rdblue commented on a change in pull request #2654:
URL: https://github.com/apache/iceberg/pull/2654#discussion_r652200065
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File path: site/docs/spec.md
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@@ -176,6 +178,15 @@ Columns in Iceberg data files are selected by field id.
The table schema's colum
For example, a file may be written with schema `1: a int, 2: b string, 3: c
double` and read using projection schema `3: measurement, 2: name, 4: a`. This
must select file columns `c` (renamed to `measurement`), `b` (now called
`name`), and a column of `null` values called `a`; in that order.
+#### Identifier Field IDs
+
+A schema can optionally track the set of primitive fields that identify rows
in a table, using the property `identifier-field-ids` (see JSON encoding in
Appendix C).
+
+Two rows are the "same"---that is, the rows represent the same entity---if the
identifier fields are equal. However, uniqueness of rows by this identifier is
not guaranteed or required by Iceberg because it is needlessly inefficient in
some cases. Reasonable steps to enforce uniqueness are the responsibility of
processing engines.
+
+Optional fields can be used as identifier fields. Identifier fields may be
nested in structs but may not be nested within maps or lists; a nested
identifier field is considered null if it or any parent struct is null. For
identifier comparison, null is considered equal to itself.
Review comment:
I'm going to remove this. My originally logic was that we the feature is
limited if we can only add required fields, but that is only because we don't
currently support default values. If we supported default values, then I think
we would choose to avoid the ambiguity and only allow required columns. Since
we plan to add default values, I think that it makes less sense to work around
it by allowing optional fields here.
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