On Sun, Nov 23, 2008 at 12:34:21AM -0300, Alvaro Herrera wrote:
> Gregory Stark wrote:
>
> > WITH RECURSIVE Z(IX, IY, CX, CY, X, Y, I) AS (
> > [elided]
>
> FWIW you can halve the running time by restricting I to 27 instead of
> 100 in the recursive term, and obtain the same result.
I found it easier to read this way:
WITH RECURSIVE
Z(Ix, Iy, Cx, Cy, X, Y, I)
AS (
SELECT Ix, Iy, X::float, Y::float, X::float, Y::float, 0
FROM
(SELECT -2.2 + 0.031 * i, i FROM generate_series(0,101) AS i) AS
xgen(x,ix)
CROSS JOIN
(SELECT -1.5 + 0.031 * i, i FROM generate_series(0,101) AS i) AS
ygen(y,iy)
UNION ALL
SELECT Ix, Iy, Cx, Cy, X * X - Y * Y + Cx AS X, Y * X * 2 + Cy, I + 1
FROM Z
WHERE X * X + Y * Y < 16::float
AND I < 27
),
Zt (Ix, Iy, I) AS (
SELECT Ix, Iy, MAX(I) AS I
FROM Z
GROUP BY Iy, Ix
ORDER BY Iy, Ix
)
SELECT array_to_string(
array_agg(
SUBSTRING(' .,,,-----++++%%%%@@@@#### ', LEAST(GREATEST(I,1),27), 1)
),''
)
FROM Zt
GROUP BY Iy
ORDER BY Iy;
Cheers,
David.
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