yonatan-sevenai commented on code in PR #21593:
URL: https://github.com/apache/datafusion/pull/21593#discussion_r3101832311
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datafusion/sql/src/unparser/dialect.rs:
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@@ -211,6 +206,15 @@ pub trait Dialect: Send + Sync {
false
}
+ /// Unparse the unnest plan as `LATERAL FLATTEN(INPUT => expr, ...)`.
+ ///
+ /// Snowflake uses FLATTEN as a table function instead of the SQL-standard
UNNEST.
+ /// When this returns `true`, the unparser emits
+ /// `LATERAL FLATTEN(INPUT => <col>, OUTER => <bool>)` in the FROM clause.
+ fn unnest_as_lateral_flatten(&self) -> bool {
+ false
+ }
Review Comment:
I implemented all the changes mentioned below except moving the
`FlattenRelationBuilder` inside the dialect.
I'd love to get the idea of dialect specific hooks implemented, but I'm not
sure what's the best way to get there.
We can easily move the `FlattenRelationBuilder` into the dialect, but it's
currently created in `try_unnest_to_lateral_flatten_sql` which does structural
traversal and then used again in `try_projection_unnest_as_lateral_flatten`,
both do structural traversal and unpeeling of the plan so they still belong to
`plan.rs`.
Therefore, if we simply move the `FlattenRelationBuilder` we either have a
dialect generic method that returns a `Box<dyn RelationBuilder>` or hard code
the pairing into `SnowflakeDialect::FlattenRelationBuilder`.
Neither of those feel attractive to me.
A long term option to really think if there's a more generic way to do
dialect specific transforms, and bring *both* the structural analysis (plan
traversal, unpeeling, etc) and AST builder into the dialects, but I feel this
is a pretty invasive change with a large blast radius that I don't want to bind
into this PR. Consider, for example, how BQ handles `UNNEST` is also not
entirely similar to how Postgres (for example) handles `UNNEST`s
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