Mihai Budiu created CALCITE-6939:
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Summary: 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
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-1306], 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.
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