DOC: Added note about groupby excluding Decimal columns by default
Also included example of how to explicitly aggregate by Decimal columns.

- closes #13821

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18953?src=pr&el=h1) Report
> Merging [#18953](https://codecov.io/gh/pandas-dev/pandas/pull/18953?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c8ce3d01e9ffafc24c6f9dd568cd9eb7e42c610c?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

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```diff
@@           Coverage Diff           @@
##           master   #18953   +/-   ##
=======================================
  Coverage    92.2%    92.2%           
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  Files         169      169           
  Lines       50924    50924           
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  Hits        46952    46952           
  Misses       3972     3972
```

| Flag | Coverage Δ | |
|---|---|---|
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| #single | `42.3% <ø> (ø)` | :arrow_up: |


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 I'm not so sure Pandas should mention ``Decimal`` on the ``groupby`` page.

Operations on ``Decimal``s will by nature be slow, and people could misunderstand the slowness as a slowness of Pandas. IMO Pandas should promote using the various numpy and pandas types and python ``str``, very seldomly other types.

Maybe it could go to the FAQ or maybe just to StackOverflow? If it goes in the pandas docs, there should IMO be a visible disclaimer regarding slowness and the backsides of doing math operations on python objects  rather than numpy objects. Made requested changes. Fix for issue #17027. can you rebase this closing as stale @jreback Can certainly rebase. 🙂  @gfyoung i am not sure this is in the right direction
needs significant edits > i am not sure this is in the right direction

Hmm...I see.  I was under the impression, given the conversation, that this just needed rebasing.  What do you mean by the "right direction" ? needs to be way way simpler my point is i would start over I can revisit this. Hello not interseted to learn

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Subject: Re: [pandas-dev/pandas] Added note about groupby excluding Decimal columns by default (#18953)


closing as stale

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 @jorisvandenbossche Will do. @jorisvandenbossche & @jreback Requested changes made. you need to merge master and force push, you are picking up all commits since july. Fixed @jreback is this ready to be merged after the last chages? so I am not averse to expanding this section, however decimals DO now work on master if they are typed as EAs. So maybe a reference to that possibility here. @pdpark do you have time to update based on the last comment? > however decimals DO now work on master if they are typed as EAs

The decimals EA is not publicly exposed, only for testing purposes for now. I updated this PR (slight rewording + simplified and fixed the example a little bit), should be ready to merge now. > Some lines are >80, but I think in rst files we don't care

We care a little bit, to the extent that it fits in the code block box without generating scroll bars, but I think the limit is a bit higher than 80 for that.
 Merging since it is a `/pandas/doc/source` change only (the CI is still an old version that is failing) @pdpark Thanks a lot for the PR!