perf: Collapse cross-joins to faster joins (#18633)

crates/polars-lazy/src/frame/mod.rs

@@ -145,6 +145,12 @@ impl LazyFrame {
         self
     }
 
+    /// Toggle collapse joins optimization.
+    pub fn with_collapse_joins(mut self, toggle: bool) -> Self {
+        self.opt_state.set(OptFlags::COLLAPSE_JOINS, toggle);
+        self
+    }
+
     /// Toggle predicate pushdown optimization.
     pub fn with_predicate_pushdown(mut self, toggle: bool) -> Self {
         self.opt_state.set(OptFlags::PREDICATE_PUSHDOWN, toggle);

crates/polars-plan/src/frame/opt_state.rs

@@ -33,6 +33,8 @@ bitflags! {
         const ROW_ESTIMATE = 1 << 13;
         /// Replace simple projections with a faster inlined projection that skips the expression engine.
         const FAST_PROJECTION = 1 << 14;
+        /// Collapse slower joins with filters into faster joins.
+        const COLLAPSE_JOINS = 1 << 15;
     }
 }
 

crates/polars-plan/src/plans/expr_ir.rs

@@ -152,6 +152,10 @@ impl ExprIR {
         self.output_name = OutputName::Alias(name)
     }
 
+    pub(crate) fn set_columnlhs(&mut self, name: PlSmallStr) {
+        self.output_name = OutputName::ColumnLhs(name)
+    }
+
     pub fn output_name_inner(&self) -> &OutputName {
         &self.output_name
     }