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Spec: require monotonically increasing snapshot timestamp-ms in v4 #16294

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Spec: require monotonically increasing snapshot timestamp-ms in v4 #16294

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10 changes: 10 additions & 0 deletions format/spec.md
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Expand Up @@ -751,6 +751,8 @@ A snapshot consists of the following fields:
| | | _required_ | **`added-rows`** | The upper bound of the number of rows with assigned row IDs, see [Row Lineage](#row-lineage) |
| | | _optional_ | **`key-id`** | ID of the encryption key that encrypts the manifest list key metadata |

In format version 4 and above, a snapshot's `timestamp-ms` must be strictly greater than the `timestamp-ms` of its parent snapshot on the same branch. The requirement applies only to snapshots committed while the table is at format version 4 or higher; root snapshots, and snapshots that existed before a table was upgraded to v4, are not constrained.

The snapshot summary's `operation` field is used by some operations, like snapshot expiration, to skip processing certain snapshots. Possible `operation` values are:

* `append` -- Only data files were added and no files were removed.
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## Appendix E: Format version changes

### Version 4

Snapshot timestamp changes:

* A snapshot's `timestamp-ms` must be strictly greater than the `timestamp-ms` of its parent snapshot on the same branch.
* The requirement applies only to snapshots committed while the table is at format version 4 or higher; snapshots that existed before a table was upgraded to v4 are not constrained.
* Root snapshots (snapshots with no parent) are not constrained by this requirement.

### Version 3

Default values are added to struct fields in v3.
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