[GitHub] [arrow-rs] Ted-Jiang commented on a diff in pull request #4216: Prefetch page index (#4090)

["Ted-Jiang (via GitHub)" <gi...@apache.org>]

Ted-Jiang commented on code in PR #4216:
URL: https://github.com/apache/arrow-rs/pull/4216#discussion_r1196013421


##########
parquet/src/arrow/async_reader/metadata.rs:
##########
@@ -15,95 +15,250 @@
 // specific language governing permissions and limitations
 // under the License.
 
+use crate::arrow::async_reader::AsyncFileReader;
 use crate::errors::{ParquetError, Result};
-use crate::file::footer::{decode_footer, decode_metadata};
+use crate::file::footer::{decode_footer, read_metadata};
 use crate::file::metadata::ParquetMetaData;
-use bytes::{BufMut, Bytes, BytesMut};
+use crate::file::page_index::index::Index;
+use crate::file::page_index::index_reader::{
+    acc_range, decode_column_index, decode_offset_index,
+};
+use bytes::Bytes;
+use futures::future::BoxFuture;
+use futures::FutureExt;
 use std::future::Future;
+use std::io::Read;
 use std::ops::Range;
 
-/// Fetches parquet metadata
-///
-/// Parameters:
-/// * fetch: an async function that can fetch byte ranges
-/// * file_size: the total size of the parquet file
-/// * footer_size_hint: footer prefetch size (see comments below)
-///
-/// The length of the parquet footer, which contains file metadata, is not
-/// known up front. Therefore this function will first issue a request to read
-/// the last 8 bytes to determine the footer's precise length, before
-/// issuing a second request to fetch the metadata bytes
-///
-/// If a hint is set, this method will read the specified number of bytes
-/// in the first request, instead of 8, and only issue a second request
-/// if additional bytes are needed. This can therefore eliminate a
-/// potentially costly additional fetch operation
-pub async fn fetch_parquet_metadata<F, Fut>(
-    mut fetch: F,
-    file_size: usize,
-    footer_size_hint: Option<usize>,
-) -> Result<ParquetMetaData>
-where
-    F: FnMut(Range<usize>) -> Fut,
-    Fut: Future<Output = Result<Bytes>>,
-{
-    if file_size < 8 {
-        return Err(ParquetError::EOF(format!(
-            "file size of {file_size} is less than footer"
-        )));
+/// A data source that can be used with [`MetadataLoader`] to load [`ParquetMetaData`]
+pub(crate) trait MetadataFetch {
+    fn fetch(&mut self, range: Range<usize>) -> BoxFuture<'_, Result<Bytes>>;
+}
+
+impl<'a, T: AsyncFileReader> MetadataFetch for &'a mut T {
+    fn fetch(&mut self, range: Range<usize>) -> BoxFuture<'_, Result<Bytes>> {
+        self.get_bytes(range)
     }
+}
+
+/// An asynchronous interface to load [`ParquetMetaData`] from an async source
+pub(crate) struct MetadataLoader<F> {
+    /// Function that fetches byte ranges asynchronously
+    fetch: F,
+    /// The in-progress metadata
+    metadata: ParquetMetaData,
+    /// The offset and bytes of remaining unparsed data
+    remainder: Option<(usize, Bytes)>,
+}
+
+impl<F: MetadataFetch> MetadataLoader<F> {
+    /// Create a new [`MetadataLoader`] by reading the footer information
+    ///
+    /// Parameters:
+    /// * fetch: an async function that can fetch byte ranges
+    /// * file_size: the total size of the parquet file
+    /// * footer_size_hint: footer prefetch size (see comments below)
+    ///
+    /// The length of the parquet footer, which contains file metadata, is not
+    /// known up front. Therefore this function will first issue a request to read
+    /// the last 8 bytes to determine the footer's precise length, before
+    /// issuing a second request to fetch the metadata bytes
+    ///
+    /// If a `prefetch` is `Some`, this will read the specified number of bytes
+    /// in the first request, instead of 8, and only issue further requests
+    /// if additional bytes are needed. Providing a hint can therefore significantly
+    /// reduce the number of `fetch` requests, and consequently latency
+    pub async fn load(
+        mut fetch: F,
+        file_size: usize,
+        prefetch: Option<usize>,
+    ) -> Result<Self> {
+        if file_size < 8 {
+            return Err(ParquetError::EOF(format!(
+                "file size of {file_size} is less than footer"
+            )));
+        }
 
-    // If a size hint is provided, read more than the minimum size
-    // to try and avoid a second fetch.
-    let footer_start = if let Some(size_hint) = footer_size_hint {
-        file_size.saturating_sub(size_hint)
-    } else {
-        file_size - 8
-    };
+        // If a size hint is provided, read more than the minimum size
+        // to try and avoid a second fetch.
+        let footer_start = if let Some(size_hint) = prefetch {
+            file_size.saturating_sub(size_hint)
+        } else {
+            file_size - 8
+        };
 
-    let suffix = fetch(footer_start..file_size).await?;
-    let suffix_len = suffix.len();
+        let suffix = fetch.fetch(footer_start..file_size).await?;
+        let suffix_len = suffix.len();
 
-    let mut footer = [0; 8];
-    footer.copy_from_slice(&suffix[suffix_len - 8..suffix_len]);
+        let mut footer = [0; 8];
+        footer.copy_from_slice(&suffix[suffix_len - 8..suffix_len]);
 
-    let length = decode_footer(&footer)?;
+        let length = decode_footer(&footer)?;
 
-    if file_size < length + 8 {
-        return Err(ParquetError::EOF(format!(
-            "file size of {} is less than footer + metadata {}",
-            file_size,
-            length + 8
-        )));
+        if file_size < length + 8 {
+            return Err(ParquetError::EOF(format!(
+                "file size of {} is less than footer + metadata {}",
+                file_size,
+                length + 8
+            )));
+        }
+
+        // Did not fetch the entire file metadata in the initial read, need to make a second request
+        let (metadata, remainder) = if length > suffix_len - 8 {
+            let metadata_start = file_size - length - 8;
+            let remaining_metadata = fetch.fetch(metadata_start..footer_start).await?;
+
+            let reader = remaining_metadata.as_ref().chain(&suffix[..suffix_len - 8]);
+            (read_metadata(reader)?, None)
+        } else {
+            let metadata_start = file_size - length - 8 - footer_start;

Review Comment:
   why need minus  `footer_start ` 🤔 . Not consist with datafusion https://github.com/apache/arrow-datafusion/blob/6e819d6c2b9280198c67fa16df3e54c79ce46ca2/datafusion/core/src/datasource/file_format/parquet.rs#L420



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