Performance decrease of groupby.first for datetime64 in 0.14 · Issue #7555 · pandas-dev/pandas (original) (raw)

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Description

I have seen a dramatic (around 100x) slowdown of groupby.first method for datetime64 type after updating to 0.14.

Here is an example:

from time import time d = pd.DataFrame({'a': pd.date_range('1/1/2011', periods=100000, freq='s'), 'b': range(100000)}) t = time() d.groupby('b').first() print(time() - t)

Result:

Same for int type

from time import time d = pd.DataFrame({'a': range(100000), 'b': range(100000)}) t = time() d.groupby('b').first() print(time() - t)

Result: