datafusion 4.0.0

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// to you under the Apache License, Version 2.0 (the
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// with the License.  You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
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// software distributed under the License is distributed on an
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// under the License.

//! Data source traits

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

use crate::error::Result;
use crate::logical_plan::Expr;
use crate::physical_plan::ExecutionPlan;
use crate::{arrow::datatypes::SchemaRef, scalar::ScalarValue};

/// This table statistics are estimates.
/// It can not be used directly in the precise compute
#[derive(Debug, Clone, Default)]
pub struct Statistics {
    /// The number of table rows
    pub num_rows: Option<usize>,
    /// total byte of the table rows
    pub total_byte_size: Option<usize>,
    /// Statistics on a column level
    pub column_statistics: Option<Vec<ColumnStatistics>>,
}
/// This table statistics are estimates about column
#[derive(Clone, Debug, PartialEq)]
pub struct ColumnStatistics {
    /// Number of null values on column
    pub null_count: Option<usize>,
    /// Maximum value of column
    pub max_value: Option<ScalarValue>,
    /// Minimum value of column
    pub min_value: Option<ScalarValue>,
    /// Number of distinct values
    pub distinct_count: Option<usize>,
}

/// Indicates whether and how a filter expression can be handled by a
/// TableProvider for table scans.
#[derive(Debug, Clone)]
pub enum TableProviderFilterPushDown {
    /// The expression cannot be used by the provider.
    Unsupported,
    /// The expression can be used to help minimise the data retrieved,
    /// but the provider cannot guarantee that all returned tuples
    /// satisfy the filter. The Filter plan node containing this expression
    /// will be preserved.
    Inexact,
    /// The provider guarantees that all returned data satisfies this
    /// filter expression. The Filter plan node containing this expression
    /// will be removed.
    Exact,
}

/// Source table
pub trait TableProvider: Sync + Send {
    /// Returns the table provider as [`Any`](std::any::Any) so that it can be
    /// downcast to a specific implementation.
    fn as_any(&self) -> &dyn Any;

    /// Get a reference to the schema for this table
    fn schema(&self) -> SchemaRef;

    /// Create an ExecutionPlan that will scan the table.
    fn scan(
        &self,
        projection: &Option<Vec<usize>>,
        batch_size: usize,
        filters: &[Expr],
        // limit can be used to reduce the amount scanned
        // from the datasource as a performance optimization.
        // If set, it contains the amount of rows needed by the `LogicalPlan`,
        // The datasource should return *at least* this number of rows if available.
        limit: Option<usize>,
    ) -> Result<Arc<dyn ExecutionPlan>>;

    /// Returns the table Statistics
    /// Statistics should be optional because not all data sources can provide statistics.
    fn statistics(&self) -> Statistics;

    /// Tests whether the table provider can make use of a filter expression
    /// to optimise data retrieval.
    fn supports_filter_pushdown(
        &self,
        _filter: &Expr,
    ) -> Result<TableProviderFilterPushDown> {
        Ok(TableProviderFilterPushDown::Unsupported)
    }
}