Jim Pivarski created AVRO-1922:
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Summary: Fixed dimension for array
Key: AVRO-1922
URL: https://issues.apache.org/jira/browse/AVRO-1922
Project: Avro
Issue Type: New Feature
Components: spec
Affects Versions: 1.8.1
Reporter: Jim Pivarski
Fix For: 1.9.0
This is a feature request for future versions of the Avro specification.
We have found one kind of data structure that is hard to express in Avro:
tensors. Although we can (and do) build matrices as {"type": "array", "items":
{"type": "array", "items": "double"}}, this type does not specify that the grid
of numbers is rectangular. We believe that rectangular arrays of numbers (or
other nested types) would be a strong addition to Avro, both as a type system
and as a serialization format. With the total size of all dimensions fixed in
the schema, they would not need to be repeated in each serialized datum.
For instance, suppose there was an extension of type "array" to specify
dimensions:
{"type": "array", "dimensions": [3, 3, 3, 3], "items": "double"}
This 3-by-3-by-3-by-3 tensor (representing, for instance, the Riemann curvature
tensor in 3-space) specifies that 81 double-precision numbers (3*3*3*3) are
expected for each datum. With nested arrays, the size, "3," would have to be
separately encoded 40 times (1 + 3*(1 + 3*(1 + 3))) for each datum, even though
they would never change in a dataset of Riemann tensors. With a "dimensions"
attribute in the schema, only the content needs to be serialized. Moreover,
this extension can clearly be used with any other "items" type, to make dense
tables of strings, for instance.
Avro has been extended in a similar way in the past. The "fixed" type is a
"bytes" without the need to specify the number of bytes for each datum. Our
proposal provides a similar packing for structured objects that can be
significant for large numbers of dimensions, as shown above. The advantage to
consumers of Avro data is that we can write functions that do not need to check
all array sizes at runtime (for operations like tensor contractions and
products).
We have searched the web and the Avro JIRA site for similar proposals and found
none, so we're adding this proposal to JIRA in addition to this e-mail. Please
let us know if you have any comments, or if we can provide any more information.
Thank you!
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