diff options
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/ref/pgbench.sgml | 19 |
1 files changed, 14 insertions, 5 deletions
diff --git a/doc/src/sgml/ref/pgbench.sgml b/doc/src/sgml/ref/pgbench.sgml index 15ee7c0f2ba..9d185248346 100644 --- a/doc/src/sgml/ref/pgbench.sgml +++ b/doc/src/sgml/ref/pgbench.sgml @@ -1604,15 +1604,24 @@ f(x) = PHI(2.0 * parameter * (x - mu) / (max - min + 1)) / in (1, 1000), a rejection method is used, based on "Non-Uniform Random Variate Generation", Luc Devroye, p. 550-551, Springer 1986. The distribution is not defined when the parameter's - value is 1.0. The drawing performance is poor for parameter values + value is 1.0. The function's performance is poor for parameter values close and above 1.0 and on a small range. </para> <para> - <replaceable>parameter</replaceable> - defines how skewed the distribution is. The larger the <replaceable>parameter</replaceable>, the more - frequently values to the beginning of the interval are drawn. + <replaceable>parameter</replaceable> defines how skewed the distribution + is. The larger the <replaceable>parameter</replaceable>, the more + frequently values closer to the beginning of the interval are drawn. The closer to 0 <replaceable>parameter</replaceable> is, - the flatter (more uniform) the access distribution. + the flatter (more uniform) the output distribution. + The distribution is such that, assuming the range starts from 1, + the ratio of the probability of drawing <replaceable>k</replaceable> + versus drawing <replaceable>k+1</replaceable> is + <literal>((<replaceable>k</replaceable>+1)/<replaceable>k</replaceable>)**<replaceable>parameter</replaceable></literal>. + For example, <literal>random_zipfian(1, ..., 2.5)</literal> produces + the value <literal>1</literal> about <literal>(2/1)**2.5 = + 5.66</literal> times more frequently than <literal>2</literal>, which + itself is produced <literal>(3/2)*2.5 = 2.76</literal> times more + frequently than <literal>3</literal>, and so on. </para> </listitem> </itemizedlist> |