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PostgreSQLCursor for handling large Result Sets ↑

<img src=“https://badge.fury.io/rb/postgresql_cursor.svg” alt=“Gem Version” />

PostgreSQLCursor extends ActiveRecord to allow for efficient processing of queries returning a large number of rows.

Why use this? ↑

ActiveRecord is designed and optimized for web performance. In a web transaction, only a “page” of around 20 rows is returned to the user. When you do this

Model.find(:all, :conditions=>["id>0"]

The database returns all matching result set rows to ActiveRecord, which instantiates each row with the data returned. This function returns an array of all these rows to the caller.

Asyncronous, Background, or Offline processing may require processing a large amount of data. When there is a very large number of rows, this requires a lot more memory to hold the data. Ruby does not return that memory after processing the array, and the causes your process to “bloat”. If you don’t have enough memory, it will cause an exception.

Enter find_each ↑

To solve this problem, ActiveRecord gives us two alternative methods that work in “chunks” of your data:

Model.where("id>0").find_each { |model| model.process! }

Model.where("id>0").find_in_batches do |batch|
  batch.each { |model| model.process! }
end

Optionally, you can specify a :batch_size option as the size of the “chunk”, and defaults to 1000.

There are drawbacks with these methods:

  • You cannot specify the order, it will be ordered by the primary key (usually id)

  • The primary key must be numeric

  • The query is rerun for each chunk (1000 rows), starting at the next id sequence.

  • You cannot use overly complex queries as that will be rerun and incur more overhead.

PostgreSQLCursor FTW! ↑

PostgreSQLCursor was developed to take advantage of PostgreSQL’s cursors. Cursors allow the program to declare a cursor to run a given query returning “chunks” of rows to the application program while retaining the position of the full result set in the database. This overcomes all the disadvantages of using find_each and find_in_batches.

Also, with PostgreSQL, you have on option to have raw hashes of the row returned instead of the instantiated models. An informal benchmark showed that returning instances is a factor of 4 times slower than returning hashes. If you are can work with the data in this form, you will find better performance.

With PostgreSQL, you can work with cursors as follows:

Model.where("id>0").each_row { |hash| Model.process(hash) }

Model.where("id>0").each_instance { |model| model.process! }
Model.where("id>0").each_instance(block_size:100000) { |model| model.process! }

Model.each_row_by_sql("select * from models") { |hash| Model.process(hash) }

Model.each_instance_by_sql("select * from models") { |model| model.process }

All these methods take an options hash to control things more:

block_size:n      The number of rows to fetch from the database each time (default 1000)
while:value       Continue looping as long as the block returns this value
until:value       Continue looping until the block returns this value
connection:conn   Use this connection instead of the current model connection
fraction:float    A value to set for the cursor_tuple_fraction variable.
                  PostgreSQL uses 0.1 (optimize for 10% of result set)
                  This library uses 1.0 (Optimize for 100% of the result set)
                  Do not override this value unless you understand it.

Notes:

  • Use cursors only for large result sets. It has more overhead with the database than ActiveRecord selecting all matching records.

  • Aliases each_hash and each_hash_by_sql are provided ifor each_row and each_row_by_sql if you prefer to express what types are being returned.

Hashes vs. Instances ↑

The each_row method returns the Hash of strings for speed (as this allows you to process a lot of rows). Hashes are returned with String values, and you must take care of any type conversion.

When you use each_instance, ActiveRecord lazily casts these strings into Ruby types (Time, Fixnum, etc.) only when you read the attribute.

If you find you need the types cast for your attributes, consider using each_instance insead. ActiveRecord’s read casting algorithm will only cast the values you need and has become more efficient over time.

Authors ↑

Allen Fair, @allenfair, github.com/afair

Thanks to:

  • Iulian Dogariu, github.com/iulianu (Fixes)

  • Julian Mehnle, julian@mehnle.net (Suggestions)

  • …And all the other contributers!

Note on Patches/Pull Requests ↑

  • Fork the project.

  • Make your feature addition or bug fix.

  • Add tests for it. This is important so I don’t break it in a future version unintentionally.

  • Commit, do not mess with rakefile, version, or history. (if you want to have your own version, that is fine but bump version in a commit by itself I can ignore when I pull)

  • Send me a pull request. Bonus points for topic branches.

Copyright ↑

Copyright © 2010-2014 Allen Fair. See LICENSE for details.