Question

基表

CREATE TABLE IF NOT EXISTS test_sessions
(
    session_id   UInt64,
    session_name String,
    created_at   DateTime
)
ENGINE = MergeTree()
PARTITION BY toYYYYMM(created_at)
ORDER BY (session_id);

有以下数据

INSERT INTO test_sessions (session_id, session_name, created_at) VALUES
(1, 'start', '2021-01-31 00:00:00'),
(1, 'stop', '2021-01-31 01:00:00'),
(2, 'start', '2021-01-31 01:00:00')
;

创建了 2 个物化视图来关闭会话

CREATE MATERIALIZED VIEW IF NOT EXISTS test_session_aggregate_states
(
    session_id UInt64,
    started_at AggregateFunction(minIf, DateTime, UInt8),
    stopped_at AggregateFunction(maxIf, DateTime, UInt8)
)
ENGINE = AggregatingMergeTree
PARTITION BY tuple()
ORDER BY (session_id)
POPULATE AS
SELECT session_id,
       minIfState(created_at, session_name = 'start') AS started_at,
       maxIfState(created_at, session_name = 'stop')  AS stopped_at
FROM test_sessions
GROUP BY session_id;

CREATE VIEW IF NOT EXISTS test_session_completed
(
    session_id UInt64,
    started_at DateTime,
    stopped_at DateTime
)
AS
SELECT session_id,
       minIfMerge(started_at) AS started_at,
       maxIfMerge(stopped_at) AS stopped_at
FROM test_session_aggregate_states
GROUP BY session_id
HAVING (started_at != '0000-00-00 00:00:00') AND
       (stopped_at != '0000-00-00 00:00:00')
;

正常工作：返回 1 行已存在的“开始”和“停止”

SELECT * FROM test_session_completed;
-- 1,2021-01-31 00:00:00,2021-01-31 01:00:00

尝试创建基于 test_session_completed 的物化视图并连接到其他表（示例中没有连接）

CREATE MATERIALIZED VIEW IF NOT EXISTS test_mv
(
    session_id UInt64
)
ENGINE = MergeTree
PARTITION BY tuple()
ORDER BY (session_id)
POPULATE AS
SELECT session_id
FROM test_session_completed
;

编写测试查询来测试 test_mv

INSERT INTO test_sessions (session_id, session_name, created_at) VALUES
(3, 'start', '2021-01-31 02:00:00'),
(3, 'stop', '2021-01-31 03:00:00');

SELECT * FROM test_session_completed;
-- SUCCESS
-- 3,2021-01-31 02:00:00,2021-01-31 03:00:00
-- 1,2021-01-31 00:00:00,2021-01-31 01:00:00

SELECT * FROM test_mv;
-- FAILURE
-- 1
-- EXPECTED RESULT
-- 3
-- 1

如何根据test_mv填写test_session_completed？

ClickHouse 版本：20.11.4.13

Answer 1

无法在视图中创建 MV。
MV 是一个插入触发器，在同一个表中没有状态 completed 的情况下不可能获得状态 started。如果您不需要检查 started 发生在 completed 之前，那么您可以制作更简单的 MV，只需检查 where completed。
您不需要minIfState，您可以使用min (SimpleAggregateFunction)。它将减少存储的数据并提高性能。
我觉得第二个MV太过分了。

检查这个： https://den-crane.github.io/Everything_you_should_know_about_materialized_views_commented.pdf

https://youtu.be/ckChUkC3Pns?list=PLO3lfQbpDVI-hyw4MyqxEk3rDHw95SzxJ&t=9371

我会这样做：

CREATE TABLE IF NOT EXISTS test_sessions
(
    session_id   UInt64,
    session_name String,
    created_at   DateTime
)
ENGINE = MergeTree()
PARTITION BY toYYYYMM(created_at)
ORDER BY (session_id);


CREATE MATERIALIZED VIEW IF NOT EXISTS test_session_aggregate_states
(
    session_id UInt64,
    started_at SimpleAggregateFunction(min, DateTime),
    stopped_at SimpleAggregateFunction(max, DateTime)
)
ENGINE = AggregatingMergeTree
PARTITION BY tuple()
ORDER BY (session_id)
POPULATE AS
SELECT session_id,
       minIf(created_at, session_name = 'start') AS started_at,
       maxIf(created_at, session_name = 'stop')  AS stopped_at
FROM test_sessions
GROUP BY session_id;

INSERT INTO test_sessions (session_id, session_name, created_at) VALUES
(3, 'start', '2021-01-31 02:00:00'),
(3, 'stop', '2021-01-31 03:00:00');


completed sessions:

SELECT session_id,
       min(started_at) AS started_at,
       max(stopped_at) AS stopped_at
FROM test_session_aggregate_states
GROUP BY session_id
HAVING (started_at != '0000-00-00 00:00:00') AND
       (stopped_at != '0000-00-00 00:00:00');

┌─session_id─┬──────────started_at─┬──────────stopped_at─┐
│          1 │ 2021-01-31 00:00:00 │ 2021-01-31 01:00:00 │
└────────────┴─────────────────────┴─────────────────────┘

并且使用 argMaxState 您可以在一个 session_id 内聚合多个 start stop

基于聚合物化视图创建物化视图

1 个答案: