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from sqlglot import exp
from sqlglot.helper import tsort
from sqlglot.optimizer.simplify import simplify
def optimize_joins(expression):
"""
Removes cross joins if possible and reorder joins based on predicate dependencies.
"""
for select in expression.find_all(exp.Select):
references = {}
cross_joins = []
for join in select.args.get("joins", []):
name = join.this.alias_or_name
tables = other_table_names(join, name)
if tables:
for table in tables:
references[table] = references.get(table, []) + [join]
else:
cross_joins.append((name, join))
for name, join in cross_joins:
for dep in references.get(name, []):
on = dep.args["on"]
on = on.replace(simplify(on))
if isinstance(on, exp.Connector):
for predicate in on.flatten():
if name in exp.column_table_names(predicate):
predicate.replace(exp.TRUE)
join.on(predicate, copy=False)
expression = reorder_joins(expression)
expression = normalize(expression)
return expression
def reorder_joins(expression):
"""
Reorder joins by topological sort order based on predicate references.
"""
for from_ in expression.find_all(exp.From):
head = from_.expressions[0]
parent = from_.parent
joins = {join.this.alias_or_name: join for join in parent.args.get("joins", [])}
dag = {head.alias_or_name: []}
for name, join in joins.items():
dag[name] = other_table_names(join, name)
parent.set(
"joins",
[joins[name] for name in tsort(dag) if name != head.alias_or_name],
)
return expression
def normalize(expression):
"""
Remove INNER and OUTER from joins as they are optional.
"""
for join in expression.find_all(exp.Join):
if join.kind != "CROSS":
join.set("kind", None)
return expression
def other_table_names(join, exclude):
return [name for name in (exp.column_table_names(join.args.get("on") or exp.TRUE)) if name != exclude]
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