25708 performance degradation when dumping large dataframe with datetime to csv

[hksonngan]

• closes Performance degradation when dumping large dataframe with np.datetime to csv #25708
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry
  Slicing in line 2181 only slice of i8values that make a whole of values array is processed in _is_dates_only function.
  This change was from pull diff reduction for 24024 #24543

        values = self.values
        i8values = self.values.view('i8')

        if slicer is not None:
            i8values = i8values[..., slicer]

        from pandas.io.formats.format import _get_format_datetime64_from_values
        fmt = _get_format_datetime64_from_values(values, date_format)

[codecov]

Codecov Report

Merging #25764 into master will decrease coverage by 49.5%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           master   #25764       +/-   ##
===========================================
- Coverage   91.25%   41.74%   -49.51%     
===========================================
  Files         172      172               
  Lines       52977    52968        -9     
===========================================
- Hits        48342    22113    -26229     
- Misses       4635    30855    +26220

Flag	Coverage Δ
#multiple	?
#single	41.74% <0%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/io/parsers.py	48.27% <ø> (-47.08%)	⬇️
pandas/plotting/_core.py	17.71% <0%> (-65.95%)	⬇️
pandas/core/internals/blocks.py	51.89% <0%> (-42.2%)	⬇️
pandas/core/reshape/merge.py	9.51% <0%> (-84.98%)	⬇️
pandas/io/formats/latex.py	0% <0%> (-100%)	⬇️
pandas/io/sas/sas_constants.py	0% <0%> (-100%)	⬇️
pandas/tseries/plotting.py	0% <0%> (-100%)	⬇️
pandas/tseries/converter.py	0% <0%> (-100%)	⬇️
pandas/io/formats/html.py	0% <0%> (-99.36%)	⬇️
pandas/core/groupby/categorical.py	0% <0%> (-95.46%)	⬇️
... and 133 more

---

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[codecov]

Codecov Report

Merging #25764 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #25764      +/-   ##
==========================================
+ Coverage   91.25%   91.25%   +<.01%     
==========================================
  Files         172      172              
  Lines       52977    52968       -9     
==========================================
- Hits        48342    48334       -8     
+ Misses       4635     4634       -1

Flag	Coverage Δ
#multiple	89.82% <100%> (-0.01%)	⬇️
#single	41.74% <0%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/io/parsers.py	95.34% <ø> (ø)	⬆️
pandas/plotting/_core.py	83.65% <100%> (ø)	⬆️
pandas/core/internals/blocks.py	94.08% <100%> (ø)	⬆️
pandas/core/reshape/merge.py	94.4% <100%> (-0.08%)	⬇️
pandas/util/testing.py	89.08% <0%> (+0.09%)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[hksonngan]

Sorry, my mistake git rebase. I close this.