Merging upstream version 7.1.3.

Signed-off-by: Daniel Baumann <daniel@debian.org>

sqlglot/optimizer/eliminate_ctes.py

@@ -0,0 +1,42 @@
+from sqlglot.optimizer.scope import Scope, build_scope
+
+
+def eliminate_ctes(expression):
+    """
+    Remove unused CTEs from an expression.
+
+    Example:
+        >>> import sqlglot
+        >>> sql = "WITH y AS (SELECT a FROM x) SELECT a FROM z"
+        >>> expression = sqlglot.parse_one(sql)
+        >>> eliminate_ctes(expression).sql()
+        'SELECT a FROM z'
+
+    Args:
+        expression (sqlglot.Expression): expression to optimize
+    Returns:
+        sqlglot.Expression: optimized expression
+    """
+    root = build_scope(expression)
+
+    ref_count = root.ref_count()
+
+    # Traverse the scope tree in reverse so we can remove chains of unused CTEs
+    for scope in reversed(list(root.traverse())):
+        if scope.is_cte:
+            count = ref_count[id(scope)]
+            if count <= 0:
+                cte_node = scope.expression.parent
+                with_node = cte_node.parent
+                cte_node.pop()
+
+                # Pop the entire WITH clause if this is the last CTE
+                if len(with_node.expressions) <= 0:
+                    with_node.pop()
+
+                # Decrement the ref count for all sources this CTE selects from
+                for _, source in scope.selected_sources.values():
+                    if isinstance(source, Scope):
+                        ref_count[id(source)] -= 1
+
+    return expression