Archive

Archive for September, 2010

Play sqldf with RPostgreSQL

September 28, 2010 3 comments

This post is obsolete, the content in this post is not correct, please see Gabor’s comment.

 

In the previous post Manipulate data in R via SQL interface, I introduced how to use RpgSQL as DB driver to interface sqldf and PostgreSQL.

But there are some problems when using RpgSQL.

> options(sqldf.driver = “pgSQL”)
> sqldf(“select Species from iris where Species like ‘set%’ limit 3″)
Error in .verify.JDBC.result(r, “Unable to retrieve JDBC result set for “, :
Unable to retrieve JDBC result set for select Species from iris where Species like ‘set%’ limit 3 (ERROR: column “species” does not exist
Position: 8)
>

And RPostgreSQL works fine

> options(sqldf.driver = “PostgreSQL”)
> sqldf(“select Species from iris limit 3″)
Species
1 setosa
2 setosa
3 setosa
> sqldf(“select Species from iris where Species like ‘set%’ limit 3″)
Species
1 setosa
2 setosa
3 setosa

If you have trouble in installing RPostgreSQL under Mac OSX, please refer to Install R Package RPostgreSQL in Mac OSX.

Manipulate data in R via SQL interface

September 28, 2010 2 comments

I has been looking for a R package providing SQL-like interface for data manipulation. Though, there are already usable DBI/RDBI package we can use to talk to SQL interfacing database already, they are still not the convenient tools for manipulate data outside the database, like in-memory frames.

After several rounds of googling, I finally find sqldf package, an seamless interface between R’s frame objects and SQL flavors, provided by specific database back-ends, like SQLite (via RSQLite) and PostgreSQL (via RpgSQL).

Personally, I prefer PostgreSQL over RSQLite, because of the SQL language features. So I tried my best to make sqldf work on my Mac.

Installation


install.packages('RpgSQL')
install.packages('sqldf')

By looking at the code of sqldf package, it tries to use ‘RpgSQL’, ‘RMySQL’, ‘RH2′ and ‘RSQLite’ in order, so if you have RpgSQL installed, it will connect the ‘test’ database of your local postgreSQL instance by default.

Use PostgreSQL as Database Back-end


library(RpgSQL)

sqldf can detect the loaded RpgSQL packge and use it. If you have no other DBI package loaded, sqldf will use RSQLite as a fallback.

Configure RpgSQL on Mac

Configuration to make RpgSQL work might be not needed for other platforms, but in my Mac box with Mac OSX 10.5 it is needed.

1. Make sure postgresql-jdbc port is installed, or

sudo port instal postgresql-jdbc

2. Change jar file name

Without changing the jar file name, RpgSQL is unable to locate the jar file automatically.


cd /opt/local/share/java/
sudo ln -s postgresql.jar postgresql.jdbc4.jar

3. Set R options

Tell R where to find the jar files.


options(RpgSQL_JAR='/opt/local/share/java/')

4. Create ‘test’ database in your local PostgreSQL, or you can set R option RpgSQL.dbname, or provide dbname paramter each time calling sqldf

5. Everything is ready, run to test

sqldf('select * from iris limit 5')

Change sqldf driver


options(sqldf.driver = "")

I tested following drivers with simple select * from iris limit 5 SQL.

Other options

1. To avoid loading tcl/tk

options(gsubfn.engine = "R")

Categories: PostgreSQL, R, Statistics Tags: , , , ,

Useful Python Resources for Setting Up Django Website

September 9, 2010 Leave a comment

Recently, I am developing a website and need practical solution to setting up a real-life production web serving environment.

I googled and found following links that are useful for my final decision.

Application Servers: Benchmark of Python Web Servers
Cache Mechanisms: Evaluating Django Caching Options

Other resources can be found also from the links contained in above articles.

Yet, I haven’t decide which app server and cache will be used, but obviously, nginx(uwsgi) + gevent powered app server + staticgenerator or django built-in cache middleware or varnish would be the most possible choose.

