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# PostgreSQL Histogram (for ActiveRecord)

This gem allows for you to efficiently create a histogram from large data sets in your Rails applications.

It uses PostgreSQL's [width_bucket](http://www.postgresql.org/docs/9.3/static/functions-math.html) function to handle the majority of the processing in the database, and only requires 3 database queries (and only one query if min and max values are specified).

## Installation

Add this line to your application's Gemfile:

gem 'pg_histogram'

And then execute:

$ bundle

Or install it yourself as:

$ gem install pg_histogram

## Usage

Create a Histogram object using the following parameters:

1. ActiveRecord Relation (query) to use.
2. Name of column to count frequency of. Also allows for aliased queries such as `'price*discount as final_price'` to create histograms on expressions.
3. Options hash (optional). Not all combinations are allowed. For example, if `:buckets` is specified, `:min` and `:max` are required and `:bucket_size` is ignored, and calculated. If `:buckets` is not specified, the number of buckets depends on `:bucket_size`, and `:min` and `:max` are optional.
    - `:buckets`: number of buckets (integer)
    - `:min` and `:max`: See [width_bucket](http://www.postgresql.org/docs/9.3/static/functions-math.html)'s docs for exact meaning (defaults to the min and max values of the column).
    - `:bucket_size`: Width of each bucket (defaults to 1).

### Example
Create sample data:

5.times do { Widget.create(price: 1.2) }
    10.times do { Widget.create(price: 2.9 ) }

Create the histogram object:
    
    histogram = PgHistogram::Histogram.new(Widget.all, 'price', 0.5)

Call the results method to retrieve a Hash of bucket minimums and frequency counts:

@histogram_data = histogram.results
     => {1.0=>5, 2.5=>10}

The results can be used by your favorite charting libary, such as [Chartkick](https://github.com/ankane/chartkick), to plot the data:

<%= column_chart @histogram_data %>

## Dependencies

This gem has been tested with Ruby 2.1.3 and ActiveRecord 4.1.6. Please open an issue or PR if you experience issues with other versions.

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