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Motivation

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Like snapshot, a new directory `/tag` will be created under table directory for storing tags. The qualified path for a tag file is `/path/to/table/tag/<tag-name>`, and the tag name is specified by user. 

New Classes

It's not necessary to introduce a new `Tag` class because the tag is very similar to snapshot, we can just reuse the `Snapshot`.  When we create a tag from a snapshot, we can just copy the corresponding snapshot file to the tag directory with tag name; when we read a tag, we can deserialize the tag file to a snapshot.

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SELECT * FROM t VERSION AS OF tag-name

Note: The tag name can not be an numeric string to make a difference from snapshot. This limitation will be checked when creating tag and documented.

Flink

SELECT * FROM t /*+ OPTIONS('scan.tag-name'='<name>') */

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