Fred Wolfe <[email protected]> asked about the implementation
of the -keep(<varlist>)- option of -merge-:
> Stata has a keep() option that limits the merging of data in the using data
> set to selected variables. My question is, Does Stata first bring all of the
> using variables into memory or does it somehow drop the non-selected using
> variables before merging.
Stata does *NOT* bring in the entire using dataset and then merge;
it does bring in, one at a time, entire observations from the using
dataset. The basic loop is this
do until all of the using data is read
read *ONE* observation of the using dataset
does it merge?
record the keep() variables in the appropriate
observation in memory
end
-- Bill
[email protected]
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