MINOR: Add summary table of encodings and supported types (in Encodings.md) (#550)

[nkaki]

Rationale for this change

To make it easier for readers to get an overall picture of Parquet encodings.

What changes are included in this PR?

Adds a summary table to Encodings.md that lists the encoding types (each linked to its description), enums and targets for different Parquet format versions.

See rendered format here: https://github.com/nkaki/parquet-format/blob/master/Encodings.md

Do these changes have PoC implementations?

No - Documentation change only

Closes #550

[alamb]

Thanks @nkaki -- this is a great start

[alamb]

I think we have been trying to avoid the nomenclature of "parquet 2.0" as its definition is not universally agreed upon.

I recommend we remove the separate columns and instead focus on helping people navigate the current version of the spec

I am also not sure about the differences in different encoding targets (e.g. PLAIN_DICTIONARY) --- maybe we can simply not include that in the table as it has been deprecated?

[nkaki]

@alamb
Thank you for the review!

I think we have been trying to avoid the nomenclature of "parquet 2.0" as its definition is not universally agreed upon.　
I recommend we remove the separate columns and instead focus on helping people navigate the current version of the spec

I agree on focusing on current versions spec. At some point it would be great to make the parquet site able to see the previous versions easily. For the table I will remove the last column and rename the thrid one.

And just a question, would Data Page V2 (header?) would be a better term in this case?

I am also not sure about the differences in different encoding targets (e.g. PLAIN_DICTIONARY) --- maybe we can simply not include that in the table as it has been deprecated?

For PLAIN_DICTIONARY and RLE_DICTIONARY, I will merge the rows and mark PLAIN_DICTIONARY enum as deprecated.

For BIT_PACKED, since the deprecated encodings are still explained in the document and it is linked by other encodings , I thought it should be in the table and linked to the details. I think there are few options.

1. Remove BIT_PACKED encoding from the table (your suggestion)
2. Remove BIT_PACKED encoding description from the page and from the table (this may break links).
3. Seperate currently supported and deprecated encodings as seperate tables, and change the layout of the page.

• Layout A:
  supported encodings table
  deprecated encodings table (only BIT_PACKED)
  supported + deprecated encodings descriptions (current order)
• Layout B:
  supported encodings table
  supported encodings descriptions (current order with out BIT_PACKED)
  deprecated encodings table (only BIT_PACKED)
  deprecated encodings descriptions (only BIT_PACKED)
• Layout C:
  supported encodings table
  deprecated encodings table (only BIT_PACKED)
  supported encodings descriptions (current order with out BIT_PACKED)
  deprecated encodings descriptions (only BIT_PACKED)

Also about Encoding Targets column should I just list the physical types? removing other encoding targets (e.g. Repetition and definition levels)

[nkaki]

I removed v1 columns and seperated the table. If the deprecated encodings table is not needed I will remove it.

Link to the rendered page: https://github.com/nkaki/parquet-format/blob/master/Encodings.md