〇〇不知道从哪里搞来了一套极耗脑细胞的题目：
将1-19这19个自然数填入圆圈中，使每一直线上的几个数之和都相等，请用SQL/PLSQL求解。
其中一个答案如下：
想了一晚没什么好办法可以投机取巧，今天早上将题目的所有约束条件放到SQL中，然后闭上眼睛让Oracle跑：
set linesize 150;
set pagesize 500;
col num format 999 ;
set timing on ;
set autotrace on ;
with points as (
select level num from dual connect by level <= 19
) -- select num from points
select /*+ parallel(dual 4) */
p1.num,p2.num,p3.num,
p4.num,p5.num,p6.num,p7.num,
p8.num,p9.num,p10.num,p11.num,p12.num,
p13.num,p14.num,p15.num,p16.num,
p17.num,p18.num,p19.num
from
points p1,points p2,points p3,
points p4,points p5,points p6,points p7,
points p8,points p9,points p10,points p11,points p12,
points p13,points p14,points p15,points p16,
points p17,points p18,points p19
where
p1.num not in ( p2.num,p3.num,p4.num,p5.num,p6.num,p7.num,p8.num,p9.num,p10.num,p11.num,p12.num,p13.num,p14.num,p15.num,p16.num,p17.num,p18.num,p19.num )
and p2.num not in ( p1.num, p3.num,p4.num,p5.num,p6.num,p7.num,p8.num,p9.num,p10.num,p11.num,p12.num,p13.num,p14.num,p15.num,p16.num,p17.num,p18.num,p19.num )
and p3.num not in ( p1.num,p2.num, p4.num,p5.num,p6.num,p7.num,p8.num,p9.num,p10.num,p11.num,p12.num,p13.num,p14.num,p15.num,p16.num,p17.num,p18.num,p19.num )
and p4.num not in ( p1.num,p2.num,p3.num, p5.num,p6.num,p7.num,p8.num,p9.num,p10.num,p11.num,p12.num,p13.num,p14.num,p15.num,p16.num,p17.num,p18.num,p19.num )
and p5.num not in ( p1.num,p2.num,p3.num,p4.num, p6.num,p7.num,p8.num,p9.num,p10.num,p11.num,p12.num,p13.num,p14.num,p15.num,p16.num,p17.num,p18.num,p19.num )
and p6.num not in ( p1.num,p2.num,p3.num,p4.num,p5.num, p7.num,p8.num,p9.num,p10.num,p11.num,p12.num,p13.num,p14.num,p15.num,p16.num,p17.num,p18.num,p19.num )
and p7.num not in ( p1.num,p2.num,p3.num,p4.num,p5.num,p6.num, p8.num,p9.num,p10.num,p11.num,p12.num,p13.num,p14.num,p15.num,p16.num,p17.num,p18.num,p19.num )
and p8.num not in ( p1.num,p2.num,p3.num,p4.num,p5.num,p6.num,p7.num, p9.num,p10.num,p11.num,p12.num,p13.num,p14.num,p15.num,p16.num,p17.num,p18.num,p19.num )
and p9.num not in ( p1.num,p2.num,p3.num,p4.num,p5.num,p6.num,p7.num,p8.num, p10.num,p11.num,p12.num,p13.num,p14.num,p15.num,p16.num,p17.num,p18.num,p19.num )
and p10.num not in ( p1.num,p2.num,p3.num,p4.num,p5.num,p6.num,p7.num,p8.num,p9.num, p11.num,p12.num,p13.num,p14.num,p15.num,p16.num,p17.num,p18.num,p19.num )
and p11.num not in ( p1.num,p2.num,p3.num,p4.num,p5.num,p6.num,p7.num,p8.num,p9.num,p10.num, p12.num,p13.num,p14.num,p15.num,p16.num,p17.num,p18.num,p19.num )
and p12.num not in ( p1.num,p2.num,p3.num,p4.num,p5.num,p6.num,p7.num,p8.num,p9.num,p10.num,p11.num, p13.num,p14.num,p15.num,p16.num,p17.num,p18.num,p19.num )
and p13.num not in ( p1.num,p2.num,p3.num,p4.num,p5.num,p6.num,p7.num,p8.num,p9.num,p10.num,p11.num,p12.num, p14.num,p15.num,p16.num,p17.num,p18.num,p19.num )
and p14.num not in ( p1.num,p2.num,p3.num,p4.num,p5.num,p6.num,p7.num,p8.num,p9.num,p10.num,p11.num,p12.num,p13.num, p15.num,p16.num,p17.num,p18.num,p19.num )
