rust/datafusion/src/physical_plan/empty.rs

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//
//   http://www.apache.org/licenses/LICENSE-2.0
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//! EmptyRelation execution plan

use std::any::Any;
use std::sync::Arc;

use crate::error::{DataFusionError, Result};
use crate::physical_plan::memory::MemoryStream;
use crate::physical_plan::{Distribution, ExecutionPlan, Partitioning};
use arrow::array::NullArray;
use arrow::datatypes::{DataType, Field, Schema, SchemaRef};
use arrow::record_batch::RecordBatch;

use super::SendableRecordBatchStream;

use async_trait::async_trait;

/// Execution plan for empty relation (produces no rows)
#[derive(Debug)]
pub struct EmptyExec {
    /// Specifies whether this exec produces a row or not
    produce_one_row: bool,
    /// The schema for the produced row
    schema: SchemaRef,
}

impl EmptyExec {
    /// Create a new EmptyExec
    pub fn new(produce_one_row: bool, schema: SchemaRef) -> Self {
        EmptyExec {
            produce_one_row,
            schema,
        }
    }

    /// Specifies whether this exec produces a row or not
    pub fn produce_one_row(&self) -> bool {
        self.produce_one_row
    }
}

#[async_trait]
impl ExecutionPlan for EmptyExec {
    /// Return a reference to Any that can be used for downcasting
    fn as_any(&self) -> &dyn Any {
        self
    }

    fn schema(&self) -> SchemaRef {
        self.schema.clone()
    }

    fn children(&self) -> Vec<Arc<dyn ExecutionPlan>> {
        vec![]
    }

    fn required_child_distribution(&self) -> Distribution {
        Distribution::UnspecifiedDistribution
    }

    /// Get the output partitioning of this plan
    fn output_partitioning(&self) -> Partitioning {
        Partitioning::UnknownPartitioning(1)
    }

    fn with_new_children(
        &self,
        children: Vec<Arc<dyn ExecutionPlan>>,
    ) -> Result<Arc<dyn ExecutionPlan>> {
        match children.len() {
            0 => Ok(Arc::new(EmptyExec::new(false, self.schema.clone()))),
            _ => Err(DataFusionError::Internal(
                "EmptyExec wrong number of children".to_string(),
            )),
        }
    }

    async fn execute(&self, partition: usize) -> Result<SendableRecordBatchStream> {
        // GlobalLimitExec has a single output partition
        if 0 != partition {
            return Err(DataFusionError::Internal(format!(
                "EmptyExec invalid partition {} (expected 0)",
                partition
            )));
        }

        // Makes a stream only contains one null element if needed
        let data = if self.produce_one_row {
            vec![RecordBatch::try_new(
                Arc::new(Schema::new(vec![Field::new(
                    "placeholder",
                    DataType::Null,
                    true,
                )])),
                vec![Arc::new(NullArray::new(1))],
            )?]
        } else {
            vec![]
        };

        Ok(Box::pin(MemoryStream::try_new(
            data,
            self.schema.clone(),
            None,
        )?))
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::physical_plan::common;
    use crate::test;

    #[tokio::test]
    async fn empty() -> Result<()> {
        let schema = test::aggr_test_schema();

        let empty = EmptyExec::new(false, schema.clone());
        assert_eq!(empty.schema(), schema);

        // we should have no results
        let iter = empty.execute(0).await?;
        let batches = common::collect(iter).await?;
        assert!(batches.is_empty());

        Ok(())
    }

    #[test]
    fn with_new_children() -> Result<()> {
        let schema = test::aggr_test_schema();
        let empty = EmptyExec::new(false, schema);

        let empty2 = empty.with_new_children(vec![])?;
        assert_eq!(empty.schema(), empty2.schema());

        let too_many_kids = vec![empty2];
        assert!(
            empty.with_new_children(too_many_kids).is_err(),
            "expected error when providing list of kids"
        );
        Ok(())
    }

    #[tokio::test]
    async fn invalid_execute() -> Result<()> {
        let schema = test::aggr_test_schema();
        let empty = EmptyExec::new(false, schema);

        // ask for the wrong partition
        assert!(empty.execute(1).await.is_err());
        assert!(empty.execute(20).await.is_err());
        Ok(())
    }

    #[tokio::test]
    async fn produce_one_row() -> Result<()> {
        let schema = test::aggr_test_schema();
        let empty = EmptyExec::new(true, schema);

        let iter = empty.execute(0).await?;
        let batches = common::collect(iter).await?;

        // should have one item
        assert_eq!(batches.len(), 1);

        Ok(())
    }
}