Materialized View的数据块触发时机答案

【问题标题】：Data block trigger timing of Materialized ViewMaterialized View的数据块触发时机
【发布时间】：2022-01-18 13:42:42
【问题描述】：

我们知道物化视图是由插入触发的。一次插入的大量数据会被分成多个block，每个block会触发一个MV select，根据this doc.

在收到块的所有行后会触发 MV 选择吗？还是取决于选择？说，如果选择尝试计算插入数据的行数，直到收到所有数据才会触发？如果select只是做数据复制/反向索引，收到block的一条记录就会触发？

【问题讨论】：

标签： clickhouse

【解决方案1】：

块插入主表后，Insert 触发 MatView。所以插入只是将指向行块（在内存中）的指针传递给 MatView。

-- by default clickhouse-client forms blocks = 1048545
-- clickhouse-client by itself parses input stream and inserts into 
-- server in Native format

select value from system.settings where name = 'max_insert_block_size';
1048545


-- here is a setup which traces sizes of blocks in rows
create table test(a Int, b String) engine=Null;
create materialized view test_mv 
Engine=MergeTree order by ts as 
select now() ts, count() rows from test;

-- create file with 100mil rows in TSV format
clickhouse-client -q "select number, 'x' from numbers(100000000) format TSV" >test.tsv

clickhouse-client -q "insert into test format TSV" <test.tsv

select max(rows), min(rows), count() from test_mv;
┌─max(rows)─┬─min(rows)─┬─count()─┐
│   1947598 │        32 │      65 │
└───────────┴───────────┴─────────┘

-- 1947598 -<- several blocks were squashed into a single block because 
-- of parallel parsing
-- 65 blocks were passed

truncate test_mv;



-- let's disable parallel parsing
clickhouse-client --input_format_parallel_parsing=0 -q "insert into test format TSV" <test.tsv

select max(rows), min(rows), count() from test_mv;
┌─max(rows)─┬─min(rows)─┬─count()─┐
│   1048545 │    388225 │      96 │
└───────────┴───────────┴─────────┘

-- got 1048545 = max_insert_block_size
-- 96 blocks were passed
-- 100000000 - 95 * 1048545 = 388225 
-- (95 blocks by max_insert_block_size and a remain 1 block = 388225 rows



truncate test_mv;



-- let's grow max_insert_block_size
clickhouse-client --max_insert_block_size=10000000000  --input_format_parallel_parsing=0 -q "insert into test format TSV" <test.tsv

select max(rows), min(rows), count() from test_mv;
┌─max(rows)─┬─min(rows)─┬─count()─┐
│ 100000000 │ 100000000 │       1 │
└───────────┴───────────┴─────────┘

-- 1 block == 100 mil rows

【讨论】：