julienledem commented on code in PR #242:
URL: https://github.com/apache/parquet-format/pull/242#discussion_r1612659697
##########
src/main/thrift/parquet.thrift:
##########
@@ -914,6 +1040,32 @@ struct RowGroup {
7: optional i16 ordinal
}
+struct RowGroupV3 {
+ /** REMOVED from v1: columns.
+ * Instead, decode each FileColumnMetadataV3 individually as needed.
+ */
+
+ /** Total byte size of all the uncompressed column data in this row group **/
+ 1: required i64 total_byte_size
Review Comment:
I believe it was added before that. this commit is just referencing when we
created the separate parquet-format repo. It might well be part of the original
metadata. The `redfile.thrift` name refers to the old RedElm name before we
rename it Parquet. It is the size of the decompressed (after decompression,
before decoding) page. Agreed it's not super useful.
##########
README.md:
##########
@@ -107,12 +113,97 @@ start locations. More details on what is contained in
the metadata can be found
in the Thrift definition.
Metadata is written after the data to allow for single pass writing.
+This is especially useful when writing to backends such as S3.
Readers are expected to first read the file metadata to find all the column
chunks they are interested in. The columns chunks should then be read
sequentially.
+### Parquet 3
+
+Parquet 3 files have the following overall structure:
+
+```
+4-byte magic number "PAR1"
+4-byte magic number "PAR3"
+
+<Column 1 Chunk 1 + Column Metadata>
+<Column 2 Chunk 1 + Column Metadata>
+...
+<Column N Chunk 1 + Column Metadata>
+<Column 1 Chunk 2 + Column Metadata>
+<Column 2 Chunk 2 + Column Metadata>
+...
+<Column N Chunk 2 + Column Metadata>
+...
+<Column 1 Chunk M + Column Metadata>
+<Column 2 Chunk M + Column Metadata>
+...
+<Column N Chunk M + Column Metadata>
+
+<File-level Column 1 Metadata v3>
+...
+<File-level Column N Metadata v3>
+
+File Metadata v3
+4-byte length in bytes of File Metadata v3 (little endian)
+4-byte magic number "PAR3"
+
+File Metadata
+4-byte length in bytes of File Metadata (little endian)
+4-byte magic number "PAR1"
+```
+
+Unlike the legacy File Metadata, the File Metadata v3 is designed to be
light-weight
+to decode, regardless of the number of columns in the file. Individual column
+metadata can be opportunistically decoded depending on actual needs.
+
+This file structure is backwards-compatible. Parquet 1 readers will read and
+decode the legacy File Metadata in the file footer, while Parquet 3 readers
+will notice the "PAR3" magic number just before the File Metadata and will
+instead read and decode the File Metadata v3.
+
+### Parquet 3 without legacy metadata
Review Comment:
I agree. There will be a time when people will agree to remove the old
metadata. I added the "PAR1" magic number specifically for this :)
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