Shantanu's Blog

Corporate Consultant

September 12, 2015

 

Redshift support of User Defined functions

The following example creates a function that compares two numbers and returns the larger value. Note that the indentation of the code between the double dollar signs ($$) is a Python requirement.

create function f_greater(a float, b float)
returns float
stable
as $$
  if a > b:
    return a
  return b
$$ language plpythonu;

The following query calls the new f_greater function to query the SALES table and return either COMMISSION or 20 percent of PRICEPAID, whichever is greater:
select f_greater(commission, pricepaid*0.20) from sales;

# Z-score calculation

create function f_z_test_by_pval (alpha float, x_bar float, test_val float, sigma float, n float)
RETURNS varchar
STABLE
AS $$

 import scipy.stats as st
 import math as math

 z = (x_bar - test_val) / (sigma / math.sqrt(n))

 p = st.norm.cdf(z)

 if p <= alpha:
   return 'Statistically significant'

 else:
   return 'May have occurred by random chance'

$$LANGUAGE plpythonu;

# string function

create function
f_return_focused_specialty(full_physician_specialty varchar)
RETURNS varchar
STABLE
AS $$

    parts = full_physician_specialty.split("/")
    return parts[len(parts) - 1].strip()

$$LANGUAGE plpythonu

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September 04, 2015

 

pandas and SQL queries

pysqldf is a useful utility that will allow to run SQL query against pandas dataframe. What it returns is also a dataframe that can be further processed using usual methods.

pip install pandasql

from pandasql import sqldf, load_meat, load_births
pysqldf = lambda q: sqldf(q, globals())
meat = load_meat()
births = load_births()
print pysqldf("SELECT * FROM meat LIMIT 10;").head()

q = """SELECT
        m.date, m.beef, b.births
     FROM
        meat m
     INNER JOIN
        births b
           ON m.date = b.date;"""

joined = pysqldf(q)
print type(joined), type(meat)

_____

If you get an error something like this then you need to edit the source code.

ProgrammingError: You must not use 8-bit bytestrings unless you use a text_factory that can interpret 8-bit bytestrings (like text_factory = str). It is highly recommended that you instead just switch your application to Unicode strings.

vi ~/anaconda/lib/python2.7/site-packages/pandasql/sqldf.py

Add one more line after creating the connection to sqlite.

    conn = sqlite.connect(dbname, detect_types=sqlite.PARSE_DECLTYPES|sqlite.PARSE_COLNAMES)
    conn.text_factory = str

This will resolve that problem.

 

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September 01, 2015

 

Adding a python package to pypi

There are times when you can simply add any python module just by a single command like this...

pip install seed

I have a package that I want to make available using pip. I need the module called "seed" that needs to be installed as shown above.

1) Create a new directory:

mkdir /easyboto
cd /easyboto

2) create files required for a package

seed create

3) Add your python scripts / files to bin/ folder and update setup.py file.

4) release the package

seed release

5) create a file where you can store your username and password

# cat ~/.pypirc
[distutils]
index-servers=pypi

[pypi]
repository = https://pypi.python.org/pypi
username:shantanu.oak
password:Inxxxxxx

6) Run the setup to upload your file to pypi

python setup.py upload

python setup.py sdist

python setup.py bdist_wheel --universal

python setup.py sdist bdist_wheel upload

That's it!

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