Example of reading and writing parquet metadata outside the file

parquet/Cargo.toml

@@ -118,6 +118,12 @@ zstd = ["dep:zstd", "zstd-sys"]
 # Display memory in example/write_parquet.rs
 sysinfo = ["dep:sysinfo"]
 
+
+[[example]]
+name = "external_metadata"
+required-features = ["arrow", "async"]
+path = "./examples/external_metadata.rs"
+
 [[example]]
 name = "read_parquet"
 required-features = ["arrow"]

parquet/examples/external_metadata.rs

@@ -0,0 +1,155 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+use arrow_array::RecordBatch;
+use arrow_cast::pretty::pretty_format_batches;
+use parquet::arrow::arrow_reader::{
+    ArrowReaderMetadata, ArrowReaderOptions, ParquetRecordBatchReaderBuilder,
+};
+use parquet::arrow::ParquetRecordBatchStreamBuilder;
+use parquet::file::metadata::{ParquetMetaData, ParquetMetaDataReader, ParquetMetaDataWriter};
+use std::path::Path;
+
+/// This example demonstrates advanced usage of the Parquet metadata APIs.
+///
+/// It shows how you can store Parquet metadata somewhere other than the Parquet
+/// file itself, and then use that metadata to read a file. This can be used,
+/// for example, to store metadata for parquet files on remote object storage
+/// (e.g. S3)  in a local file or an in-memory cache, use a query engine like
+/// DataFusion to analyze the metadata to determine which files may contain
+/// relevant data, and then read only those the required files with a single
+/// object store request.
+///
+/// The example:
+/// 1. Reads the metadata of a Parquet file
+/// 2. Removes some column statistics from the metadata (to make them smaller)
+/// 3. Stores the metadata in a separate file
+/// 4. Reads the metadata from the separate file and uses that to read the Parquet file
+///
+/// Without this API, to implement the functionality you would need to implement
+/// a conversion of the `ParquetMetaData` and related structures to/from some
+/// other structs that can be serialized/deserialized.
+
+#[tokio::main(flavor = "current_thread")]
+async fn main() -> parquet::errors::Result<()> {
+    let testdata = arrow::util::test_util::parquet_test_data();
+    let parquet_path = format!("{testdata}/alltypes_plain.parquet");
+    let metadata_path = "thrift_metadata.dat"; // todo tempdir for now use local file to inspect it
+
+    let metadata = get_metadata_from_parquet_file(&parquet_path).await;
+    println!(
+        "Read metadata from Parquet file into memory: {} bytes",
+        metadata.memory_size()
+    );
+    let metadata = prepare_metadata(metadata);
+    write_metadata_to_file(metadata, &metadata_path);
+
+    // now read the metadata from the file and use it to read the Parquet file
+    let metadata = read_metadata_from_file(&metadata_path);
+    println!("Read metadata from file: {metadata:#?}");
+
+    let batches = read_parquet_file_with_metadata(&parquet_path, metadata);
+
+    // display the results
+    let batches_string = pretty_format_batches(&batches).unwrap().to_string();
+    let batches_lines: Vec<_> = batches_string.split('\n').collect();
+
+    assert_eq!(batches_lines,
+               [
+                   "+----+----------+-------------+--------------+---------+------------+-----------+------------+------------------+------------+---------------------+",
+                   "| id | bool_col | tinyint_col | smallint_col | int_col | bigint_col | float_col | double_col | date_string_col  | string_col | timestamp_col       |",
+                   "+----+----------+-------------+--------------+---------+------------+-----------+------------+------------------+------------+---------------------+",
+                   "| 4  | true     | 0           | 0            | 0       | 0          | 0.0       | 0.0        | 30332f30312f3039 | 30         | 2009-03-01T00:00:00 |",
