merges two record sets by looping through every record in the first set and trying to find a match in the second set. All matching records are returned. 遍历模式 索引模式 5、聚合 - Aggregate group by 操作 groups records t
A hash function is applied to the join column and the result dictates which consumer parallel server process should receive the row. BROADCAST: Broadcast redistribution happens when one of the two result sets in a join operation is much smaller than the other result set. Instead of re...
Parquet文件是以二进制方式存储的,不能直接读取的,文件中包括实际数据和元数据,Parquet格式文件是自解析的。create table if not exists uaction_parquet( userid string, itemid string, behaviortype int, geohash string, itemcategory string, time string) stored as parquet; insert overwrite table uaction_pa...
ClickHouse是俄罗斯的Yandex于2016年开源的列式存储数据库(DBMS),使用C++语言编写,主要用于在线分析处理查询(OLAP),能够使用SQL查询实时生成分析数据报告。 OLAP(On-Line Analytical Processing)翻译为联机分析处理,专注于分析处理,从对数据库操作来看,OLAP是对数据的查询; OLTP(on-line transaction processing)翻译为联机...
14 KEY `index_user_id` (`user_id`) USING HASH 15) ENGINE=MEMORY DEFAULT CHARSET=utf8mb4; 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. (3)创建普通表 创建普通表,参数设置和内存表相同,否则从内存表往普通标导入数据会报错。
REPLICATE –Copies the row data to all workers. Distributed Exchange types The following exchange types indicate the layout of the data when they are exchanged between nodes in a distributed plan. HASH –The exchange distributes data to multiple destinations using a hash function. SINGLE –The exc...
Groups here are defined by the join variables used to build the hash index (See DFEHashIndexBuild for where this hash index might be built.) DFEHashIndexBuild – Builds a hash index over a set of variables as a side-effect. This hash index is typically reused in later operations. (See...
* a preliminary copy of the source querytree. This prevents problems in * the case that the EXPLAIN is in a portal or plpgsql function and is * executed repeatedly. (See also the same hack in DECLARE CURSOR and * PREPARE.) XXX FIXME someday. */ rewritten = QueryRewrite(castNode(Query...
The relative cost of the plan nodes is based on wall time, which might or might not be correlated to CPU time. For each plan node you can see some additional statistics (For example, average input per node instance, average number of hash collisions for relevant plan nodes). Such ...
In computing, a hash table (hash map) is a data structure used to implement an associative array, a structure that can map keys to values. A hash table uses a hash function to compute anindexinto an array ofbucketsorslots, from which the desired value can be found. ...