rdblue commented on code in PR #9661:
URL: https://github.com/apache/iceberg/pull/9661#discussion_r1501923473
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format/spec.md:
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@@ -1128,16 +1142,14 @@ Each partition field in the fields list is stored as an
object. See the table fo
|**`month`**|`JSON string: "month"`|`"month"`|
|**`day`**|`JSON string: "day"`|`"day"`|
|**`hour`**|`JSON string: "hour"`|`"hour"`|
-|**`Partition Field`** [1,2]|`JSON object: {`<br /> `"source-id":
<id int>,`<br /> `"field-id": <field id int>,`<br
/> `"name": <name string>,`<br /> `"transform":
<transform JSON>`<br />`}`|`{`<br /> `"source-id": 1,`<br
/> `"field-id": 1000,`<br /> `"name": "id_bucket",`<br
/> `"transform": "bucket[16]"`<br />`}`|
In some cases partition specs are stored using only the field list instead of
the object format that includes the spec ID, like the deprecated
`partition-spec` field in table metadata. The object format should be used
unless otherwise noted in this spec.
The `field-id` property was added for each partition field in v2. In v1, the
reference implementation assigned field ids sequentially in each spec starting
at 1,000. See Partition Evolution for more details.
-Notes:
+Transforms that accept multiple arguments specify source field IDs using
`source-ids` instead of `source-id`. Writers producing these transforms in v1
and v2 metadata should additionally produce the `source-id` field by setting it
to the first ID from the `source-ids` list. Writers producing these transforms
in v3 metadata should populate only the `source-ids` field because v3 readers
will fully-support multi-arg transforms by reading this field.
-1. For partition fields with a transform with a single argument, the ID of the
source field is set on `source-id`, and `source-ids` is omitted.
-2. For partition fields with a transform of multiple arguments, the IDs of the
source fields are set on `source-ids`. To preserve backward compatibility,
`source-id` is set to -1.
+Older versions of the reference implementation can read tables with transforms
unknown to it, without the ability to push down filters or write. But other
implementations may break if they encounter unknown transforms.
Review Comment:
Should we state that in v3, readers are required to ignore unknown
transforms and are not allowed to write to tables that use unknown transforms
in the target partition spec?
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