房地产门户网站建设,网站建设合同需注意什么,株洲信息网,个人做网站法律风险ClickHouse 的 SQL 优化规则是基于RBO(Rule Based Optimization)#xff0c;下面是一些优化规则 1 准备测试用表 1#xff09;上传官方的数据集 将visits_v1.tar和hits_v1.tar上传到虚拟机#xff0c;解压到clickhouse数据路径下 // 解压到clickhouse数据路径 sudo tar -xvf… ClickHouse 的 SQL 优化规则是基于RBO(Rule Based Optimization)下面是一些优化规则 1 准备测试用表 1上传官方的数据集 将visits_v1.tar和hits_v1.tar上传到虚拟机解压到clickhouse数据路径下 // 解压到clickhouse数据路径 sudo tar -xvf hits_v1.tar -C /var/lib/clickhouse
sudo tar -xvf visits_v1.tar -C /var/lib/clickhouse//修改所属用户
sudo chown -R clickhouse:clickhouse /var/lib/clickhouse/data/datasets
sudo chown -R clickhouse:clickhouse /var/lib/clickhouse/metadata/datasets2重启clickhouse-server sudo clickhouse restart 3执行查询 clickhouse-client --query SELECT COUNT(*) FROM datasets.hits_v1 clickhouse-client --query SELECT COUNT(*) FROM datasets.visits_v1 注意官方的tar包包含了建库、建表语句、数据内容这种方式不需要手动建库、建表最方便。 hits_v1表有130多个字段880多万条数据 visits_v1表有180多个字段160多万条数据 2 COUNT 优化 在调用 count 函数时如果使用的是 count() 或者 count(*)且没有 where 条件则会直接使用 system.tables 的 total_rows例如: EXPLAIN SELECT count()FROM datasets.hits_v1;UnionExpression (Projection)Expression (Before ORDER BY and SELECT)MergingAggregatedReadNothing (Optimized trivial count)注意 Optimized trivial count 这是对 count 的优化。 如果 count 具体的列字段则不会使用此项优化 EXPLAIN SELECT count(CounterID) FROM datasets.hits_v1;UnionExpression (Projection)Expression (Before ORDER BY and SELECT)AggregatingExpression (Before GROUP BY)ReadFromStorage (Read from MergeTree)3 消除子查询重复字段 下面语句子查询中有两个重复的 id 字段会被去重: EXPLAIN SYNTAX SELECT a.UserID,b.VisitID,a.URL,b.UserIDFROMhits_v1 AS a LEFT JOIN ( SELECT UserID, UserID as HaHa, VisitID FROM visits_v1) AS b USING (UserID)limit 3;//返回优化语句
SELECT UserID,VisitID,URL,b.UserID
FROM hits_v1 AS a
ALL LEFT JOIN
(SELECT UserID,VisitIDFROM visits_v1
) AS b USING (UserID)
LIMIT 34 谓词下推 当group by有having子句但是没有with cube、with rollup 或者with totals修饰的时候having过滤会下推到where提前过滤。例如下面的查询HAVING name变成了WHERE name在group by之前过滤 EXPLAIN SYNTAX SELECT UserID FROM hits_v1 GROUP BY UserID HAVING UserID 8585742290196126178;//返回优化语句
SELECT UserID
FROM hits_v1
WHERE UserID \8585742290196126178\
GROUP BY UserID子查询也支持谓词下推 EXPLAIN SYNTAX
SELECT *
FROM
(SELECT UserIDFROM visits_v1
)
WHERE UserID 8585742290196126178//返回优化后的语句
SELECT UserID
FROM
(SELECT UserIDFROM visits_v1WHERE UserID \8585742290196126178\
)
WHERE UserID \8585742290196126178\ 再来一个复杂例子 //返回优化后的语句
SELECT UserID
FROM
(SELECT UserIDFROM (SELECT UserIDFROM visits_v1WHERE UserID \8585742290196126178\)WHERE UserID \8585742290196126178\UNION ALLSELECT UserIDFROM (SELECT UserIDFROM visits_v1WHERE UserID \8585742290196126178\)WHERE UserID \8585742290196126178\
)
WHERE UserID \8585742290196126178\5 聚合计算外推 聚合函数内的计算会外推例如 EXPLAIN SYNTAX
SELECT sum(UserID * 2)
FROM visits_v1//返回优化后的语句
SELECT sum(UserID) * 2
FROM visits_v16 聚合函数消除 如果对聚合键也就是 group by key 使用 min、max、any 聚合函数则将函数消除例如 EXPLAIN SYNTAX
SELECTsum(UserID * 2),max(VisitID),max(UserID)
FROM visits_v1
GROUP BY UserID//返回优化后的语句
SELECT sum(UserID) * 2,max(VisitID),UserID
FROM visits_v1
GROUP BY UserID7 删除重复的 order by key 例如下面的语句重复的聚合键 id 字段会被去重: EXPLAIN SYNTAX
SELECT *
FROM visits_v1
ORDER BYUserID ASC,UserID ASC,VisitID ASC,
VisitID ASC//返回优化后的语句
select……
FROM visits_v1
ORDER BY UserID ASC,
VisitID ASC8 删除重复的 limit by key 例如下面的语句重复声明的 name 字段会被去重 EXPLAIN SYNTAX
SELECT *
FROM visits_v1
LIMIT 3 BYVisitID,VisitID
LIMIT 10//返回优化后的语句
select……
FROM visits_v1
LIMIT 3 BY VisitID
LIMIT 109 删除重复的 USING Key 例如下面的语句重复的关联键 id 字段会被去重 EXPLAIN SYNTAX
SELECTa.UserID,a.UserID,b.VisitID,a.URL,b.UserID
FROM hits_v1 AS a
LEFT JOIN visits_v1 AS b USING (UserID, UserID)//返回优化后的语句
SELECT UserID,UserID,VisitID,URL,b.UserID
FROM hits_v1 AS a
ALL LEFT JOIN visits_v1 AS b USING (UserID)10 标量替换 如果子查询只返回一行数据在被引用的时候用标量替换例如下面语句中的 total_disk_usage 字段 EXPLAIN SYNTAX
WITH (SELECT sum(bytes)FROM system.partsWHERE active) AS total_disk_usage
SELECT(sum(bytes) / total_disk_usage) * 100 AS table_disk_usage,table
FROM system.parts
GROUP BY table
ORDER BY table_disk_usage DESC
LIMIT 10;//返回优化后的语句
WITH CAST(0, \UInt64\) AS total_disk_usage
SELECT (sum(bytes) / total_disk_usage) * 100 AS table_disk_usage,table
FROM system.parts
GROUP BY table
ORDER BY table_disk_usage DESC
LIMIT 1011 三元运算优化 如果开启了 optimize_if_chain_to_multiif 参数三元运算符会被替换成 multiIf 函数例如 EXPLAIN SYNTAX
SELECT number 1 ? hello : (number 2 ? world : atguigu)
FROM numbers(10)
settings optimize_if_chain_to_multiif 1;// 返回优化后的语句
SELECT multiIf(number 1, \hello\, number 2, \world\, \atguigu\)
FROM numbers(10)
SETTINGS optimize_if_chain_to_multiif 1
