emkornfield commented on code in PR #8982:
URL: https://github.com/apache/iceberg/pull/8982#discussion_r1686982629
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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:
IIUC conformant REST catalogs as of now must have a link to require the JSON
metadata file to be present, so I don't think this directly reflects them. If
REST catalogs do not have a first class REST API for retrieving this
information (or a way to parameterize the API via a timestamp to get the
metadata) that seems like a new feature for the REST catalog, which is out of
scope. Hopefully the proposal to remove the requirement for JSON metadata
file is not approved until issues like this are sorted out.
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