1 files changed, 220 insertions, 0 deletions
diff --git a/sqlglot/optimizer/unnest_subqueries.py b/sqlglot/optimizer/unnest_subqueries.py
new file mode 100644
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+++ b/sqlglot/optimizer/unnest_subqueries.py
@@ -0,0 +1,220 @@
+import itertools
+
+from sqlglot import exp
+from sqlglot.optimizer.scope import traverse_scope
+
+
+def unnest_subqueries(expression):
+    """
+    Rewrite sqlglot AST to convert some predicates with subqueries into joins.
+
+    Convert the subquery into a group by so it is not a many to many left join.
+    Unnesting can only occur if the subquery does not have LIMIT or OFFSET.
+    Unnesting non correlated subqueries only happens on IN statements or = ANY statements.
+
+    Example:
+        >>> import sqlglot
+        >>> expression = sqlglot.parse_one("SELECT * FROM x AS x WHERE (SELECT y.a AS a FROM y AS y WHERE x.a = y.a) = 1 ")
+        >>> unnest_subqueries(expression).sql()
+        'SELECT * FROM x AS x LEFT JOIN (SELECT y.a AS a FROM y AS y WHERE TRUE GROUP BY y.a)\
+ AS "_u_0" ON x.a = "_u_0".a WHERE ("_u_0".a = 1 AND NOT "_u_0".a IS NULL)'
+
+    Args:
+        expression (sqlglot.Expression): expression to unnest
+    Returns:
+        sqlglot.Expression: unnested expression
+    """
+    sequence = itertools.count()
+
+    for scope in traverse_scope(expression):
+        select = scope.expression
+        parent = select.parent_select
+        if scope.external_columns:
+            decorrelate(select, parent, scope.external_columns, sequence)
+        else:
+            unnest(select, parent, sequence)
+
+    return expression
+
+
+def unnest(select, parent_select, sequence):
+    predicate = select.find_ancestor(exp.In, exp.Any)
+
+    if not predicate or parent_select is not predicate.parent_select:
+        return
+
+    if len(select.selects) > 1 or select.find(exp.Limit, exp.Offset):
+        return
+
+    if isinstance(predicate, exp.Any):
+        predicate = predicate.find_ancestor(exp.EQ)
+
+        if not predicate or parent_select is not predicate.parent_select:
+            return
+
+    column = _other_operand(predicate)
+    value = select.selects[0]
+    alias = _alias(sequence)
+
+    on = exp.condition(f'{column} = "{alias}"."{value.alias}"')
+    _replace(predicate, f"NOT {on.right} IS NULL")
+
+    parent_select.join(
+        select.group_by(value.this, copy=False),
+        on=on,
+        join_type="LEFT",
+        join_alias=alias,
+        copy=False,
+    )
+
+
+def decorrelate(select, parent_select, external_columns, sequence):
+    where = select.args.get("where")
+
+    if not where or where.find(exp.Or) or select.find(exp.Limit, exp.Offset):
+        return
+
+    table_alias = _alias(sequence)
+    keys = []
+
+    # for all external columns in the where statement,
+    # split out the relevant data to convert it into a join
+    for column in external_columns:
+        if column.find_ancestor(exp.Where) is not where:
+            return
+
+        predicate = column.find_ancestor(exp.Predicate)
+
+        if not predicate or predicate.find_ancestor(exp.Where) is not where:
+            return
+
+        if isinstance(predicate, exp.Binary):
+            key = (
+                predicate.right
+                if any(node is column for node, *_ in predicate.left.walk())
+                else predicate.left
+            )
+        else:
+            return
+
+        keys.append((key, column, predicate))
+
+    if not any(isinstance(predicate, exp.EQ) for *_, predicate in keys):
+        return
+
+    value = select.selects[0]
+    key_aliases = {}
+    group_by = []
+
+    for key, _, predicate in keys:
+        # if we filter on the value of the subquery, it needs to be unique
+        if key == value.this:
+            key_aliases[key] = value.alias
+            group_by.append(key)
+        else:
+            if key not in key_aliases:
+                key_aliases[key] = _alias(sequence)
