dimas-b commented on code in PR #8982:
URL: https://github.com/apache/iceberg/pull/8982#discussion_r1686695087
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format/spec.md:
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@@ -1370,3 +1370,16 @@ Writing v2 metadata:
* `sort_columns` was removed
Note that these requirements apply when writing data to a v2 table. Tables
that are upgraded from v1 may contain metadata that does not follow these
requirements. Implementations should remain backward-compatible with v1
metadata requirements.
+
+## Appendix F: Implementation Notes
+
+This section covers topics not required by the specification but
recommendations for systems implementing the Iceberg specification
+to help maintain a uniform experience.
+
+### Point in Time Reads (Time Travel)
+
+Iceberg supports two types of histories for tables. A history of previous
"current snapshots" stored in ["snapshot-log" table
metadata](#table-metadata-fields) and [parent-child lineage stored in
"snapshots"](#table-metadata-fields). These two histories
+might indicate different snapshot IDs for a specific timestamp. The
discrepancies can be caused by a variety of table operations (e.g. updating the
`current-snapshot-id` of the table).
+
+When processing point in time queries the Iceberg community has chosen to use
"snapshot-log" metadata to lookup the table state
Review Comment:
I think this recommendation affects REST catalog implementations.
I believe this text is not normative, therefore REST catalog implementations
that do not expose the full snapshot history in metadata JSON will not violate
the spec.
However, for the sake of clarity and considering that the field is option, I
propose adding a phrase like "when the catalog makes the snapshot history
available in the metadata JSON, time travel queries should be executed like
this.... [existing spec text]. If a catalog does not expose the full snapshot
history, time travel queries should provide clear messages in case they cannot
find the appropriate snapshot".
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