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The following CTE runs almost instantly on MySQL, but hangs in MariaDB - tested on 10.3.10 and 10.2.18:
WITH RECURSIVE x(i) AS (
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SELECT CAST(0 AS DECIMAL(13, 10))
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UNION ALL
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SELECT i + 1
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FROM x
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WHERE i < 101
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),
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Z(Ix, Iy, Cx, Cy, X, Y, I) AS (
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SELECT Ix, Iy, X, Y, X, Y, 0
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FROM (SELECT CAST(-2.2 + 0.031 * i AS DECIMAL(13, 10)) AS X,
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i AS Ix FROM x) AS xgen
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CROSS JOIN (
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SELECT CAST(-1.5 + 0.031 * i AS DECIMAL(13, 10)) AS Y,
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i AS iY FROM x
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) AS ygen
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UNION ALL
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SELECT Ix, Iy, Cx, Cy,
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CAST(X * X - Y * Y + Cx AS DECIMAL(13, 10)) AS X,
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CAST(Y * X * 2 + Cy AS DECIMAL(13, 10)), I + 1
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FROM Z
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WHERE X * X + Y * Y < 16.0
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AND I < 27
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),
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Zt (Ix, Iy, I) AS (
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SELECT Ix, Iy, MAX(I) AS I
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FROM Z
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GROUP BY Iy, Ix
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ORDER BY Iy, Ix
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)
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SELECT GROUP_CONCAT(
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SUBSTRING(
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' .,,,-----++++%%%%@@@@#### ',
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GREATEST(I, 1),
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1
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) ORDER BY Ix SEPARATOR ''
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) AS 'Mandelbrot Set'
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FROM Zt
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GROUP BY Iy
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ORDER BY Iy;
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