+            # all predicates that are equalities must also be in the unique
+            # so that we don't do a many to many join
+            if isinstance(predicate, exp.EQ) and key not in group_by:
+                group_by.append(key)
+
+    parent_predicate = select.find_ancestor(exp.Predicate)
+
+    # if the value of the subquery is not an agg or a key, we need to collect it into an array
+    # so that it can be grouped
+    if not value.find(exp.AggFunc) and value.this not in group_by:
+        select.select(
+            f"ARRAY_AGG({value.this}) AS {value.alias}", append=False, copy=False
+        )
+
+    # exists queries should not have any selects as it only checks if there are any rows
+    # all selects will be added by the optimizer and only used for join keys
+    if isinstance(parent_predicate, exp.Exists):
+        select.args["expressions"] = []
+
+    for key, alias in key_aliases.items():
+        if key in group_by:
+            # add all keys to the projections of the subquery
+            # so that we can use it as a join key
+            if isinstance(parent_predicate, exp.Exists) or key != value.this:
+                select.select(f"{key} AS {alias}", copy=False)
+        else:
+            select.select(f"ARRAY_AGG({key}) AS {alias}", copy=False)
+
+    alias = exp.column(value.alias, table_alias)
+    other = _other_operand(parent_predicate)
+
+    if isinstance(parent_predicate, exp.Exists):
+        if value.this in group_by:
+            parent_predicate = _replace(parent_predicate, f"NOT {alias} IS NULL")
+        else:
+            parent_predicate = _replace(parent_predicate, "TRUE")
+    elif isinstance(parent_predicate, exp.All):
+        parent_predicate = _replace(
+            parent_predicate.parent, f"ARRAY_ALL({alias}, _x -> _x = {other})"
+        )
+    elif isinstance(parent_predicate, exp.Any):
+        if value.this in group_by:
+            parent_predicate = _replace(parent_predicate.parent, f"{other} = {alias}")
+        else:
+            parent_predicate = _replace(
+                parent_predicate, f"ARRAY_ANY({alias}, _x -> _x = {other})"
+            )
+    elif isinstance(parent_predicate, exp.In):
+        if value.this in group_by:
+            parent_predicate = _replace(parent_predicate, f"{other} = {alias}")
+        else:
+            parent_predicate = _replace(
+                parent_predicate,
+                f"ARRAY_ANY({alias}, _x -> _x = {parent_predicate.this})",
+            )
+    else:
+        select.parent.replace(alias)
+
+    for key, column, predicate in keys:
+        predicate.replace(exp.TRUE)
+        nested = exp.column(key_aliases[key], table_alias)
+
+        if key in group_by:
+            key.replace(nested)
+            parent_predicate = _replace(
+                parent_predicate, f"({parent_predicate} AND NOT {nested} IS NULL)"
+            )
+        elif isinstance(predicate, exp.EQ):
+            parent_predicate = _replace(
+                parent_predicate,
+                f"({parent_predicate} AND ARRAY_CONTAINS({nested}, {column}))",
+            )
+        else:
+            key.replace(exp.to_identifier("_x"))
+            parent_predicate = _replace(
+                parent_predicate,
+                f'({parent_predicate} AND ARRAY_ANY({nested}, "_x" -> {predicate}))',
+            )
+
+    parent_select.join(
+        select.group_by(*group_by, copy=False),
+        on=[predicate for *_, predicate in keys if isinstance(predicate, exp.EQ)],
+        join_type="LEFT",
+        join_alias=table_alias,
+        copy=False,
+    )
+
+
+def _alias(sequence):
+    return f"_u_{next(sequence)}"
+
+
+def _replace(expression, condition):
+    return expression.replace(exp.condition(condition))
+
+
+def _other_operand(expression):
+    if isinstance(expression, exp.In):
+        return expression.this
+
+    if isinstance(expression, exp.Binary):
+        return expression.right if expression.arg_key == "this" else expression.left
+
+    return None