pitrou commented on code in PR #242: URL: https://github.com/apache/parquet-format/pull/242#discussion_r1608770331
########## 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. Review Comment: The goal here is twofold: 1. achieve better metadata parsing performance for PAR3 readers 2. keep compatibility with PAR1 readers This is why this proposal creates a separate array of structure types: so that PAR3 readers don't have to eagerly decode those pesky columns, while letting PAR1 readers correctly access column information. I don't think any of your two proposals is able of achieving of two goals simultaneously, are they? (admittedly, I'm not sure I understand proposal number 2, though it seems to require hand-coded Thrift parsing which doesn't sound like a tremendous idea) ########## 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. Review Comment: The goal here is twofold: 1. achieve better metadata parsing performance for PAR3 readers 2. keep compatibility with PAR1 readers This is why this proposal creates a separate array of structure types: so that PAR3 readers don't have to eagerly decode those pesky columns, while letting PAR1 readers correctly access column information. I don't think any of your two proposals is able of achieving those two goals simultaneously, are they? (admittedly, I'm not sure I understand proposal number 2, though it seems to require hand-coded Thrift parsing which doesn't sound like a tremendous idea) -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: issues-unsubscr...@parquet.apache.org For queries about this service, please contact Infrastructure at: us...@infra.apache.org --------------------------------------------------------------------- To unsubscribe, e-mail: issues-unsubscr...@parquet.apache.org For additional commands, e-mail: issues-h...@parquet.apache.org
