Can We Replace Right Join with Left Join

Why is right join required when we have left join | use of right join | why use right join instead of left join | is right outer join ever required | difference between left outer join and right outer join in sql | do we really need both left join and right join | Left vs right sql join

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Apache Spark’s Join Algorithms

One of the most frequently used transformations in Apache Spark is Join operation. Joins in Apache Spark allow the developer to combine two or more data frames based on certain (sortable) keys. The syntax for writing a join operation is simple but some times what goes on behind the curtain is lost. Internally, for Joins Apache Spark proposes a couple of Algorithms and then chooses one of them. Not knowing what these internal algorithms are, and which one does spark choose might make a simple Join operation expensive.

While opting for a Join Algorithm, Spark looks at the size of the data frames involved. It considers the Join type and condition specified, and hint (if any) to finally decide upon the algorithm to use. In most of the cases, Sort Merge join and Shuffle Hash join are the two major power horses that drive the Spark SQL joins. But if spark finds the size of one of the data frames less than a certain threshold, Spark puts up Broadcast Join as it’s top contender.

Broadcast Hash Join

Looking at the Physical plan of a Join operation, a Broadcast Hash Join in Spark looks like this

The above plan shows that the data frame from one of the branches broadcasts to every node containing the other data frame. In each node, Spark then performs the final Join operation. This is Spark’s per-node communication strategy.

Spark uses the Broadcast Hash Join when one of the data frame’s size is less than the threshold set in spark.sql.autoBroadcastJoinThreshold. It’s default value is 10 Mb, but can be changed using the following code

spark.conf.set("spark.sql.autoBroadcastJoinThreshold", 100 * 1024 * 1024)

This algorithm has the advantage that the other side of the join doesn’t require any shuffle. If this other side is very large, not doing the shuffle will bring notable speed-up as compared to other algorithms that would have to do the shuffle.

Broadcasting large datasets can also lead to timeout errors. A configuration spark.sql.broadcastTimeout sets the maximum time that a broadcast operation should take, past which the operation fails. The default timeout value is 5 minutes, but it can be set as follows:

spark.conf.set("spark.sql.broadcastTimeout", time_in_sec)

Sort Merge Join

If neither of the data frames can be broadcasted, then Spark resorts to Sort Merge Join. This algorithm uses the node-node communication strategy, where Spark shuffles the data across the cluster.

Sort Merge Join requires both sides of the join to have correct partitioning and order. Generally, this is ensured by** shuffle and sort** in both branches of the join as depicted below

#apache spark #scala #tech blogs #broadcast join #join opertaions #join optimization #joins in spark #shuffled hash join #sort merge join

Left Outer Join in SQL Example | SQL Left Join Keyword

SQL Left Outer Join returns all records from the left table (table1) and the matched records from the right table (table2). SQL LEFT JOIN clause allows us to query data from the multiple tables. It returns all the rows from a left table and the matching rows from the right table. This means that the left join returns all the values from a left table, plus matched values from a right table or NULL in the case of no matching join predicate.

In some SQL databased LEFT OUTER JOIN is also called as LEFT JOIN.

Left Outer Join in SQL

The syntax of SQL Left Join is the following.

SELECT column_name(s)
FROM table1
LEFT JOIN table2
ON table1.column_name = table2.column_name;

In this syntax, table1 and table2 are the left and right tables, respectively.

For each row from the table1 table, the query compares it with all the rows from the table2 table. If the pair of rows causes the join predicate to evaluate to true, the column values from these rows will be in the combined form to the new row, which then included in the final result set.

#sql #left join #sql left join

Wlapi: Ruby Based API for The Project Wortschatz Leipzig

WLAPI

DESCRIPTION

WLAPI is a programmatic API for web services provided by the project Wortschatz, University of Leipzig. These services are a great source of linguistic knowledge for morphological, syntactic and semantic analysis of German both for traditional and Computational Linguistics (CL).

Use this API to gain data on word frequencies, left and right neighbours, collocations and semantic similarity. Check it out if you are interested in Natural Language Processing (NLP) and Human Language Technology (HLT).

This library is a set of Ruby bindings for the following featuren. You may also be interested in other language specific bindings:

Perl

Python

PHP

C#

The original Java based clients with many examples can be found on the project overview page.

