ClickHouse学习(八)物化视图

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什么是物化视图

在数据库中视图指的是通过一张或者多张表查询出来的逻辑表,就类似网页版的拳皇一样按UIO就会放招而不是用上下上下BABA那样按,视图就是UIO记录了招式的过程但是并不记录数据.
而clickhouse的物化视图则是把查询的结果和相应的引擎存入到磁盘或内存中,类似形成了一张新的表,但是表中会对自己数据进行重新组织.

使用物化视图好处

可以避免对基础表的频繁查询并复用结果，从而显著提升查询的性能

基本语法

CREATE [MATERIALIZED] VIEW [IF NOT EXISTS] [db.]table_name [TO[db.]name] [ENGINE = engine] [POPULATE] AS SELECT …

• 若使用POPULATE则会将历史记录数据全部导入进视图中,最好别用,想要将历史记录导入则使用insert插入数据就行

创建物化视图

创建源表

CREATE TABLE hits_test
(
 EventDate Date, 
 CounterID UInt32, 
 UserID UInt64, 
 URL String, 
 Income UInt8
)
ENGINE = MergeTree()
PARTITION BY toYYYYMM(EventDate)
ORDER BY (CounterID, EventDate, intHash32(UserID))
SAMPLE BY intHash32(UserID)
SETTINGS index_granularity = 8192;

向表中插入数据

INSERT INTO hits_test 
 SELECT 
 EventDate,
 CounterID,
 UserID,
 URL,
 Income 
FROM hits_v1 
limit 10000;

创建物化视图

CREATE MATERIALIZED VIEW hits_mv 
ENGINE=SummingMergeTree
PARTITION BY toYYYYMM(EventDate) ORDER BY (EventDate, intHash32(UserID)) 
AS SELECT
UserID,
EventDate,
count(URL) as ClickCount,
sum(Income) AS IncomeSum
FROM hits_test
WHERE EventDate >= '2014-03-20'
GROUP BY UserID,EventDate;

框起来的为物化视图自己的数据规则

向物化视图中增加数据

INSERT INTO hits_test 
SELECT 
 EventDate,
 CounterID,
 UserID,
 URL,
 Income 
FROM hits_v1 
WHERE EventDate >= '2014-03-23' 
limit 10;

导入历史数据

最好别用POPULATE导入历史数据
将hits_test表中的数据导入物化视图中

INSERT INTO hits_mv
SELECT
 UserID,
 EventDate,
 count(URL) as ClickCount,
 sum(Income) AS IncomeSum
FROM hits_test
WHERE EventDate = '2014-03-20'
GROUP BY UserID,EventDate;