[
https://issues.apache.org/jira/browse/CALCITE-6939?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17941211#comment-17941211
]
Julian Hyde edited comment on CALCITE-6939 at 4/5/25 8:11 AM:
--------------------------------------------------------------
I don’t think you should resolve left-to-right. This would make some
refactorings tricky. Allow backward and forward references, but check for
cycles. I proposed something similar in CALCITE-4496.
I also don’t agree with the “only if all the other name lookup methods have
failed” strategy. An unqualified column reference x should prefer a select
expression to a column in one of the tables in the FROM clause. Otherwise the
behavior of the query would change if someone added a column x to one of the
tables. If this is to be a proper feature, it should use a sensible scoping
order.
I think you can leave it unspecified whether expressions are repeatable. If
people want deterministic behavior they can use a CTE.
was (Author: julianhyde):
I don’t think you should resolve left-to-right. This would make some
refactorings tricky. Allow backward and forward references, but check for
cycles. I proposed something similar in CALCITE-4496.
I think you can leave it unspecified whether expressions are repeatable. If
people want deterministic behavior they can use a CTE.
> Add support for Lateral Column Alias
> ------------------------------------
>
> Key: CALCITE-6939
> URL: https://issues.apache.org/jira/browse/CALCITE-6939
> Project: Calcite
> Issue Type: Wish
> Components: core
> Affects Versions: 1.39.0
> Reporter: Mihai Budiu
> Priority: Minor
> Labels: pull-request-available
>
> Several SQL dialects allow a SELECT to refer to aliases introduced previously
> in the same select.
> Examples are: Spark, BigQuery, Snowflake. Other names for this feature are
> "column aliasing with immediate reference", or "column alias visibility in
> the same SELECT statement".
> This feature was already proposed and implemented by [CALCITE-5248], but that
> issue and PR contains multiple other unrelated features.
> I am also proposing adding a conformance flag `isSelectAlias` which enables
> this feature. This is strictly a validator feature.
> The semantics has to be clearly defined; we do not want to change the
> semantics of any existing query. My proposal is to allow a name to be
> resolved in the current SELECT statement only if all the other name lookup
> methods have failed.
> I also think it's a good idea to introduce new aliases from left to right, so
> a expression can only refer to aliases introduced to the left of it. This
> removes a few unpleasant corner cases too.
> Another difficult problem is the handling of nondeterministic expressions.
> When you refer to a prior expression by an alias, can/is the expression
> re-evaluated? Unfortunately I believe that the current Calcite IR makes it
> difficult to represent the program in such a way that re-evaluation is
> avoided. (But this is a problem with many other Calcite rewrite rules as
> well.) This feature is still useful when applied to deterministic expressions.
--
This message was sent by Atlassian Jira
(v8.20.10#820010)
