emkornfield commented on code in PR #8579:
URL: https://github.com/apache/iceberg/pull/8579#discussion_r1468916964
##########
format/spec.md:
##########
@@ -1128,12 +1128,17 @@ 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`**|`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 />`}`|
+|**`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:
+
+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.
Review Comment:
I think using a valid source ID here would lead to incorrect results for old
clients if a predicate is specified on the column. IIUC invalid ID here makes
sure reads should always be correct or fail which seems like better semantics
if the aim is forwards compatibility
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