nuno-faria commented on code in PR #19221:
URL: https://github.com/apache/datafusion/pull/19221#discussion_r2628395045
##########
datafusion/sql/src/unparser/ast.rs:
##########
@@ -140,7 +140,15 @@ impl Default for QueryBuilder {
pub struct SelectBuilder {
distinct: Option<ast::Distinct>,
top: Option<ast::Top>,
- projection: Vec<ast::SelectItem>,
+ /// Projection items for the SELECT clause.
+ ///
+ /// This field uses `Option` to distinguish between three distinct states:
+ /// - `None`: No projection has been set (not yet initialized)
+ /// - `Some(vec![])`: Empty projection explicitly set (generates `SELECT
FROM ...` or `SELECT 1 FROM ...`)
+ /// - `Some(vec![...])`: Non-empty projection with specific
columns/expressions
+ ///
+ /// Use `projection()` to set this field and `already_projected()` to
check if it has been set.
+ projection: Option<Vec<ast::SelectItem>>,
Review Comment:
For anyone else reviewing this: projection needs to be an `Option` to be
able to distinguish between explicitly empty (`SELECT FROM`) and select all
(`SELECT * FROM`).
##########
datafusion/sql/src/unparser/plan.rs:
##########
@@ -98,6 +98,16 @@ pub fn plan_to_sql(plan: &LogicalPlan) ->
Result<ast::Statement> {
}
impl Unparser<'_> {
+ /// Generates appropriate projection expression for empty projection lists.
+ /// Returns an empty vec for dialects supporting empty select lists,
+ /// or a dummy literal `1` for other dialects.
+ fn empty_projection_fallback(&self) -> Vec<Expr> {
+ if self.dialect.supports_empty_select_list() {
+ Vec::new()
+ } else {
+ vec![Expr::Literal(ScalarValue::Int64(Some(1)), None)]
+ }
+ }
Review Comment:
nit: Perhaps it's better to move this method to the end of Unparser, so
`plan_to_sql` remains at the top.
##########
datafusion/sql/src/unparser/dialect.rs:
##########
@@ -218,6 +218,46 @@ pub trait Dialect: Send + Sync {
fn timestamp_with_tz_to_string(&self, dt: DateTime<Tz>, _unit: TimeUnit)
-> String {
dt.to_string()
}
+
+ /// Whether the dialect supports an empty select list such as `SELECT FROM
table`.
+ ///
+ /// An empty select list returns rows without any column data, which is
useful for:
+ /// - Counting rows: `SELECT FROM users WHERE active = true` (combined
with `COUNT(*)`)
+ /// - Testing row existence without retrieving column data
+ /// - Performance optimization when only row counts or existence checks
are needed
+ ///
+ /// # Supported Databases
+ ///
+ /// - **PostgreSQL**: Fully supported. Returns rows with zero columns.
+ /// - **DataFusion**: Supported natively.
Review Comment:
nit: Since there isn't a DataFusion dialect (and the `DefaultDialect`
returns false) it might be better to remove it from the documentation to avoid
any confusion.
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