feat: API for collecting statistics/index for metadata of a parquet f…

…ile + tests (apache#10537)

* test: some tests to write data to a parquet file and read its metadata

* feat: API to convert parquet stats to arrow stats

* Refine statistics extraction API and tests

* Implement null counts

* port test

* test: add more tests for the arrow statistics

* chore: fix format and test output

* chore: rename test helpers

* chore: Apply suggestions from code review

Co-authored-by: Andrew Lamb <[email protected]>

* Apply suggestions from code review

* Apply suggestions from code review

---------

Co-authored-by: Andrew Lamb <[email protected]>

datafusion/core/src/datasource/physical_plan/parquet/mod.rs

@@ -72,6 +72,7 @@ mod statistics;
 
 pub use metrics::ParquetFileMetrics;
 pub use schema_adapter::{SchemaAdapter, SchemaAdapterFactory, SchemaMapper};
+pub use statistics::{RequestedStatistics, StatisticsConverter};
 
 /// Execution plan for scanning one or more Parquet partitions
 #[derive(Debug, Clone)]
@@ -482,7 +483,6 @@ struct ParquetOpener {
 impl FileOpener for ParquetOpener {
     fn open(&self, file_meta: FileMeta) -> Result<FileOpenFuture> {
         let file_range = file_meta.range.clone();
-
         let file_metrics = ParquetFileMetrics::new(
             self.partition_index,
             file_meta.location().as_ref(),

datafusion/core/src/datasource/physical_plan/parquet/statistics.rs

@@ -20,11 +20,15 @@
 // TODO: potentially move this to arrow-rs: https://github.com/apache/arrow-rs/issues/4328
 
 use arrow::{array::ArrayRef, datatypes::DataType};
-use arrow_array::new_empty_array;
-use arrow_schema::{FieldRef, Schema};
-use datafusion_common::{Result, ScalarValue};
+use arrow_array::{new_empty_array, new_null_array, UInt64Array};
+use arrow_schema::{Field, FieldRef, Schema};
+use datafusion_common::{
+    internal_datafusion_err, internal_err, plan_err, Result, ScalarValue,
+};
+use parquet::file::metadata::ParquetMetaData;
 use parquet::file::statistics::Statistics as ParquetStatistics;
 use parquet::schema::types::SchemaDescriptor;
+use std::sync::Arc;
 
 // Convert the bytes array to i128.
 // The endian of the input bytes array must be big-endian.
@@ -210,6 +214,152 @@ fn collect_scalars<I: Iterator<Item = Option<ScalarValue>>>(
     }
 }
 
