Add TableProvider.statistics method (apache#3986)

datafusion/common/src/stats.rs

@@ -15,11 +15,11 @@
 // specific language governing permissions and limitations
 // under the License.
 
-//! This module provides an interface for plan level statistics.
+//! This module provides data structures to represent statistics
 
 use crate::ScalarValue;
 
-/// Statistics for a physical plan node
+/// Statistics for a relation
 /// Fields are optional and can be inexact because the sources
 /// sometimes provide approximate estimates for performance reasons
 /// and the transformations output are not always predictable.
@@ -37,7 +37,7 @@ pub struct Statistics {
  pub is_exact: bool,
 }
 
-/// This table statistics are estimates about column
+/// Statistics for a column within a relation
 #[derive(Clone, Debug, Default, PartialEq, Eq)]
 pub struct ColumnStatistics {
  /// Number of null values on column

datafusion/core/src/datasource/datasource.rs

@@ -21,6 +21,7 @@ use std::any::Any;
 use std::sync::Arc;
 
 use async_trait::async_trait;
+use datafusion_common::Statistics;
 use datafusion_expr::LogicalPlan;
 pub use datafusion_expr::{TableProviderFilterPushDown, TableType};
 
@@ -77,6 +78,11 @@ pub trait TableProvider: Sync + Send {
  ) -> Result<TableProviderFilterPushDown> {
  Ok(TableProviderFilterPushDown::Unsupported)
  }
+
+ /// Get statistics for this table, if available
+ fn statistics(&self) -> Option<Statistics> {
+ None
+ }
 }
 
 /// A factory which creates [`TableProvider`]s at runtime given a URL.