databendlabs/databend · error

_ => unreachable!()

Error message

_ => unreachable!()

What it means

HashJoinFactory::create_grace_join panics when the join type is not one of the nine explicitly handled JoinType variants (Inner, Left, LeftSemi, LeftAnti, Right, RightSemi, RightAnti, and cross/nested-loop variants). It is an internal exhaustiveness guard: grace join construction assumes the planner only produces supported join types.

Solutions

1. Inspect the JoinType value passed in and ensure create_grace_join is only called for grace-supported types
2. Add the missing JoinType arm to the match at hash_join_factory.rs:302 if the type should be supported
3. Route unsupported types (e.g. cross/nested-loop) to create_memory_join or the nested-loop executor instead
4. Add a compile-time exhaustiveness test or match on the enum without a wildcard arm

Example fix

// before
match typ {
    JoinType::Inner => { ... }
    ...
    JoinType::RightAnti => { ... }
    _ => unreachable!(),
}
// after
JoinType::Cross => return Err(ErrorCode::Unimplemented(
    "grace join does not support cross join")),
_ => unreachable!(),

debug_assert!(matches!(typ, JoinType::Inner | JoinType::Left | JoinType::LeftSemi | JoinType::LeftAnti | JoinType::Right | JoinType::RightSemi | JoinType::RightAnti), "grace join unsupported type: {:?}", typ);

fn supports_grace_join(t: &JoinType) -> bool {
    !matches!(t, JoinType::Cross)
}

When it happens

Trigger: Calling HashJoinFactory::create_grace_join with a JoinType not covered by the match (e.g. a newly added JoinType variant, or a NestedLoop/Cross join passed into the grace-join path).

Common situations: Adding a new join type to the planner without extending the factory, constructing joins directly in tests with an exotic JoinType, or rebasing code where JoinType gained a variant.

Related errors

• _ => unreachable!()
• _ => unreachable!()
• HashJoinHashTable::NestedLoop(_) => unreachable!()
• HashJoinHashTable::NestedLoop(_) => unreachable!()
• _ => unreachable!()

AI-assisted analysis of databendlabs/databend@288d84d76e (2026-09-11). Data as JSON: /api/errors/cb0bb6c566134cb8. Report an issue: GitHub.

Appendix: source

Thrown at src/query/service/src/pipelines/processors/transforms/new_hash_join/hash_join_factory.rs:302

                let anti_right_hash_join = AntiRightHashJoin::create(
                    &self.ctx,
                    self.function_ctx.clone(),
                    self.hash_method.clone(),
                    self.desc.clone(),
                    self.create_basic_state(id)?,
                )?;

                Ok(Box::new(GraceHashJoin::create(
                    self.ctx.clone(),
                    self.function_ctx.clone(),
                    self.hash_method.clone(),
                    self.desc.clone(),
                    self.create_grace_state(id + 1)?,
                    anti_right_hash_join,
                    0,
                )?))
            }
            _ => unreachable!(),
        }
    }

    /// Create a basic memory join (used internally by HybridHashJoin)
    pub fn create_memory_join(
        self: &Arc<Self>,
        typ: JoinType,
        level: usize,
    ) -> Result<Box<dyn GraceMemoryJoin>> {
        let basic_state = self.create_basic_state(level)?;
        match typ {
            JoinType::Inner => {
                let settings = self.ctx.get_settings();
                let nested_loop_desc = self
                    .desc
                    .create_nested_loop_desc(&settings, &self.function_ctx)?;

                let inner = InnerHashJoin::create(

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