avgMergeState

avgMergeState 组合器使用示例

​

描述

MergeState 组合器 可用于 avg 函数，以合并类型为 AverageFunction(avg, T) 的部分聚合状态，并 返回一个新的中间聚合状态。

​

示例用法

MergeState 组合器在多级聚合 场景中特别有用：当你希望合并预聚合状态，并继续将其保留为 状态 (而不是将其最终化) 以便后续处理时，尤其如此。为了说明这一点，我们来看 一个示例：如何将单台服务器的性能指标 转换为跨多个层级的分层聚合：服务器级 → 区域级 → 数据中心级。

首先，我们创建一个表来存储原始数据：

CREATE TABLE raw_server_metrics
(
    timestamp DateTime DEFAULT now(),
    server_id UInt32,
    region String,
    datacenter String,
    response_time_ms UInt32
)
ENGINE = MergeTree()
ORDER BY (region, server_id, timestamp);

我们将创建一个服务器级聚合目标表，并定义一个充当该表插入触发器的 Incremental materialized view：

CREATE TABLE server_performance
(
    server_id UInt32,
    region String,
    datacenter String,
    avg_response_time AggregateFunction(avg, UInt32)
)
ENGINE = AggregatingMergeTree()
ORDER BY (region, server_id);

CREATE MATERIALIZED VIEW server_performance_mv
TO server_performance
AS SELECT
    server_id,
    region,
    datacenter,
    avgState(response_time_ms) AS avg_response_time
FROM raw_server_metrics
GROUP BY server_id, region, datacenter;

我们也将在区域和数据中心层级执行相同的操作：

CREATE TABLE region_performance
(
    region String,
    datacenter String,
    avg_response_time AggregateFunction(avg, UInt32)
)
ENGINE = AggregatingMergeTree()
ORDER BY (datacenter, region);

CREATE MATERIALIZED VIEW region_performance_mv
TO region_performance
AS SELECT
    region,
    datacenter,
    avgMergeState(avg_response_time) AS avg_response_time
FROM server_performance
GROUP BY region, datacenter;

-- 数据中心级别的表和 materialized view

CREATE TABLE datacenter_performance
(
    datacenter String,
    avg_response_time AggregateFunction(avg, UInt32)
)
ENGINE = AggregatingMergeTree()
ORDER BY datacenter;

CREATE MATERIALIZED VIEW datacenter_performance_mv
TO datacenter_performance
AS SELECT
      datacenter,
      avgMergeState(avg_response_time) AS avg_response_time
FROM region_performance
GROUP BY datacenter;

然后，我们会将示例原始数据插入源表中：

INSERT INTO raw_server_metrics (timestamp, server_id, region, datacenter, response_time_ms) VALUES
    (now(), 101, 'us-east', 'dc1', 120),
    (now(), 101, 'us-east', 'dc1', 130),
    (now(), 102, 'us-east', 'dc1', 115),
    (now(), 201, 'us-west', 'dc1', 95),
    (now(), 202, 'us-west', 'dc1', 105),
    (now(), 301, 'eu-central', 'dc2', 145),
    (now(), 302, 'eu-central', 'dc2', 155);

我们将为每个级别分别编写三个查询：

服务级别

SELECT
    server_id,
    region,
    avgMerge(avg_response_time) AS avg_response_ms
FROM server_performance
GROUP BY server_id, region
ORDER BY region, server_id;

┌─server_id─┬─region─────┬─avg_response_ms─┐
│       301 │ eu-central │             145 │
│       302 │ eu-central │             155 │
│       101 │ us-east    │             125 │
│       102 │ us-east    │             115 │
│       201 │ us-west    │              95 │
│       202 │ us-west    │             105 │
└───────────┴────────────┴─────────────────┘

区域级别

SELECT
    region,
    datacenter,
    avgMerge(avg_response_time) AS avg_response_ms
FROM region_performance
GROUP BY region, datacenter
ORDER BY datacenter, region;

┌─region─────┬─datacenter─┬────avg_response_ms─┐
│ us-east    │ dc1        │ 121.66666666666667 │
│ us-west    │ dc1        │                100 │
│ eu-central │ dc2        │                150 │
└────────────┴────────────┴────────────────────┘

数据中心级别

SELECT
    datacenter,
    avgMerge(avg_response_time) AS avg_response_ms
FROM datacenter_performance
GROUP BY datacenter
ORDER BY datacenter;

┌─datacenter─┬─avg_response_ms─┐
│ dc1        │             113 │
│ dc2        │             150 │
└────────────┴─────────────────┘

我们可以再插入更多数据：

INSERT INTO raw_server_metrics (timestamp, server_id, region, datacenter, response_time_ms) VALUES
    (now(), 101, 'us-east', 'dc1', 140),
    (now(), 201, 'us-west', 'dc1', 85),
    (now(), 301, 'eu-central', 'dc2', 135);

我们再来看看数据中心层面的性能。请注意，整个 聚合链已自动更新：

SELECT
    datacenter,
    avgMerge(avg_response_time) AS avg_response_ms
FROM datacenter_performance
GROUP BY datacenter
ORDER BY datacenter;

┌─datacenter─┬────avg_response_ms─┐
│ dc1        │ 112.85714285714286 │
│ dc2        │                145 │
└────────────┴────────────────────┘

​

另请参见

• avg
• AggregateFunction
• Merge
• MergeState