and p15.num not in ( p1.num,p2.num,p3.num,p4.num,p5.num,p6.num,p7.num,p8.num,p9.num,p10.num,p11.num,p12.num,p13.num,p14.num, p16.num,p17.num,p18.num,p19.num )
and p16.num not in ( p1.num,p2.num,p3.num,p4.num,p5.num,p6.num,p7.num,p8.num,p9.num,p10.num,p11.num,p12.num,p13.num,p14.num,p15.num, p17.num,p18.num,p19.num )
and p17.num not in ( p1.num,p2.num,p3.num,p4.num,p5.num,p6.num,p7.num,p8.num,p9.num,p10.num,p11.num,p12.num,p13.num,p14.num,p15.num,p16.num, p18.num,p19.num )
and p18.num not in ( p1.num,p2.num,p3.num,p4.num,p5.num,p6.num,p7.num,p8.num,p9.num,p10.num,p11.num,p12.num,p13.num,p14.num,p15.num,p16.num,p17.num, p19.num )
and p19.num not in ( p1.num,p2.num,p3.num,p4.num,p5.num,p6.num,p7.num,p8.num,p9.num,p10.num,p11.num,p12.num,p13.num,p14.num,p15.num,p16.num,p17.num,p18.num )
and p1.num +p2.num + p3.num = 38
and p3.num +p7.num + p12.num = 38
and p12.num + p16.num + p19.num = 38
and p17.num + p18.num + p19.num = 38
and p8.num + p13.num + p17.num = 38
and p1.num + p4.num + p8.num = 38
and p2.num + p6.num + p11.num + p16.num = 38
and p4.num + p9.num + p14.num + p18.num = 38
and p2.num + p5.num + p9.num + p13.num = 38
and p7.num + p11.num + p15.num + p18.num = 38
and p4.num + p5.num + p6.num + p7.num = 38
and p13.num + p14.num + p15.num + p16.num = 38
and p1.num + p5.num + p10.num + p15.num + p19.num = 38
and p3.num + p6.num + p10.num + p14.num + p17.num = 38
and p8.num + p9.num + p10.num + p11.num + p12.num = 38
/
NUM NUM NUM NUM NUM NUM NUM NUM NUM NUM NUM NUM NUM NUM NUM NUM NUM NUM NUM ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- 15 13 10 14 8 4 12 9 6 5 2 16 11 1 7 19 18 17 3 15 14 9 13 8 6 11 10 4 5 1 18 12 2 7 17 16 19 3 16 12 10 19 2 4 13 3 7 5 8 15 17 1 6 14 18 11 9 16 19 3 12 2 7 17 10 4 5 1 18 13 8 6 11 15 14 9 18 11 9 17 1 6 14 3 7 5 8 15 19 2 4 13 16 12 10 18 17 3 11 1 7 19 9 6 5 2 16 14 8 4 12 15 13 10 3 17 18 19 7 1 11 16 2 5 6 9 12 4 8 14 10 13 15 3 19 16 17 7 2 12 18 1 5 4 10 11 6 8 13 9 14 15 9 11 18 14 6 1 17 15 8 5 7 3 13 4 2 19 10 12 16 9 14 15 11 6 8 13 18 1 5 4 10 17 7 2 12 3 19 16 10 12 16 13 4 2 19 15 8 5 7 3 14 6 1 17 9 11 18 10 13 15 12 4 8 14 16 2 5 6 9 19 7 1 11 3 17 18 12 rows selected. Elapsed: 00:00:40.15
注：第一行那3个点，从左到右 p1 p2 p3 第二行从左到右 p4 p5 p6 p7 ,如此类推。
我尝试在Oracle 10g中运行这个结果，可是等了2个小时结果还是跑不出来，将查询放在Oracle 11g中几十秒后就有结果。
以上结果是在一台CPU 是 Intel XEON 2.8GHz 的 DELL PE2850 服务器上跑得的。突发好奇拿到一台CPU 是 Intel XEON 5110 频率仅有 1.6GHz 的HP DL380 G5 上跑，用时 00:00:30.53 ，频率高一大截但是成绩却差一大截，这让我非常惊讶，看老白的《Oracle RAC日记》中提到一个案例也是有类似的情况——频率高性能反而低。
难道我人品这么好今天就遇到了？
将这个查询拿到一台CPU为SPARC 频率为1.5GHz 的 Sun Fire V890 小型机上面跑，用时 00:01:05.66。
看来频率并不是决定性能的绝对参考指标，同样根据老白的办法，写一个冒泡排序的C程序，然后拿到多个平台上跑一次：
Intel Xeon 2.8GHz 0m33.831s Intel Xeon 3.0GHz 0m31.692s Intel Xeon 5110 1.6GHz 0m31.137s Intel Xeon 5160 3.0GHz 0m18.842s Intel Xeon 5450 3.0GHz 0m16.655s SUN SPARC 1.5GHz 0m42.787s IBM Power5 1.9GHz 0m37.84s
从测试结果来看，同样架构下频率越高性能的确越好，但是不同的架构即使频率相同性能孰优孰劣还是说不准。
后来我才知道这条数学题叫做“A Unique Magic Hexagon”（Google翻译：独特的魔术六角，有人称为“六角幻方”），这里有精妙的解答过程绝非我这种暴力方法