+/// What type of statistics should be extracted?
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub enum RequestedStatistics {
+    /// Minimum Value
+    Min,
+    /// Maximum Value
+    Max,
+    /// Null Count, returned as a [`UInt64Array`])
+    NullCount,
+}
+
+/// Extracts Parquet statistics as Arrow arrays
+///
+/// This is used to convert Parquet statistics to Arrow arrays, with proper type
+/// conversions. This information can be used for pruning parquet files or row
+/// groups based on the statistics embedded in parquet files
+///
+/// # Schemas
+///
+/// The schema of the parquet file and the arrow schema are used to convert the
+/// underlying statistics value (stored as a parquet value) into the
+/// corresponding Arrow  value. For example, Decimals are stored as binary in
+/// parquet files.
+///
+/// The parquet_schema and arrow _schema do not have to be identical (for
+/// example, the columns may be in different orders and one or the other schemas
+/// may have additional columns). The function [`parquet_column`] is used to
+/// match the column in the parquet file to the column in the arrow schema.
+///
+/// # Multiple parquet files
+///
+/// This API is designed to support efficiently extracting statistics from
+/// multiple parquet files (hence why the parquet schema is passed in as an
+/// argument). This is useful when building an index for a directory of parquet
+/// files.
+///
+#[derive(Debug)]
+pub struct StatisticsConverter<'a> {
+    /// The name of the column to extract statistics for
+    column_name: &'a str,
+    /// The type of statistics to extract
+    statistics_type: RequestedStatistics,
+    /// The arrow schema of the query
+    arrow_schema: &'a Schema,
+    /// The field (with data type) of the column in the arrow schema
+    arrow_field: &'a Field,
+}
+
+impl<'a> StatisticsConverter<'a> {
+    /// Returns a [`UInt64Array`] with counts for each row group
+    ///
+    /// The returned array has no nulls, and has one value for each row group.
+    /// Each value is the number of rows in the row group.
+    pub fn row_counts(metadata: &ParquetMetaData) -> Result<UInt64Array> {
+        let row_groups = metadata.row_groups();
+        let mut builder = UInt64Array::builder(row_groups.len());
+        for row_group in row_groups {
+            let row_count = row_group.num_rows();
+            let row_count: u64 = row_count.try_into().map_err(|e| {
+                internal_datafusion_err!(
+                    "Parquet row count {row_count} too large to convert to u64: {e}"
+                )
+            })?;
+            builder.append_value(row_count);
+        }
+        Ok(builder.finish())
+    }
+
+    /// create an new statistics converter
+    pub fn try_new(
+        column_name: &'a str,
+        statistics_type: RequestedStatistics,
+        arrow_schema: &'a Schema,
+    ) -> Result<Self> {
+        // ensure the requested column is in the arrow schema
+        let Some((_idx, arrow_field)) = arrow_schema.column_with_name(column_name) else {
+            return plan_err!(
+                "Column '{}' not found in schema for statistics conversion",
+                column_name
+            );
+        };
+
+        Ok(Self {
+            column_name,
+            statistics_type,
+            arrow_schema,
+            arrow_field,
+        })
+    }
+
+    /// extract the statistics from a parquet file, given the parquet file's metadata
+    ///
+    /// The returned array contains 1 value for each row group in the parquet
+    /// file in order
+    ///
+    /// Each value is either
+    /// * the requested statistics type for the column
+    /// * a null value, if the statistics can not be extracted
+    ///
+    /// Note that a null value does NOT mean the min or max value was actually
+    /// `null` it means it the requested statistic is unknown
+    ///
+    /// Reasons for not being able to extract the statistics include:
+    /// * the column is not present in the parquet file
+    /// * statistics for the column are not present in the row group
+    /// * the stored statistic value can not be converted to the requested type
+    pub fn extract(&self, metadata: &ParquetMetaData) -> Result<ArrayRef> {
+        let data_type = self.arrow_field.data_type();
+        let num_row_groups = metadata.row_groups().len();
+
+        let parquet_schema = metadata.file_metadata().schema_descr();
+        let row_groups = metadata.row_groups();
+
+        // find the column in the parquet schema, if not, return a null array
+        let Some((parquet_idx, matched_field)) =
+            parquet_column(parquet_schema, self.arrow_schema, self.column_name)
+        else {
+            // column was in the arrow schema but not in the parquet schema, so return a null array
+            return Ok(new_null_array(data_type, num_row_groups));
+        };
+
+        // sanity check that matching field matches the arrow field
+        if matched_field.as_ref() != self.arrow_field {
+            return internal_err!(
+                "Matched column '{:?}' does not match original matched column '{:?}'",
+                matched_field,
+                self.arrow_field
+            );
+        }
+
+        // Get an iterator over the column statistics
+        let iter = row_groups
+            .iter()
+            .map(|x| x.column(parquet_idx).statistics());
+
+        match self.statistics_type {
+            RequestedStatistics::Min => min_statistics(data_type, iter),
+            RequestedStatistics::Max => max_statistics(data_type, iter),
+            RequestedStatistics::NullCount => {
+                let null_counts = iter.map(|stats| stats.map(|s| s.null_count()));
+                Ok(Arc::new(UInt64Array::from_iter(null_counts)))
+            }
+        }
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::*;