docs: private AbstractExpression optimizeExistsExpression(AbstractExpression existsExpr) {
          assert(ExpressionType.OPERATOR_EXISTS == existsExpr.getExpressionType());
          assert(existsExpr.getLeft() != null);
  
          if (existsExpr.getLeft() instanceof SelectSubqueryExpression) {
              SelectSubqueryExpression subqueryExpr = (SelectSubqueryExpression) existsExpr.getLeft();
              AbstractParsedStmt subquery = subqueryExpr.getSubqueryStmt();
              if (subquery instanceof ParsedSelectStmt) {
                  ParsedSelectStmt selectSubquery = (ParsedSelectStmt) subquery;
                  return selectSubquery.simplifyExistsSubqueryStmt(existsExpr);
              }
          }
          return existsExpr;
      }