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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (AVRO-519) Efficient sparse optional fields support
Date Thu, 29 Apr 2010 18:25:54 GMT

    [ https://issues.apache.org/jira/browse/AVRO-519?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12862322#action_12862322
] 

Doug Cutting commented on AVRO-519:
-----------------------------------

> Is there a place where 2.0 proposals are collected? Maybe a wiki page?

Not yet.  Feel free to start one.  It would be good to have.

> Efficient sparse optional fields support
> ----------------------------------------
>
>                 Key: AVRO-519
>                 URL: https://issues.apache.org/jira/browse/AVRO-519
>             Project: Avro
>          Issue Type: New Feature
>          Components: spec
>            Reporter: John Plevyak
>
> One of the nice features of protobuf is efficient support for very sparse optional fields,
> for example large number of tags potentially associated with a document the vast
> majority of which are empty.
> Avro does support optional fields as part of differing specifications, but not on a per-record
> level after a protocol has been agreed upon.  Avro does have support for arrays and maps
> however both of these require homogeneous types.
> I would suggest adding an additional field attribute:
>    * "optional" - with values "true"/"false" (where "false" is assumed)
> For the encoding I would suggest that that any record which includes optional fields
> would be prefixed by an presence map which would be a sequence of int8 x* where:
>   x > 0 : the lower 7 bits are presence bits for the next 7 optional fields (low bit
first)
>   -128 < x < 0 : the next present field is position x + 135 (as x runs from 0 to
-127 and the first 7
>               must be empty otherwise we would use the x > 0 encoding) 
>   x == -128: no optional fields present in the next 134 optional fields
>   x = 0 : end of sequence
>   further, if the map has covered all the options, the end-of-sequence marker can be
>   elided.  For example, a type with 3 optional fields would require only a single byte.

> This will permit encoding at 8/7 of a bit per present entry (worst case) and at a cost
of
> 8/134 (0.06) bits/entry per all but last not-present (7.5 bytes / 1000 optional fields).
> This encoding is backward compatible as well as schema's which do not contain optional
> elements do not have the presence map and the encoding is therefore identical.  Backward
> compatibility can be maintained by simply using the default value for not-present fields.
> Language APIs:
> Efficient support could include either an explicit presence test or a function which
returns the value
> or default value (if the field is not present).
>  

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