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 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'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, 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.