q - 通过枚举进行数据规范化没有速度增益

Question

在q for mortals chapter on data normalisation中，即消除列表中重复的任务，它建议使用枚举来查找列表中的不同值，因为遍历整数比遍历可变长度的符号更快

u:`g`ibm`intl`msft / unique list of tickers
v:1000000?u / list with duplicate tickers
k:u?v / positions in u
\t:10 distinct v / performing distinct on symbols 10 times and timing 
\t:10 distinct k / performing distinct on positions 10 times and timing

我发现 distinct v 比 distinct k 快很多，这与承诺的不符。

感谢您的帮助。

Answer 1

枚举通常用于保存到磁盘的数据以帮助压缩等这就是您将看到更大性能提升的地方。

KDB+ 3.5 2017.04.06 Copyright (C) 1993-2017 Kx Systems

Welcome to kdb+ 32bit edition
For support please see http://groups.google.com/d/forum/personal-kdbplus
Tutorials can be found at http://code.kx.com/wiki/Tutorials
To exit, type \
To remove this startup msg, edit q.q
u:`g`ibm`intl`msft / unique list of tickers
v:1000000?u / list with duplicate tickers
q)k:`u$v //enumerate v against u
q)k
`u$`g`g`intl`ibm`intl`ibm`intl`msft`intl`ibm`g`msft`ibm`intl`intl`ibm`g`ibm`i..
q)save `:k
`:k
q)save `:u
`:u
q)save `:v
`:v
q)\

KDB+ 3.5 2017.04.06 Copyright (C) 1993-2017 Kx Systems

Welcome to kdb+ 32bit edition
For support please see http://groups.google.com/d/forum/personal-kdbplus
Tutorials can be found at http://code.kx.com/wiki/Tutorials
To exit, type \
To remove this startup msg, edit q.q
q)u:get `:u
q)\ts:10 distinct get `:v
462 8388848
q)\ts:10 distinct get `:k
37 4194544
q)

但是您确实提出了一个有趣的问题，即为什么符号列表（在内存中）比整数列表更快。

q - 通过枚举进行数据规范化没有速度增益

q - No speed gain for data normalisation via enumerations

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