July 2023
Intermediate to advanced
278 pages
5h 40m
Chinese
Content preview from Trino 权威指南：原书第2版
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Trino
架构
|
61
化为
Θ
[
row_count
×
log(
limit
)]
，空间复杂度简化为
Θ
(
limit
)
。现在查询计算的总代价是
Ω
[
O
+ (
R
×
N
) +
N
]
。
4.6.4
局部聚合
Trino
无须将
orders
表中的全部行都进行
Join
，因为不会用到那些单独的
order
。我们
的查询案例会在每个
nation
上执行累加
totalprice
的聚合操作。因此，如示例
4-5
所
示，我们可以预聚合这些行。通过聚合数据，可以减少流经下游
Join
的数据量。这里返
回的结果还不完整，因此称为预聚合，但它可以大大减少产生的数据量，显著地提高查
询性能。
示例 4-5：局部预聚合可以显著提高性能
- Aggregate[by nationkey...; orders_sum := sum(totalprice)]
- InnerJoin[c.custkey = o.custkey]
- InnerJoin[n.nationkey = c.nationkey]
- TableScan[nation]
- TableScan[customer]
- Aggregate[by custkey; totalprice := sum(totalprice)]
- TableScan[orders]
为了提高并行度，这类预聚合采用不同的局部聚合（
partial aggregation
）方式来实现。
这里只展示简化的查询计划，如果实际运行
EXPLAIN
命令查看查询计划，展示的结果会
与这里最终的聚合不同。
示例
4-5
中显示的这 ...
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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.