Preserving column names through the optimization process, and many rewrite rules being applied, is very hard if not impossible.
Instead I would approach this as the RelToSqlConverter does, and try to produce the best/most concise/most human-readable SQL possible given a RelRoot. Flattening the subquery that is generated to project/rename the output columns of a Sort seems a reasonable thing to do for RelToSqlConverter to do. My approach would be to write a unit test and torture the code until it passes. :) > On Jun 25, 2025, at 3:07 AM, Yanjing Wang <zhuangzixiao...@gmail.com> wrote: > > Hi all, I'd like to discuss a challenge regarding the preservation of > column aliases after RelNode optimization in Apache Calcite. Let me outline > the specific problem and potential approaches. Problem Statement: > Currently, when applying RelNode optimization rules, Calcite doesn't > preserve the original output column aliases. While using RelRoot seems like > a potential solution, it introduces complications when the optimal RelNode > is a Sort or similar node. Consider this scenario with a best rel: ` SELECT > column1, ... FROM ... ORDER BY column1 DESC ` If we try to preserve aliases > using RelRoot, it might generate SQL like: ` SELECT column1 AS alias1, ... > FROM ( SELECT ... ORDER BY column1 DESC ) t1 ` This transformation can > break the ORDER BY clause functionality in many compute engines. Questions > for Discussion: 1. Is there an existing mature solution in Calcite for > maintaining output alias consistency after optimization? 2. If not, what > would be the recommended approach when dealing with Sort nodes (or similar > RelNodes) that could be affected by RelRoot-based alias preservation? 3. If > we implement a new solution, should we introduce additional RelOptRules to > optimize the resulting RelNode structure? This would ensure we maintain > both alias consistency and query performance. > I'd appreciate your thoughts and suggestions on this matter, especially > from those who have encountered similar challenges. Best regards, Yanjing > Wang
