When applied to "numeric" input, stddev_pop produces the same result as
stddev_samp, and var_pop produces the same result as var_samp. This
is because whoever wrote numeric_stddev_internal() forgot that the
divisor is different for the sample case. This mistake is in fact
visible in the regression test outputs for the function, so those
outputs weren't checked very carefully :-(
regards, tom lane
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TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
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