databendlabs/databend · error

not implemented

Error message

not implemented

What it means

In PhysicalSort's build_pipeline2, when a sort step leaves the main pipeline with more than one output and max_threads == 1, the code deliberately panics with unimplemented!() — the comment notes the query would otherwise hang in MultiSortMergeProcessor. So single-threaded execution of this sort shape is unsupported and raises 'not implemented'.

Solutions

1. Increase max_threads to >= 2 for the session/query (SET max_threads = 2 or higher).
2. Remove/adjust settings or resource-group limits that pin execution to a single thread.
3. Rewrite the query to avoid the multi-output sort shape, or add an explicit LIMIT that lets the planner collapse outputs.
4. For maintainers: replace the panic by merging outputs into one stream (resize pipeline) or fixing MultiSortMergeProcessor for max_threads == 1.

Example fix

-- before
SET max_threads = 1;  -- then ORDER BY query panics
-- after
SET max_threads = 4;

-- ensure multi-threaded execution for sort-heavy queries
SELECT if(current_setting('max_threads')::int < 2, 'raise max_threads to >= 2', 'ok') AS check;

fn single_threaded(ctx: &dyn TableContext) -> bool { ctx.get_settings().get_max_threads().unwrap_or(1) <= 1 }

match build_pipeline2(builder, ctx) {
    Err(e) if e.to_string().contains("not implemented") && ctx.get_max_threads() == 1 => {
        retry_with_min_threads(2)?
    }
    other => other?,
}

When it happens

Trigger: Executing a query with a sort (SortStep) whose pipeline output_len != 1 while running with max_threads == 1 (e.g. single-threaded query settings, constrained executor config, or resource-group limiting threads to 1).

Common situations: Environments that force single-threaded execution: setting max_threads=1 / max_threads setting to 1 in the query context, CI tests with single-thread runtimes, or resource-constrained deployments; certain ORDER BY queries then panic instead of completing.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• internal error: entered unreachable code
• internal error: entered unreachable code
• internal error: entered unreachable code
• Unsupported column type for encoding
• Unsupported column type for length calculation

AI-assisted analysis of databendlabs/databend@288d84d76e (2026-09-11). Data as JSON: /api/errors/289a5d7245978c5e. Report an issue: GitHub.

Appendix: source

Thrown at src/query/service/src/physical_plans/physical_sort.rs:290

                Ok(())
            }
            SortStep::Shuffled => {
                if Exchange::check_physical_plan(&self.input) {
                    let exchange = TransformSortBuilder::exchange_injector();
                    let old_inject = std::mem::replace(&mut builder.exchange_injector, exchange);
                    self.input.build_pipeline(builder)?;
                    builder.exchange_injector = old_inject;
                } else {
                    self.input.build_pipeline(builder)?;
                }

                if builder.main_pipeline.output_len() == 1 {
                    return Ok(());
                }

                if max_threads == 1 {
                    // TODO(Winter): the query will hang in MultiSortMergeProcessor when max_threads == 1 and output_len != 1
                    unimplemented!();
                }
                sort_builder.build_bounded_merge_sort(&mut builder.main_pipeline)
            }
            SortStep::Route => {
                if builder.main_pipeline.output_len() == 1 {
                    builder
                        .main_pipeline
                        .add_transformer(TransformSortBuilder::build_dummy_route);
                    Ok(())
                } else {
                    TransformSortBuilder::add_route(&mut builder.main_pipeline)
                }
            }
        }
    }
}

impl Sort {

View on GitHub (pinned to 288d84d76e)