szehon-ho commented on code in PR #18057:
URL: https://github.com/apache/iceberg/pull/18057#discussion_r4021219829
##########
format/udf-spec.md:
##########
@@ -118,19 +122,26 @@ following fields required. Any other fields must be
ignored.
e.g., `{ "type": "struct", "fields": [ { "name": "id", "type": "int" }, {
"name": "name", "type": "string" } ] }`
#### Definition ID
-The `definition-id` is a canonical string derived from the parameter types,
formatted as a comma-separated list with no
-spaces. Each type uses the following string representation:
+The `definition-id` is a canonical string derived from the parameter types,
formatted as a comma-separated list. The
+separators that this format adds must not be followed by a space. Each type
uses the following string representation:
* Primitives and semi-structured: the type name (e.g., `int`, `variant`)
* List: `list<element-type>` (e.g., `list<int>`)
* Map: `map<key-type,value-type>` (e.g., `map<string,int>`)
* Struct: `struct<name1:type1,name2:type2,...>` with field names and types
(e.g., `struct<id:int,name:string>`)
+In a struct field name, `\`, `:`, `,`, `<`, and `>` must each be escaped with
a preceding `\`. Without escaping, a
+field name that contains a separator would produce the same string as a
different set of fields.
+
Examples of complete definition-id signatures:
* `int` – single int parameter
* `int,string` – two parameters: int and string
* `int,list<int>,struct<id:int,name:string>` – three parameters: an int, a
list and a struct
+* `decimal(9, 2),geometry(srid: 3857)` – two parameterized parameters, each
keeping its own spaces
+* `geometry(epsg:4326)` – a geometry parameter; CRS values are lowercased, so
`geometry(EPSG:4326)` yields this id
Review Comment:
actually we also should skip mention 'are lowercased'?
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