Implemented Features

You can use the following search methods:

#baseform;

#cooccurrences;

#cooccurrences_all;

#crossword;

#domain;

#experimental_synonyms;

#frequencies;

#left_collocation_finder;

#left_neighbours;

#right_collocation_finder;

#right_neighbours;

#sentences;

#similarity;

#synonyms;

#thesaurus;

#wordforms;

#ngrams;

#ngram_references;

The services NGrams and NGramReferences are under development and will be available soon. Both methods throw an NotImplementedError for now.

The interface will be slightly changed in the version 1.0 to be more readable. For example, #cooccurrences_all may become #all_cooccurrences.

There are two additional services by Wortschatz Leipzig: MARS and Kookurrenzschnitt. They will not be implemented due to internal restrictions of the service provider.

INSTALLATION

WLAPI is provided as a .gem package. Simply install it via RubyGems.

To install WLAPI ussue the following command:

$ gem install wlapi

The current version of WLAPI works with the second Savon generation. You might want to install versions prior to 0.8.0, if you are bound on the old implementations of savon:

$ gem install wlapi -v 0.7.4

If you want to do a system wide installation, do this as root (possibly using sudo).

Alternatively use your Gemfile for dependency management.

We are working on a .deb package, which will be released soon.

SYNOPSIS

Basic usage is very simple:

$ require 'wlapi'
$ api = WLAPI::API.new
$ api.synonyms('Haus', 15) # returns an array with string values (UTF8 encoded)
$ api.domain('Auto') # => Array

If you are going to send mass requests, please contact the support team of the project Wortschatz, get your private credentials and instantiate an authenticated client:

$ require 'wlapi'
$ api = WLAPI::API.new(username, password)

See documentation in the WLAPI::API class for details on particular search methods.

EXCEPTION HIERARCHY

While using WLAPI you can face following errors:

WLAPI::UserError;

WLAPI::ExternalError.

The errors here are presented in the order they may occur during WLAPI's work.

First WLAPI checks the user input and throws a WLAPI::UserError if the arguments are not appropriate.

Then it fetches a response from a remote server, it can result in a WLAPI::ExternalError. In most cases it will be a simple wrapper around other errors, e.g. Savon::SOAP::Fault.

All of them are subcalsses of WLAPI::Error which is in turn a subclass of the standard RuntimeError.

If you want to intercept any and every exception thrown by WLAPI simply rescue WLAPI::Error.

SUPPORT

If you have question, bug reports or any suggestions, please drop me an email :) Any help is deeply appreciated!

If you need some new functionality please contact me or provide a pull request. You code should be complete and tested. Please use local_* and remote_* naming convention for your tests.

Supported Ruby Versions

The library is testend on the following Ruby interpreters:

MRI 1.8.7

MRI 1.9.3

MRI 2.0.x

MRI 2.1.x

JRuby (both 1.8 and 1.9 modes)

Rubinius

CHANGELOG

For details on future plan and working progress see CHANGELOG.

CAUTION

This library is work in process! Though the interface is mostly complete, you might face some not implemented features.

Please contact me with your suggestions, bug reports and feature requests.

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DISCLAIMER We are working on the new RESTful client. Please be patient!

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Author: Arbox
Source Code: https://github.com/arbox/wlapi 
License: MIT license

#ruby #nlp #naturallanguageprocessing 

SQL Right Join Example | Right Outer Join In SQL

SQL Right join returns all records from the right table (table2), and the matched records from the left table (table1). The result is NULL from the left side when there is no match. RIGHT JOIN performs a join starting with the second (right-most) table and then any matching first (left-most) table records.

RIGHT JOIN and RIGHT OUTER JOIN are the same.

SQL Right Join Tutorial With Example

SQL RIGHT JOIN keyword returns all the records from the right table (table2), and the matched records from a left table (table1). The result is NULL from the left side when there is no match. In some databases, the RIGHT JOIN is called RIGHT OUTER JOIN. The RIGHT JOIN clause allows us to  query data from the multiple tables.

#sql #sql right #right join

Your Ultimate Guide to SQL Join: CROSS JOIN

CROSS JOIN is in the spotlight. This article finishes our small series of SQL JOIN-related publications.

SQL Server CROSS JOIN is the simplest of all joins. It implements a combination of 2 tables without a join condition. If you have 5 rows in one table and 3 rows in another, you get 15 combinations. Another definition is a Cartesian Product.

Now, why would you want to combine tables without a join condition? Hang on a bit because we are getting there. First, let’s refer to the syntax.

#sql server #cross join #inner join #outer join #sql join #sql