1. 概述

转载：Clickhouse查询语句 sample

注意：

sample子句只能用于MergeTree系列引擎的数据表，并且在create table的时候就声明sample by 抽样表达式。

sample子句提供了近似计算的功能，能够实现数据采样的功能，使查询仅仅返回采样数据而不是全部数据，从而有效减少查询负载。

sample子句的采样设计是一种幂等设计，即在数据发生变化的时候使用相同的采样规则能返回相同的数据。这种特性非常适合那些可以接受近似查询结果的场景。

官方提供了如下的使用场景：

1. When you have strict timing requirements (like <100ms) but you can’t justify the cost of additional hardware
   resources to meet them.
2. When your raw data is not accurate, so approximation doesn’t noticeably degrade the quality.
3. Business requirements target approximate results (for cost-effectiveness, or to market exact results to
   premium users).

Clickhouse> create table clicks(CounterID UInt64,EventDate DATE, UserID UInt64) engine=MergeTree() 
order by (CounterID,intHash32(UserID)) sample by intHash32(UserID);
 
CREATE TABLE clicks
(
    `CounterID` UInt64,
    `EventDate` DATE,
    `UserID` UInt64
)
ENGINE = MergeTree()
ORDER BY (CounterID, intHash32(UserID))
SAMPLE BY intHash32(UserID)

插入测试数据：

Clickhouse> insert into clicks select CounterID,EventDate,UserID from hits_v1;
 
INSERT INTO clicks SELECT 
    CounterID,
    EventDate,
    UserID
FROM hits_v1
 
Ok.
 
0 rows in set. Elapsed: 1.003 sec. Processed 8.87 million rows, 124.23 MB (8.85 million rows/s., 123.88 MB/s.) 
 

clicks表的定义按照intHash32(UserID) 分布后的结果采样查询。
声明Sample KEY的时候有两点需要注意：

1. sample by 所声明的表达式必须同时包含在主键的声明内
2. sample key必须UInt类型，若不是可以定义但是查询的时候会抛出异常。

SAMPLE 子句支持三种格式：

1.sample k
k表示因子系数，采样因子，取值范围【0,1】，若在0–1之间的小数则表示采样，若为0或者1则等同于不采样。

select CounterID from clicks sample 0.1
等同于：
select CounterID from clicks sample 1/10

查询获取近似结果:

Clickhouse> select count() from clicks;
 
SELECT count()
FROM clicks
 
┌─count()─┐
│ 8873898 │
└─────────┘
 
1 rows in set. Elapsed: 0.003 sec. 
 
Clickhouse> select count() from clicks sample 0.1;
 
SELECT count()
FROM clicks
SAMPLE 1 / 10
 
┌─count()─┐
│  839889 │
└─────────┘
 
1 rows in set. Elapsed: 0.029 sec. Processed 5.89 million rows, 94.27 MB (201.86 million rows/s., 3.23 GB/s.) 
 
 
Clickhouse> select CounterID,_sample_factor from clicks sample 0.1 limit 2;
 
SELECT 
    CounterID,
    _sample_factor
FROM clicks
SAMPLE 1 / 10
LIMIT 2
 
┌─CounterID─┬─_sample_factor─┐
│        57 │             10 │
│        57 │             10 │
└───────────┴────────────────┘
 
2 rows in set. Elapsed: 0.012 sec. 

可以通过虚拟字段_sample_factor 查询采样系数。

2.sample n
n表示采样的样本数量。n表示至少采样多少行数据。n=1表示不使用采样，n的范围从2到表的总行数。

Clickhouse> select count() from clicks sample 10000;
 
SELECT count()
FROM clicks
SAMPLE 10000
 
┌─count()─┐
│    9251 │
└─────────┘
 
1 rows in set. Elapsed: 0.025 sec. Processed 5.48 million rows, 87.72 MB (223.47 million rows/s., 3.58 GB/s.) 
 
Clickhouse> select count()*any(_sample_factor) from clicks sample 10000;
 
SELECT count() * any(_sample_factor)
FROM clicks
SAMPLE 10000
 
┌─multiply(count(), any(_sample_factor))─┐
│                      8154379.059200001 │
└────────────────────────────────────────┘
 
1 rows in set. Elapsed: 0.024 sec. Processed 5.48 million rows, 54.82 MB (229.44 million rows/s., 2.29 GB/s.) 
 
Clickhouse> select CounterID,_sample_factor from clicks sample 10000 limit 2;
 
SELECT 
    CounterID,
    _sample_factor
FROM clicks
SAMPLE 10000
LIMIT 2
 
┌─CounterID─┬────_sample_factor─┐
│      1294 │ 881.4592000000001 │
└───────────┴───────────────────┘
┌─CounterID─┬────_sample_factor─┐
│      1366 │ 881.4592000000001 │
└───────────┴───────────────────┘
 
2 rows in set. Elapsed: 0.041 sec. Processed 7.69 thousand rows, 123.01 KB (187.84 thousand rows/s., 3.01 MB/s.) 
 

数据采样的范围是一个近似值，采样数据的最小粒度有index_granularity 索引粒度决定的。
若设置一个小于索引粒度或者较小的n值没有意义。

3. sample k offset n
表示按照因子系数和偏移量采样。

Clickhouse> select CounterID,_sample_factor from clicks sample 0.4 offset 0.5 limit 1;
 
SELECT 
    CounterID,
    _sample_factor
FROM clicks
SAMPLE 4 / 10 OFFSET 5 / 10
LIMIT 1
 
┌─CounterID─┬─_sample_factor─┐
│        57 │            2.5 │
└───────────┴────────────────┘
 
1 rows in set. Elapsed: 0.017 sec. 
 
Clickhouse> select CounterID,_sample_factor from clicks sample 0.6 offset 0.5 limit 1;
 
SELECT 
    CounterID,
    _sample_factor
FROM clicks
SAMPLE 6 / 10 OFFSET 5 / 10
LIMIT 1
 
┌─CounterID─┬─────_sample_factor─┐
│        57 │ 1.6666666666666667 │
└───────────┴────────────────────┘
 
1 rows in set. Elapsed: 0.007 sec. 
 

当采样因子溢出（offset 的值+sample的值大于1）则溢出的数据则自动阶段。