Module mz_persist_types::columnar

Columnar understanding of persisted data

For efficiency/performance, we directly expose the columnar structure of persist’s internal encoding to users during encoding and decoding. For ergonomics, we wrap the arrow2 crate we use to read and write parquet data.

Some of the requirements that led to this design:

• Support a separation of data and schema because Row is not self-describing: e.g. a Datum::Null can be one of many possible column types. A RelationDesc is necessary to describe a Row schema.
• Narrow down arrow2::data_types::DataType (the arrow “logical” types) to a set we want to support in persist.
• Associate an arrow2::io::parquet::write::Encoding with each of those types.
• Do dyn Any downcasting of columns once per part, not once per update.
• Unlike arrow2, be precise about whether each column is optional or not.

The primary presentation of this abstraction is a sealed trait Data, which is implemented for the owned version of each type of data that can be stored in persist: int64, Option<String>, etc.

Under the hood, it’s necessary to store something like a map of name -> column. A natural instinct is to make Data object safe, but I couldn’t figure out a way to make that work without severe limitations. As a result, the DataType enum is introduced with a 1:1 relationship between variants and implementations of Data. This allows for easy type erasure and guardrails when downcasting the types back.

Note: The “Data” strategy is roughly how columnation works and the “DataType” strategy is roughly how arrow2 works. Doing both of them gets us the benefits of both, while the downside is code duplication and cognitive overhead.

The Data trait has associated types for the exclusive “builder” type for the column and for the shared “reader” type. These also implement some common traits to make relationships between types more structured.

Finally, the Schema trait maps an implementor of Codec to the underlying column structure. It also provides a PartEncoder and PartDecoder for amortizing any downcasting that does need to happen.

Structs

• DataType
  A description of a type understood by persist.
• OpaqueData
  Opaque binary encoded data.

Enums

• ColumnFormat
  The in-memory rust type of a column of data.

Traits

• ColumnCfg
  If necessary, whatever information beyond the type of Self needed to produce a columnar schema for this type.
• ColumnGet
  A type that may be retrieved from a column of [T].
• ColumnPush
  A type that may be added into a column of [T].
• Data
  A type understood by persist.
• PartDecoder
  A decoder for values of a fixed schema.
• PartEncoder
  An encoder for values of a fixed schema
• Schema
  A description of the structure of a crate::Codec implementor.

Functions

• validate_roundtrip
  A helper for writing tests that validate that a piece of data roundtrips through the columnar format.