dimas-b commented on code in PR #8982:
URL: https://github.com/apache/iceberg/pull/8982#discussion_r1687279691
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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:
`snapshot-log` is optional in both v1 and v2 spec:
https://iceberg.apache.org/spec/#table-metadata-fields
Given that this PR suggests how engines should process that information so
that users can expect similar behaviour from different engines, it would be
nice to provide a suggestion of how to deal with missing `snapshot-log` in a
common way too. In other words, how to produce a user-friendly message as
opposed to some internal and engine-specific error.
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