Categories: Python Tags: , ,

Turning PostgreSQL for Development Purpose

September 8, 2010 Leave a comment

I installed a PostgreSQL 9.0 port in my puny Mac Pro, and I don’t want it to be a burden for the limited 2GB memory, so I modified following parameters in the postgresql.conf file:


shared_buffers = 2MB
max_connections = 10

Actually, I don’t need much concurrent connections in a development environment.

After the tuning, the virtual memory of each postgres process reduced to 31MB, much smaller than original 57MB.

Categories: PostgreSQL Tags:

Install Postgresql in Mac OSX

September 8, 2010 1 comment

There are at least four different ways to install PostgreSQL in your Mac.

Method 1: Install from source code
This is the most boring way to install PostgreSQL, you need to deal with all the things of resolving dependencies and compilation.

Method 2: Install via binary package provided by EnterpriseDB
This is the most quick way to make your PostgreSQL instance ready to serve. But it has constraints. The binary package is a self-contained package contains all dependencies and tools, so it will not live with other packages installed via MacPorts or Fink in your system. If you want to install client packages like py26-psycopg2, you will have to install another copy of PostgreSQL via MacPorts or Fink too for the dependence need.

Besides, if you want to enable PL/Python, there is no convenient way to do this.

Method 3: Install via Fink
Fink will resolve all the dependence problems for you and get your PostgreSQL installed at prefix /sw

Method 4: Install via MacPorts

port install postgresql90-server

And follow the instructions displayed in the screen.

As there are several versions of PostgreSQL at MacPorts, you need to decide which one to install. I prefer the new features of pg90. After installation, the binary directory of PostgreSQL is not at the standard paths, like /opt/local/bin, so I put it in /etc/paths.d/pgsql:

/opt/local/lib/postgresql90/bin

Or you will not have tools like pg_dump available in hand.

Categories: Mac Impression, PostgreSQL Tags:

Install R Package RPostgreSQL in Mac OSX

September 7, 2010 17 comments

I am trying to use PostgreSQL for data warehousing data and R for interactive data exploration. So the first priority to make this happen is to connect PostgreSQL from R and read data back via customized SQL.

As discussed at stackoverflow, the RPostgreSQL seems to be the best choice for its maturity and standard compliance. But there is no official binary for Mac OSX, so I have to do it myself to make it work.

First of all, my PostgreSQL is installed via MacPorts. I am using the 8.4 version. And I am using the binary version of R downloaded from its homepage. I prefer the binary version over the MacPorts version because the binary version has a native GUI, and doesn’t depend on X11. The problem of installing RPostgreSQL comes along with the different sources between PostgreSQL and R.

I downloaded the RPostgreSQL source code from the project page and issue:

install.packages(c("/Users/<you>/Downloads/RPostgreSQL_0.1-6.tar.gz"), type="source"))

The configure script fails to find the libpq-fe.h file. That is because my PostgreSQL is installed at /opt/local/ prefix, which is not set to the default location for searching include and lib files at RPostgreSQL’s configure script.

Solution for the installation is simple. After reading the RPostgreSQL’s configure script. I found that I can use pg_config to let the configure script discover the installation of RPostgreSQL automatically. But there is still a problem that the pg_config doesn’t resides at ‘/opt/local/lib/postgresql84/bin’ which is not in the standard executable search paths. So, I made a temporary link of the pg_config, like

sudo ln -s /opt/local/lib/postgresql84/bin/pg_config /usr/local/bin/

Then re-issuing the above install command succeeded.

In sum,

  • You need to install RPostgreSQL package from source due to lack of precompiled binary in Mac OSX
  • If your R and PostgreSQL are installed from difference Sources (like me, one from binary, the other via MacPorts) or R prompts that libpq-fe.h cannot be found, check your pg_config whether is in the executable search paths.
  • If your pg_config cannot be searched, make it happen by simply making a soft link (my method) or put the path of pg_config into the PATH environment.
  • Issue the ordinary install command in R environment.

Finally, to test whether the installation succeeded, type

library(RPostgreSQL)

Categories: PostgreSQL, R, Statistics Tags: , ,
Follow

Get every new post delivered to your Inbox.