• Spark operation aggregate, aggregate ByKey


    1. aggregate function The elements in each partition are aggregated, and then the results and initial values of each partition are combined with the combine function. The final type returned by this function does not need to be the same as the element type in RDD. The seqOp operation aggregates the elements in each partition, […]

  • MongoDB aggregate uses a personal summary


    Recently, I have been using mongodb. Sometimes I need to use statistics. I have checked some information on the Internet. The most suitable way to use aggregate is to use aggregate. Here is my experience in using it. MongoDB aggregationAggregate in MongoDB is mainly used to process data (such as statistical average, sum, etc.) and […]

  • Is the python collection a variable summary


    A set is an unordered variable sequence. The elements in a collection must be hash-able, that is, immutable data types. Empty set a=set() Note that a={} creates an empty dictionary. Set — Variable sets. Elements in a collection can be added or deleted dynamically. Frozenset — an immutable set. Elements in a collection are immutable. […]

  • Redis hash and set-power node Java college collation


    Redis hash values are a mapping between string fields and string values, so they are perfect data types representing objects in Redis and can store more than 40 billion key-value pairs. Example redis> HMSET yiibai name “redis tutorial” description “redis basic commands for caching” likes 20 visitors 23000 OK redis> HGETALL yiibai 1) […]

  • Collection Writing Guide in Ruby


    Literal representations of inclined arrays and hashes (unless you need to pass parameters into their constructors). # bad arr = Array.new hash = Hash.new # good arr = [] hash = {} When you need an array of elements with no spaces or special symbols, you always use% w to define an array of strings. […]

  • The Elegant Posture of Processing Sets in Java 8-Stream


    In Java, collections and arrays are the data structures that we often use. They need to be added, deleted, changed, checked, aggregated, counted, filtered and so on. By contrast, these operations are also available in relational databases, but before Java 8, the processing of collections and arrays was not very convenient. However, this problem has […]

  • A very useful Aggregate introductory tutorial in mongodb


    Preface Aggregate translates to aggregate, but the experience of using it in practice is very much like the pipeline in linux. After each pipeline is processed, the result is handed over to the next pipeline. Your data is like water flow. Finally, you can get the data you want through each pipeline. What do we […]