from_.this
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parent = from_.parent
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joins = {join.this.alias_or_name: join for join in parent.args.get("joins", [])}
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dag = {head.alias_or_name: []}
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for name, join in joins.items():
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dag[name] = other_table_names(join, name)
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parent.set(
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"joins",
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[joins[name] for name in tsort(dag) if name != head.alias_or_name],
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)
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return expression
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def normalize(expression):
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"""
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Remove INNER and OUTER from joins as they are optional.
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"""
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for join in expression.find_all(exp.Join):
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if not any(join.args.get(k) for k in JOIN_ATTRS):
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join.set("kind", "CROSS")
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if join.kind != "CROSS":
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join.set("kind", None)
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return expression
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def other_table_names(join, exclude):
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return [
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name
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for name in (exp.column_table_names(join.args.get("on") or exp.true()))
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if name != exclude
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]
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