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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Will Pugh <willpugh@gmail.com>
cc: pgsql-sql@postgresql.org
Subject: Re: How does Numeric division determine precision?
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Comments: In-reply-to Will Pugh <willpugh@gmail.com>
	message dated "Thu, 12 Jul 2012 09:02:03 -0700"
Date: Thu, 12 Jul 2012 13:30:19 -0400
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Will Pugh <willpugh@gmail.com> writes:
> It seems that is 9.1, numerics that don't have a specified precision
> and scale are arbitrary scale/precision.
> For many operations this is straightforward.  However, when doing a
> division operation that does not terminate, I'm curious about how the
> number of digits is determined.

According to select_div_scale() in src/backend/utils/adt/numeric.c,

    /*
     * The result scale of a division isn't specified in any SQL standard. For
     * PostgreSQL we select a result scale that will give at least
     * NUMERIC_MIN_SIG_DIGITS significant digits, so that numeric gives a
     * result no less accurate than float8; but use a scale not less than
     * either input's display scale.
     */

I wouldn't necessarily claim that that couldn't be improved on,
but that's what it does now.

			regards, tom lane