rdblue commented on a change in pull request #2654:
URL: https://github.com/apache/iceberg/pull/2654#discussion_r644081533
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File path: site/docs/spec.md
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@@ -495,10 +528,12 @@ Table metadata consists of the following fields:
| | _required_ | **`last-sequence-number`**| The table's highest
assigned sequence number, a monotonically increasing long that tracks the order
of snapshots in a table. |
| _required_ | _required_ | **`last-updated-ms`**| Timestamp in milliseconds
from the unix epoch when the table was last updated. Each table metadata file
should update this field just before writing. |
| _required_ | _required_ | **`last-column-id`**| An integer; the highest
assigned column ID for the table. This is used to ensure columns are always
assigned an unused ID when evolving schemas. |
-| _required_ | _required_ | **`schema`**| The table’s current schema. |
+| _required_ | _required_ | **`schema`**| The table’s current schema. In v2,
this must be the schema identified by the `current-schema-id`. |
+| _optional_ | _required_ | **`schemas`**| A list of schemas, stored as
objects with `schema-id`. |
+| _optional_ | _required_ | **`current-schema-id`**| ID of the table's current
schema. |
| _required_ | | **`partition-spec`**| The table’s current
partition spec, stored as only fields. Note that this is used by writers to
partition data, but is not used when reading because reads use the specs stored
in manifest files. (**Deprecated**: use `partition-specs` and
`default-spec-id`instead ) |
| _optional_ | _required_ | **`partition-specs`**| A list of partition specs,
stored as full partition spec objects. |
-| _optional_ | _required_ | **`default-spec-id`**| ID of the “current” spec
that writers should use by default. |
+| _optional_ | _required_ | **`default-spec-id`**| ID of the "current" spec
that writers should use by default. |
Review comment:
This distinction is on purpose. A table can have only one schema and
that is the current one. We track old schemas for older snapshots. But a table
can have multiple valid partition specs and it is fine to write new data into
either one. That's why we track a "default" spec to use when writing if you
aren't doing something that overrides it like migrating data from one spec to
another.
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