class_docstr: Visits the SQL query plan to simplify sub-queries. On each visit, return a simplified node.
  
  Unlike SqlSubQueryReducerVisitor, this will re-write expressions to realize more reductions.
code: def _select_columns_are_column_references(
      select_columns: Tuple[SqlSelectColumn, ...]
  ) -> bool:
      for select_column in select_columns:
          if not select_column.expr.as_column_reference_expression:
              return False
      return True
  
imports: from __future__ import annotations
  import logging
  from dataclasses import dataclass
  from typing import List, Optional, Sequence, Tuple
  from metricflow.sql.optimizer.sql_query_plan_optimizer import SqlQueryPlanOptimizer
  from metricflow.sql.sql_exprs import SqlColumnAliasReferenceExpression, SqlColumnReference, SqlColumnReplacements, SqlExpressionNode, SqlExpressionTreeLineage, SqlLogicalExpression, SqlLogicalOperator
  from metricflow.sql.sql_plan import SqlJoinDescription, SqlOrderByDescription, SqlQueryPlanNode, SqlQueryPlanNodeVisitor, SqlSelectColumn, SqlSelectQueryFromClauseNode, SqlSelectStatementNode, SqlTableFromClauseNode
label_desc: Use image node_id 4 for calling the SqlRewritingSubQueryReducerVisitor obj's underlying member method code with example usage: obj._select_columns_are_column_references(select_columns) and returns: True, False