Sql Left Join Performance

We want to do not only write SQL, but also to do a good performance of the SQL, the following for the author to learn, extract, and summarized part of the information to share with you! (1) Select the most efficient table name order (valid only in the Rule-based optimizer): The ORACLE parser processes the table names in the FROM clause in Right-to-left order, and the last table in the FROM clause (the underlying table driving tables) is processed first, In the case where multiple tables are included in the FROM clause, you must select the table with the least number of records as the underlying table. If...

Hive in the official document of the query language has a very detailed description, please refer to: http://wiki.apache.org/hadoop/Hive/LanguageManual, most of the content of this article is translated from this page, Some of the things that need to be noted during the use process are added. Create tablecreate [EXTERNAL] TABLE [IF not EXISTS] table_name [col_name data_t ...

According to sort Benchmark's latest news, Databricks's spark tritonsort two systems at the University of California, San Diego, 2014 in the Daytona graysort tied sorting contest. Among them, Tritonsort is a multi-year academic project, using 186 EC2 i2.8xlarge nodes in 1378 seconds to complete the sorting of 100TB data, while Spark is a production environment general-purpose large-scale iterative computing tool, it uses 207 ...

Hive is the most widely used SQL on Hadoop tool, and recently many major data companies have introduced new SQL tools such as Impala,tez,spark, based on column or memory hot data, although many people have a view of hive, inefficient, slow query, and many bugs. But Hive is still the most widely used and ubiquitous SQL on Hadoop tool. Taobao survey analysis of the previous report, Taobao 90% of the business run in hive above. The ratio of the storm audio and video ...

1, use the index to traverse the table faster. The index created by default is a non-clustered index, but sometimes it is not optimal. Under non-clustered indexes, the data is physically stored on the data page. Reasonable index design should be based on the analysis and prediction of various inquiries. In general: a. There are a large number of duplicate values, and often range query (>, <,> =, <=) and order by, group by occurred columns, consider the establishment of cluster index; Column, and each column contains duplicate values ​​can be ...

1, use the index to traverse the table faster. The index created by default is a non-clustered index, but sometimes it is not optimal. Under non-clustered indexes, the data is physically stored on the data page. Reasonable index design should be based on the analysis and prediction of various inquiries. In general: a. A large number of duplicate values, and often range query (>, <,> =, <=) and order by, group by occurred column, consider the establishment of cluster index; b. Column, ...

Using hive, you can write complex MapReduce query logic efficiently and quickly. In some cases, however, the Hive Computing task can become very inefficient or even impossible to get results, because it is unfamiliar with data attributes or if the Hive optimization convention is not followed. A "good" hive program still needs to have a deep understanding of the hive operating mechanism. Some of the most familiar optimization conventions include the need to write large tables on the right side of the join, and try to use UDF instead of transfrom ... Like。 Here are 5 performance and logic ...

This article mainly introduces the MySQL slow query analysis method, some time ago, I have set a record in the MySQL database to query SQL statements slower than 1 second. Think of a few very set method, there are several parameters of the name of life and death memories can not afford, so re-sort it yourself as a note. For troubleshooting bottlenecks to find out the performance, the most easy to find and solve the problem is MySQL slow query and did not have to use the index query. OK, began to find out MySQL implementation is not "cool" SQL statement. ...

In the development of friends especially and MySQL have contact friends will encounter sometimes MySQL query is very slow, of course, I refer to the large amount of data millions level, not dozens of, the following we take a look at the resolution of the query slow way. -Often find developers looking up statements that are not indexed or limit n, which can have a significant impact on the database, such as a large table of tens of millions of records to scan all, or do filesort, and the database and server IO impact. This is the case above the mirrored library. And ...

From the 2008 60-man "Hadoop in China" technology salon, to the current thousands of-person scale of the industry technology feast, the seven-year BDTC (large data technology conference) has fully witnessed the transformation of China's large data technology and applications, faithfully depicting the large data field of technology hotspots, Precipitated countless valuable industry experience. At the same time, from December 2014 12 to 14th, the largest China data technology event will continue to lead the current field of technology hotspots, sharing the industry experience. In order to better understand the trend of industry development, understanding of enterprises ...