[logs] Dropping dt column

[john-bodley]

Whist looking through the Superset logs table I discovered that there was a misalignment of the date defined in the dt column vs dttm.

The reason for the inconsistency is dttm is defined in reference to the UTC timezone whereas dt use the date in reference to one's local timezone. One possible fix is

dt = Column(Date, default=ddatetime.utcnow().date())

however given that dt and dttm aren't using the same now instance, there's still no guarantee there will be consistency between these columns especially around 12:00:00 am UTC.

The fix here is simply to remove the dt column which is unnecessary as this information is completely defined by the dttm column. Note I also removed the deprecated activity_per_day which used the Log.dt column which is no longer used in the code.

Additionally tested by running:

superset db upgrade
superset db downgrade

to: @graceguo-supercat @michellethomas @mistercrunch @timifasubaa

[mistercrunch]

That's a breaking change though as dt may be used in charts/dashboards/etl . Personally I'm fine with breaking changes before we hit 1.0, but we need an UPDATING.md or equivalent with a list of breaking changes, motives and ways to handle them associated with versions. When releasing, we'd be pointing to the UPDATING.md notes on top of change log.

Here's what the Airflow UPDATING.md look like:
https://github.com/apache/incubator-airflow/blob/master/UPDATING.md

It can be simple at first, with just a list of version and related breaking changes and PR references.

[john-bodley]

@mistercrunch I added an UPDATING.md file. Let me know if you want me to add more refinement beyond a simple list of breaking PRs.

[codecov-io]

Codecov Report

Merging #4587 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4587      +/-   ##
==========================================
+ Coverage   72.63%   72.64%   +0.01%     
==========================================
  Files         205      205              
  Lines       15403    15395       -8     
  Branches     1183     1183              
==========================================
- Hits        11188    11184       -4     
+ Misses       4212     4208       -4     
  Partials        3        3

Impacted Files	Coverage Δ
superset/views/core.py	74.62% <ø> (+0.16%)	⬆️
superset/models/core.py	86.49% <100%> (-0.03%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 41c158e...ad7556d. Read the comment docs.

[michellethomas]

lgtm

[mistercrunch]

@john-bodley let's avoid backward incompatible db migrations in the future as they require scheduling downtime to upgrade the app for people who use rolling deploys. This took our production down for a moment.

[john-bodley]

@mistercrunch just to clarify what should have the logic been here? I'm simply asking to learn.

[mistercrunch]

Not sure, maybe we could have just left the column there as NULL moving forward and add a note about it being deprecated.