datafusion 0.13.0

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//
//   http://www.apache.org/licenses/LICENSE-2.0
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//! Limit relation, to limit the number of rows returned by a relation

use std::cell::RefCell;
use std::rc::Rc;
use std::sync::Arc;

use arrow::array::*;
use arrow::compute::array_ops::limit;
use arrow::datatypes::Schema;
use arrow::record_batch::RecordBatch;

use crate::error::{ExecutionError, Result};
use crate::execution::relation::Relation;

/// Implementation of a LIMIT relation
pub(super) struct LimitRelation {
    /// The relation which the limit is being applied to
    input: Rc<RefCell<Relation>>,
    /// The schema for the limit relation, which is always the same as the schema of the input relation
    schema: Arc<Schema>,
    /// The number of rows returned by this relation
    limit: usize,
    /// The number of rows that have been returned so far
    num_consumed_rows: usize,
}

impl LimitRelation {
    pub fn new(input: Rc<RefCell<Relation>>, limit: usize, schema: Arc<Schema>) -> Self {
        Self {
            input,
            schema,
            limit,
            num_consumed_rows: 0,
        }
    }
}

impl Relation for LimitRelation {
    fn next(&mut self) -> Result<Option<RecordBatch>> {
        match self.input.borrow_mut().next()? {
            Some(batch) => {
                let capacity = self.limit - self.num_consumed_rows;

                if capacity <= 0 {
                    return Ok(None);
                }

                if batch.num_rows() >= capacity {
                    let limited_columns: Result<Vec<ArrayRef>> = (0..batch.num_columns())
                        .map(|i| match limit(batch.column(i), capacity) {
                            Ok(result) => Ok(result),
                            Err(error) => Err(ExecutionError::from(error)),
                        })
                        .collect();

                    let limited_batch: RecordBatch =
                        RecordBatch::try_new(self.schema.clone(), limited_columns?)?;
                    self.num_consumed_rows += capacity;

                    Ok(Some(limited_batch))
                } else {
                    self.num_consumed_rows += batch.num_rows();
                    Ok(Some(batch))
                }
            }
            None => Ok(None),
        }
    }

    fn schema(&self) -> &Arc<Schema> {
        &self.schema
    }
}