+                   "| 5  | false    | 1           | 1            | 1       | 10         | 1.1       | 10.1       | 30332f30312f3039 | 31         | 2009-03-01T00:01:00 |",
+                   "| 6  | true     | 0           | 0            | 0       | 0          | 0.0       | 0.0        | 30342f30312f3039 | 30         | 2009-04-01T00:00:00 |",
+                   "| 7  | false    | 1           | 1            | 1       | 10         | 1.1       | 10.1       | 30342f30312f3039 | 31         | 2009-04-01T00:01:00 |",
+                   "| 2  | true     | 0           | 0            | 0       | 0          | 0.0       | 0.0        | 30322f30312f3039 | 30         | 2009-02-01T00:00:00 |",
+                   "| 3  | false    | 1           | 1            | 1       | 10         | 1.1       | 10.1       | 30322f30312f3039 | 31         | 2009-02-01T00:01:00 |",
+                   "| 0  | true     | 0           | 0            | 0       | 0          | 0.0       | 0.0        | 30312f30312f3039 | 30         | 2009-01-01T00:00:00 |",
+                   "| 1  | false    | 1           | 1            | 1       | 10         | 1.1       | 10.1       | 30312f30312f3039 | 31         | 2009-01-01T00:01:00 |",
+                   "+----+----------+-------------+--------------+---------+------------+-----------+------------+------------------+------------+---------------------+",
+               ]
+
+               , "actual output:\n\n{batches_lines:#?}");
+
+    Ok(())
+}
+
+/// Reads the metadata from a parquet file
+async fn get_metadata_from_parquet_file(file: impl AsRef<Path>) -> ParquetMetaData {
+    // pretend we are reading the metadata from a remote object store
+    let file = std::fs::File::open(file).unwrap();
+    let file = tokio::fs::File::from_std(file);
+
+    let builder = ParquetRecordBatchStreamBuilder::new(file).await.unwrap();
+
+    // The metadata is Arc'd -- since we are going to modify it we
+    // need to clone it
+    builder.metadata().as_ref().clone()
+}
+
+/// modifies the metadata to reduce its size
+fn prepare_metadata(metadata: ParquetMetaData) -> ParquetMetaData {
+    // maybe we will do this
+    metadata
+}
+
+/// writes the metadata to a file
+///
+/// The data is stored using the same thrift format as the Parquet file metadata
+fn write_metadata_to_file(metadata: ParquetMetaData, file: impl AsRef<Path>) {
+    let file = std::fs::File::create(file).unwrap();
+    let writer = ParquetMetaDataWriter::new(file, &metadata);
+    writer.finish().unwrap()
+}
+
+/// Reads the metadata from a file
+///
+/// This function reads the format written by `write_metadata_to_file`
+fn read_metadata_from_file(file: impl AsRef<Path>) -> ParquetMetaData {
+    let mut file = std::fs::File::open(file).unwrap();
+    ParquetMetaDataReader::new()
+        .with_column_indexes(true)
+        .with_offset_indexes(true)
+        .parse_and_finish(&mut file)
+        .unwrap()
+}
+
+/// Reads the Parquet file using the metadata
+///
+/// This shows how to read the Parquet file using previously read metadata
+/// instead of the metadata in the Parquet file itself. This avoids an IO /
+/// having to fetch and decode the metadata from the Parquet file before
+/// beginning to read it.
+///
+/// In this example, we read the results as Arrow record batches
+fn read_parquet_file_with_metadata(
+    file: impl AsRef<Path>,
+    metadata: ParquetMetaData,
+) -> Vec<RecordBatch> {
+    let file = std::fs::File::open(file).unwrap();
+    let options = ArrowReaderOptions::new()
+        // tell the reader to read the page index
+        .with_page_index(true);
+    // create a reader with pre-existing metadata
+    let arrow_reader_metadata = ArrowReaderMetadata::try_new(metadata.into(), options).unwrap();
+    let reader = ParquetRecordBatchReaderBuilder::new_with_metadata(file, arrow_reader_metadata)
+        .build()
+        .unwrap();
+
+    reader.collect::<arrow::error::Result<Vec<_>>>().unwrap()
+}