[R] Sampling from a database

[F. Tusell]

I use RODBC  and RpgSQL quite a lot to access  files stored in another
machine under PostgreSQL. Since I am  now using files which do not fit
into R's  memory, I would  like to take  random samples. What  I would
like is to issue a query such as

   SELECT * FROM file WHERE runif > 0.9

with "runif" being a uniformly distributed random number, generated on
the fly; but I  cannot find any way to do that  in PostgreSQL. So far,
my only idea is to create  a new table with two fields, "IDnumber" and
"Random",  fill "IDnumber"  with the  similarly named  field  from the
table to sample, populate "Random"  with random numbers generated in R
and then issue a query for  records where "IDnumber" of the two tables
match AND Random > 0.9.

Seems  to me  clumsy, and  surely there  has to  be a  better  way and
someone knowing SQL can suggest it.


ft.
-- 
Fernando TUSELL                                e-mail:
Departamento de Econometría y Estadística           etptupaf at bs.ehu.es 
Facultad de CC.EE. y Empresariales             Tel:   (+34)94.601.3733
Avenida Lendakari Aguirre, 83                  Fax:   (+34)94.601.3754
E-48015 BILBAO  (Spain)                        Secr:  (+34)94.601.3740
PGP: finger etptupaf at bsdx01.bs.ehu.es          http://etdx01.bs.ehu.es
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