Re: st: Reshape vs. merge: computing time?

[Roberto Ferrer <refp16@gmail.com>]

I think that link should read 
http://www.nber.org/stata/efficient/

Daniel Feenberg <feenberg@nber.org> wrote:
>
>On Sat, 18 Jan 2014, Dorothy Bridges wrote:
>
>> Dear all: I have a data set in which 20 million individuals appear in
>> each of ten periods (~200 million observations total). The data are
>> long and I would like them to be wide. From a computing time / memory
>> usage perspective, am I better off (a) running reshape or (b) saving
>> each of the ten periods as a separate file and then merging on person
>> ID?
>
>-reshape- is not as fast as one might expect, and wide to long can be
>done much faster by reading the file multiple times. See the link
>under "Rehape" at
>
>   http://www.nber.org/stata/efficiency
>
>But a similar trick to go long to wide won't work because -save-
>doesn't 
>allow an -if- qualifier, meaning you would have to reread the whole
>file 
>for each of the 10 saves. Also, -merge- is quite a bit slower than
>-use-.
>
>Dan Feenberg
>>
>> Thank you,
>> D
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