Method try_unnest_to_lateral_flatten_sql

datafusion/sql/src/unparser/plan.rs:1617–1639

Convert an `Unnest` logical plan node to a `LATERAL FLATTEN(INPUT => expr, ...)` table factor for Snowflake-style SQL output.

(
        &self,
        unnest: &Unnest,
    )

    /// Convert an `Unnest` logical plan node to a `LATERAL FLATTEN(INPUT => expr, ...)`
    /// table factor for Snowflake-style SQL output.
    fn try_unnest_to_lateral_flatten_sql(
        &self,
        unnest: &Unnest,
    ) -> Result<Option<FlattenRelationBuilder>> {
        let Some(projection) = Self::peel_to_inner_projection(unnest.input.as_ref())
        else {
            return Ok(None);
        };

        // For now, handle the simple case of a single expression to flatten.
        // Multi-expression would require multiple LATERAL FLATTEN calls chained together.
        let Some(first_expr) = projection.expr.first() else {
            return Ok(None);
        };

        let input_expr = self.expr_to_sql(first_expr)?;

        let mut flatten = FlattenRelationBuilder::default();
        flatten.input_expr(input_expr);
        flatten.outer(unnest.options.preserve_nulls);

        Ok(Some(flatten))
    }

    fn is_scan_with_pushdown(scan: &TableScan) -> bool {
        scan.projection.is_some() || !scan.filters.is_empty() || scan.fetch.is_some()