rust/datafusion/src/datasource/empty.rs

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// to you under the Apache License, Version 2.0 (the
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//
//   http://www.apache.org/licenses/LICENSE-2.0
//
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//! An empty plan that is usefull for testing and generating plans without mapping them to actual data.

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

use arrow::datatypes::*;

use crate::datasource::datasource::Statistics;
use crate::datasource::TableProvider;
use crate::error::Result;
use crate::logical_plan::Expr;
use crate::physical_plan::{empty::EmptyExec, ExecutionPlan};

/// A table with a schema but no data.
pub struct EmptyTable {
    schema: SchemaRef,
}

impl EmptyTable {
    /// Initialize a new `EmptyTable` from a schema.
    pub fn new(schema: SchemaRef) -> Self {
        Self { schema }
    }
}

impl TableProvider for EmptyTable {
    fn as_any(&self) -> &dyn Any {
        self
    }

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

    fn scan(
        &self,
        projection: &Option<Vec<usize>>,
        _batch_size: usize,
        _filters: &[Expr],
    ) -> Result<Arc<dyn ExecutionPlan>> {
        // even though there is no data, projections apply
        let projection = match projection.clone() {
            Some(p) => p,
            None => (0..self.schema.fields().len()).collect(),
        };
        let projected_schema = Schema::new(
            projection
                .iter()
                .map(|i| self.schema.field(*i).clone())
                .collect(),
        );
        Ok(Arc::new(EmptyExec::new(false, Arc::new(projected_schema))))
    }

    fn statistics(&self) -> Statistics {
        Statistics {
            num_rows: Some(0),
            total_byte_size: Some(0),
            column_statistics: None,
        }
    }
}