March 2021
Intermediate to advanced
265 pages
6h 50m
Chinese
Content preview from Presto实战
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102
｜
第
7
章
)
WITH
external = true,
column_mapping = 'origin:location:origin,dest:location:dest'
);
7.2.2
Accumulo
中的谓词下推
在
Accumulo
连接器中，
Presto
可以利用
Accumulo
内置的二级索引。为了实现这一点，
Accumulo
连接器需要在每个
tablet
服务器上建立一个自定义的服务器端迭代器
。该迭代器
以
JAR
文件的形式分发
，你必须将其复制到每个
tablet
服务器上的
$ACCUMULO_HOME/
lib/ext
目录中。你可以在
Presto
文档中找到具体做法。
Accumulo
中的索引用于查询行
ID
，行
ID
用来读取实际表中的值。让我们来看一个例子。
SELECT flightnum, origin
FROM flights
WHERE flightdate BETWEEN DATE '2019-10-01' AND 2019-11-05'
AND origin = 'BOS';
在没有索引的情况下，
Presto
会从
Accumulo
中读取整个数据集，然后在
Presto
内部进行
过滤。要读取的
Accumulo
范
围被分配给各个工作节点，这里的范围是整个
tablet
的范围。
如果有索引（如表
7-3
中的示例索引）
，处理范围的数量就可以大大减少。
表7-3：航班表上的索引示例
2019-08-10 flightdate_flightdate:2232 []
2019-10-19 flightdate_flightdate:2232 ...
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O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.