Re: [PR] [WIP][Proposal] PARQUET-2430: Add parquet joiner [parquet-java]

["pyckle (via GitHub)" <gi...@apache.org>]

pyckle commented on code in PR #1273:
URL: https://github.com/apache/parquet-java/pull/1273#discussion_r2258246902


##########
parquet-hadoop/src/main/java/org/apache/parquet/hadoop/rewrite/ParquetRewriter.java:
##########
@@ -904,4 +953,171 @@ public byte[] getDictPageAAD() {
       return this.dictPageAAD;
     }
   }
+
+  private static class RightColumnWriter {
+    private final Queue<TransParquetFileReader> inputFiles;
+    private final ParquetRewriter parquetRewriter;
+    private final ParquetFileWriter writer;
+    private final MessageType schema;
+    private final Map<ColumnPath, ColumnDescriptor> descriptorsMap;
+    private final Map<ColumnDescriptor, ColumnReader> colReaders = new HashMap<>();
+    private final Map<ColumnDescriptor, ColumnChunkPageWriteStore> cPageStores = new HashMap<>();
+    private final Map<ColumnDescriptor, ColumnWriteStore> cStores = new HashMap<>();
+    private final Map<ColumnDescriptor, ColumnWriter> cWriters = new HashMap<>();
+    private int rowGroupIdxIn = 0;
+    private int rowGroupIdxOut = 0;
+    private int writtenFromBlock = 0;
+
+    public RightColumnWriter(Queue<TransParquetFileReader> inputFiles, ParquetRewriter parquetRewriter)
+        throws IOException {
+      this.inputFiles = inputFiles;
+      this.parquetRewriter = parquetRewriter;
+      this.writer = parquetRewriter.writer;
+      this.schema = inputFiles.peek().getFooter().getFileMetaData().getSchema();
+      this.descriptorsMap = this.schema.getColumns().stream()
+          .collect(Collectors.toMap(x -> ColumnPath.get(x.getPath()), x -> x));
+      initReaders();
+      initWriters();
+    }
+
+    public void writeRows(int rowGroupIdx, long rowsToWrite) throws IOException {
+      if (rowGroupIdxIn != rowGroupIdx) {
+        rowGroupIdxIn = rowGroupIdx;
+        flushWriters();
+        initWriters();
+      }
+      while (rowsToWrite > 0) {
+        List<BlockMetaData> blocks = inputFiles.peek().getFooter().getBlocks();
+        BlockMetaData block = blocks.get(rowGroupIdxOut);
+        List<ColumnChunkMetaData> chunks = block.getColumns();
+        long leftInBlock = block.getRowCount() - writtenFromBlock;
+        long writeFromBlock = Math.min(rowsToWrite, leftInBlock);
+        for (ColumnChunkMetaData chunk : chunks) {
+          if (chunk.isEncrypted()) {
+            throw new IOException("Column " + chunk.getPath().toDotString() + " is encrypted");
+          }
+          ColumnDescriptor descriptor = descriptorsMap.get(chunk.getPath());
+          copyValues(descriptor, writeFromBlock);
+        }
+        rowsToWrite -= writeFromBlock;
+        writtenFromBlock += writeFromBlock;
+        if (rowsToWrite > 0 || (block.getRowCount() == writtenFromBlock)) {
+          rowGroupIdxOut++;
+          if (rowGroupIdxOut == blocks.size()) {
+            inputFiles.poll();
+            rowGroupIdxOut = 0;
+          }
+          writtenFromBlock = 0;
+          // this is called after all rows are processed
+          initReaders();
+        }
+      }
+      flushWriters();
+    }
+
+    private void flushWriters() throws IOException {
+      cStores.values().forEach(cStore -> {
+        cStore.flush();
+        cStore.close();
+      });
+      cWriters.values().forEach(ColumnWriter::close);
+      for (ColumnDescriptor descriptor : descriptorsMap.values()) {
+        if (cPageStores.containsKey(descriptor))
+          cPageStores.get(descriptor).flushToFileWriter(writer);

Review Comment:
   I encountered this same question while writing a parquet driver (https://github.com/Earnix/parquetforge). So I'll share what I dug up.
   The parquet-format spec requires the ColumnChunk order in the footer to match the schema order. That's documented here: https://github.com/apache/parquet-format/blob/1dbc814b97c9307687a2e4bee55545ab6a2ef106/src/main/thrift/parquet.thrift#L1002
   In the JIRA ticket linked to the commit message (now migrated to GH) - https://github.com/apache/parquet-java/issues/1734 it seems that at least impala accepts columns written in arbitrary orders, and ordering the Dict/DataPages of a column chunk is not required.
   With this said, be aware that this can adversely affect performance. See: https://dl.acm.org/doi/10.1145/3035918.3035930



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