DRILL-7723 - Add Excel Metadata as Implicit Fields

[cgivre]

DRILL-7723: Add Excel Metadata as Implicit Fields

Description

Excel files contain a significant number of metadata fields in them. This PR adds the ability to access these fields via implicit columns. The columns are:

_category
_content_status
_content_type;
_creator
_description
_identifier
_keywords
_last_modified_by_user
_revision
_subject
_title
_created
_last_printed
_modified

These fields are not projected in star queries so there is no effect on existing queries.

Documentation

Updated the README.md file with the implicit field information.

Testing

Added a unit test for the metadata fields.

[vvysotskyi]

@cgivre, will these "implicit" columns values be populated (but not returned) every time the file is read even they aren't specified in the query?

[cgivre]

@vvysotskyi
That is correct. This follows the model for EVF readers where data is written to the writers and the EVF framework determines which columns to actually project.

[cgivre]

@vvysotskyi
Any comments on this or is it good to go?

[vvysotskyi]

@cgivre, I'll take a look this weekend.

[vvysotskyi]

Please use naming according to JCC.

[cgivre]

@vvysotskyi
Are you referring to the field names?

[vvysotskyi]

Yes, please name elements of enum in the upper case. And do not start them with an underscore.

[vvysotskyi]

And please rename enums with correct casing.

[cgivre]

Will do. Thanks for the review :-)

[cgivre]

@vvysotskyi
To clarify, the fields here are metadata fields and I wanted to make sure that they do not conflict with fields that might actually be in the users' data. It is a fairly standard practice to begin metadata fields with an _. I've seen this done in many other platforms.

In any event, how would you like me to proceed with this?

[vvysotskyi]

Adding a field to enum wouldn't make things too complex, using enums in such way is a common practice. Regarding naming fields in other projects, the specific project has its own code-style rules.

If you have a strong objection to this, please merge the PR as it is, I've just expressed my opinion about this.

[cgivre]

@vvysotskyi
Ok.. I see what you're getting at. I'll refactor the enum with a value. I misunderstood what you were asking. In any event, do you have any objection to the actual column name starting with an underscore?

[vvysotskyi]

It is ok to have such a column name, my objection was only related to the enum element names.

[cgivre]

@vvysotskyi
Enum refactored and commits squashed. Are we good to go?

[vvysotskyi]

Thanks for making changes, LGTM, +1