March 2021
Intermediate to advanced
265 pages
6h 50m
Chinese
Content preview from Presto实战
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生产环境中的
Presto
｜
223
Presto
和其他
SQL
引擎常用的一个优化是，将部分聚合下推到
Join
之前执行，从而减少输
入
Join
的数据量。要使用这个优化，
可以设置
push_aggregation_through_join
属性。由于
下推的聚合只产生局部结果，因此，在
Join
之后仍需要执行一次最终聚合
。使用这个优化
可以获得的收益依赖于实际的数据，有时它甚至会产生更慢的查询。例如，当
Join
条件比
较严格，只会产生少量数据时，在
Join
之后再进行聚合往往更高效
。可以在当前会话上将
这个属性值设为
false
，以关闭此优化并进行性能试验。
另一种常见的启发式优化是在计算最终聚合之前先计算局部聚合：
presto:ontime> EXPLAIN SELECT count(*) FROM flights_orc;
Query Plan
---------------------------------------------------------------------
- Output[_col0]
Layout: [count:bigint]
_col0 := count
- Aggregate(FINAL)
Layout: [count:bigint]
count := "count"("count_3")
- LocalExchange[SINGLE] ()
Layout: [count_3:bigint]
- RemoteExchange[GATHER]
Layout: [count_3:bigint] ...
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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.
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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.