Re: [PATCH] Revive line type

On 6/26/13 9:34 PM, Peter Eisentraut wrote:
> Still wondering whether to use a A,B,C-based output
> format per Tom's comment.

Wouldn't it also be helpful to remove "The points used in the output are
not necessarily the points used on input" by making that not true?

There are three obvious ways you might output a line:

-Math class expectations of slope-intercept form: y = mx + b.
Intercept forms don't handle both horizontal and vertical lines though,
so that's easily out.

-Pair of points. A casual observer might get lucky and observe putting
a pair of points in and getting the same pair back again, then
incorrectly assume that's normally the case. Seems better to never make
that possible in the infinite line case. I'm reminded of how row output
usually is in order gives a bad impression about ordering guarantees in SQL.

-Ax+By+C=0

Outputting that third one, when it's also the internal form, handles any
time of line; will show any assumptions about individual points being
preserved are wrong; and avoids rounding errors too. The only downside
seems to be that bounded lines are easier to show with a pair of points.
I think that suggests an alternate, secondary output format would be
useful though, rather than that a different default one is a good idea.

--
Greg Smith 2ndQuadrant US greg(at)2ndQuadrant(dot)com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com