Review use of panic in datafusion-optimizer crate

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

"/home/andy/git/apache/arrow-datafusion/datafusion/optimizer/src/common_subexpr_eliminate.rs":111                 new_expr.pop().unwrap(),
"/home/andy/git/apache/arrow-datafusion/datafusion/optimizer/src/common_subexpr_eliminate.rs":145                 predicate: new_expr.pop().unwrap().pop().unwrap(),
"/home/andy/git/apache/arrow-datafusion/datafusion/optimizer/src/common_subexpr_eliminate.rs":167                 window_expr: new_expr.pop().unwrap(),
"/home/andy/git/apache/arrow-datafusion/datafusion/optimizer/src/common_subexpr_eliminate.rs":189             let new_aggr_expr = new_expr.pop().unwrap();
"/home/andy/git/apache/arrow-datafusion/datafusion/optimizer/src/common_subexpr_eliminate.rs":190             let new_group_expr = new_expr.pop().unwrap();
"/home/andy/git/apache/arrow-datafusion/datafusion/optimizer/src/common_subexpr_eliminate.rs":212                 expr: new_expr.pop().unwrap(),
"/home/andy/git/apache/arrow-datafusion/datafusion/optimizer/src/common_subexpr_eliminate.rs":276         let (expr, _, data_type) = expr_set.get(&id).unwrap();
"/home/andy/git/apache/arrow-datafusion/datafusion/optimizer/src/common_subexpr_eliminate.rs":557         unreachable!("Enter mark should paired with node number");
"/home/andy/git/apache/arrow-datafusion/datafusion/optimizer/src/common_subexpr_eliminate.rs":655         let (_, counter, _) = self.expr_set.get(curr_id).unwrap();
"/home/andy/git/apache/arrow-datafusion/datafusion/optimizer/src/common_subexpr_eliminate.rs":676             || self.expr_set.get(id).unwrap().1 <= 1
"/home/andy/git/apache/arrow-datafusion/datafusion/optimizer/src/filter_push_down.rs":180         _ => unreachable!("lr_is_preserved only valid for JOIN nodes"),
"/home/andy/git/apache/arrow-datafusion/datafusion/optimizer/src/filter_push_down.rs":195             JoinType::Semi | JoinType::Anti => unreachable!(
"/home/andy/git/apache/arrow-datafusion/datafusion/optimizer/src/filter_push_down.rs":200             unreachable!("on_lr_is_preserved cannot be applied to CROSSJOIN nodes")
"/home/andy/git/apache/arrow-datafusion/datafusion/optimizer/src/filter_push_down.rs":202         _ => unreachable!("on_lr_is_preserved only valid for JOIN nodes"),
"/home/andy/git/apache/arrow-datafusion/datafusion/optimizer/src/filter_push_down.rs":314             .chain(once(on_to_keep.into_iter().reduce(Expr::and).unwrap()))
"/home/andy/git/apache/arrow-datafusion/datafusion/optimizer/src/simplify_expressions.rs":172         _ => panic!("Expected boolean literal, got {:?}", expr),
"/home/andy/git/apache/arrow-datafusion/datafusion/optimizer/src/simplify_expressions.rs":393         if self.can_evaluate.pop().unwrap() {
"/home/andy/git/apache/arrow-datafusion/datafusion/optimizer/src/single_distinct_to_groupby.rs":176                             fields_set.insert(expr.name(input.schema()).unwrap());

Describe the solution you'd like
Review code that can panic and see where it makes sense to return a Result instead. For example, It is generally better to use ? than unwrapon results.

The goal is not to remove all panics but review and make sure we are using them appropriately. Bonus points for adding documentation for invariants.

Describe alternatives you've considered
None

Additional context
Generated by https://github.com/andygrove/no-need-to-panic

[andygrove]

These have all been resolved now.