PARQUET-2362: Clarify parquet encoding

[letian-jiang]

Dictionary page format: the entries in the dictionary - in dictionary order - using the plain encoding.

The dictionary entries are not sorted (or at least not always sorted).

There is no padding between values (except for the last byte) which is padded with 0s.

Minor change.

Jira

• [x] https://issues.apache.org/jira/browse/PARQUET-2362

Commits

• [x] My commits all reference Jira issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

[mapleFU]

Would you mind first create an issue like: https://issues.apache.org/jira/browse/PARQUET-2299 or use MINOR?

[mapleFU]

There is no padding between values (except for the last byte) which is padded with 0s.

This change looks good to me.

Dictionary page format: the entries in the dictionary - in dictionary order - using the plain encoding.

Does this means value in data page is same as position in dictionary page? 🤔

Also cc @wgtmac @gszadovszky

[letian-jiang]

Would you mind first create an issue like: https://issues.apache.org/jira/browse/PARQUET-2299 or use MINOR?

I will create a related issue once my JIRA account request is approved.

Does this means value in data page is same as position in dictionary page? 🤔

I think so. The data page contains dictionary code (i.e. offset in dictionary page )

[letian-jiang]

Made some updates. Please take another look. @mapleFU @gszadovszky @JFinis