Re: Statistical Analysis

Steve Heaven wrote:
>
> At 20:18 24/07/00 -0400, you wrote:
> >Sigh, forgot to mention the solution. There's an undocumented function:
> >
> > * bool oidrand (oid o, int4 X)-
> > * takes in an oid and a int4 X, and will return 'true'
> > * about 1/X of the time.
> >
> >typically used like this:
> >
> >-- select roughly 1/10 of the tuples
> >SELECT * FROM onek WHERE oidrand(onek.oid, 10);
> >
>
> It doesnt seem to work as you explain.
> For a value of 1 you expect (nearly) all the tuples and two should return
> half, but that not what I'm finding.
>
> galore=> select count(*) from topten where room='HI';
> count
> -----
> 14
> (1 row)
>
> galore=> SELECT * FROM topten WHERE room='HE' and oidrand(topten.oid, 1);
> type|data |norder|room |grp
> ----+-------------------------+------+------+---
> B |0764552503 | 1|HE |
> B |0751327190 | 1|HE |
> B |0718144392 | |HE |
> B |0500280754 | |HE |
> B |0028610091 | 1|HE |
> (5 rows)
> galore=> SELECT * FROM topten WHERE room='HE' and oidrand(topten.oid, 2);
> type|data |norder|room |grp
> ----+-------------------------+------+------+---
> B |0764552503 | 1|HE |
> B |0751327190 | 1|HE |
> B |0718144392 | |HE |
> (3 rows)
> galore=> SELECT * FROM topten WHERE room='HE' and oidrand(topten.oid, 7);
> type|data |norder|room |grp
> ----+-------------------------+------+------+---
> B |0751327190 | 1|HE |
> B |0718144392 | |HE |
> (2 rows)
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Isn't it because oidrand evals as 'random() < 1/X' ? or maybe 'random() <
1/(X+1)' ?

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Guillaume Perréal - Stagiaire MIAG
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