A Python library for working with the ClickHouse database (https://clickhouse.yandex/)
Python
Latest commit 3b82f31 Nov 9, 2016 @ishirav ishirav committed on GitHub Merge pull request #7 from tswr/master
Fixing Database class to allow readonly mode

README.rst

Overview

This project is simple ORM for working with the ClickHouse database. It allows you to define model classes whose instances can be written to the database and read from it.

Installation

To install infi.clickhouse_orm:

pip install infi.clickhouse_orm

Usage

Defining Models

Models are defined in a way reminiscent of Django's ORM:

from infi.clickhouse_orm import models, fields, engines

class Person(models.Model):

    first_name = fields.StringField()
    last_name = fields.StringField()
    birthday = fields.DateField()
    height = fields.Float32Field()

    engine = engines.MergeTree('birthday', ('first_name', 'last_name', 'birthday'))

It is possible to provide a default value for a field, instead of its "natural" default (empty string for string fields, zero for numeric fields etc.).

See below for the supported field types and table engines.

Using Models

Once you have a model, you can create model instances:

>>> dan = Person(first_name='Dan', last_name='Schwartz')
>>> suzy = Person(first_name='Suzy', last_name='Jones')
>>> dan.first_name
u'Dan'

When values are assigned to model fields, they are immediately converted to their Pythonic data type. In case the value is invalid, a ValueError is raised:

>>> suzy.birthday = '1980-01-17'
>>> suzy.birthday
datetime.date(1980, 1, 17)
>>> suzy.birthday = 0.5
ValueError: Invalid value for DateField - 0.5
>>> suzy.birthday = '1922-05-31'
ValueError: DateField out of range - 1922-05-31 is not between 1970-01-01 and 2038-01-19

Inserting to the Database

To write your instances to ClickHouse, you need a Database instance:

from infi.clickhouse_orm.database import Database

db = Database('my_test_db')

This automatically connects to http://localhost:8123 and creates a database called my_test_db, unless it already exists. If necessary, you can specify a different database URL and optional credentials:

db = Database('my_test_db', db_url='http://192.168.1.1:8050', username='scott', password='tiger')

Using the Database instance you can create a table for your model, and insert instances to it:

db.create_table(Person)
db.insert([dan, suzy])

The insert method can take any iterable of model instances, but they all must belong to the same model class.

Reading from the Database

Loading model instances from the database is simple:

for person in db.select("SELECT * FROM my_test_db.person", model_class=Person):
    print person.first_name, person.last_name

Do not include a FORMAT clause in the query, since the ORM automatically sets the format to TabSeparatedWithNamesAndTypes.

It is possible to select only a subset of the columns, and the rest will receive their default values:

for person in db.select("SELECT first_name FROM my_test_db.person WHERE last_name='Smith'", model_class=Person):
    print person.first_name

SQL Placeholders

There are a couple of special placeholders that you can use inside the SQL to make it easier to write: $db and $table. The first one is replaced by the database name, and the second is replaced by the database name plus table name (but is available only when the model is specified).

So instead of this:

db.select("SELECT * FROM my_test_db.person", model_class=Person)

you can use:

db.select("SELECT * FROM $db.person", model_class=Person)

or even:

db.select("SELECT * FROM $table", model_class=Person)

Ad-Hoc Models

Specifying a model class is not required. In case you do not provide a model class, an ad-hoc class will be defined based on the column names and types returned by the query:

for row in db.select("SELECT max(height) as max_height FROM my_test_db.person"):
    print row.max_height

This is a very convenient feature that saves you the need to define a model for each query, while still letting you work with Pythonic column values and an elegant syntax.

Counting

The Database class also supports counting records easily:

>>> db.count(Person)
117
>>> db.count(Person, conditions="height > 1.90")
6

Pagination

It is possible to paginate through model instances:

>>> order_by = 'first_name, last_name'
>>> page = db.paginate(Person, order_by, page_num=1, page_size=100)
>>> print page.number_of_objects
2507
>>> print page.pages_total
251
>>> for person in page.objects:
>>>     # do something

The paginate method returns a namedtuple containing the following fields:

  • objects - the list of objects in this page
  • number_of_objects - total number of objects in all pages
  • pages_total - total number of pages
  • number - the page number
  • page_size - the number of objects per page

You can optionally pass conditions to the query:

>>> page = db.paginate(Person, order_by, page_num=1, page_size=100, conditions='height > 1.90')

Note that order_by must be chosen so that the ordering is unique, otherwise there might be inconsistencies in the pagination (such as an instance that appears on two different pages).

Schema Migrations

Over time, your models may change and the database will have to be modified accordingly. Migrations allow you to describe these changes succinctly using Python, and to apply them to the database. A migrations table automatically keeps track of which migrations were already applied.

For details please refer to the MIGRATIONS.rst document.

Field Types

Currently the following field types are supported:

Class DB Type Pythonic Type Comments
StringField String unicode Encoded as UTF-8 when written to ClickHouse
DateField Date datetime.date Range 1970-01-01 to 2038-01-19
DateTimeField DateTime datetime.datetime Minimal value is 1970-01-01 00:00:00; Always in UTC
Int8Field Int8 int Range -128 to 127
Int16Field Int16 int Range -32768 to 32767
Int32Field Int32 int Range -2147483648 to 2147483647
Int64Field Int64 int/long Range -9223372036854775808 to 9223372036854775807
UInt8Field UInt8 int Range 0 to 255
UInt16Field UInt16 int Range 0 to 65535
UInt32Field UInt32 int Range 0 to 4294967295
UInt64Field UInt64 int/long Range 0 to 18446744073709551615
Float32Field Float32 float  
Float64Field Float64 float  
Enum8Field Enum8 Enum See below
Enum16Field Enum16 Enum See below
ArrayField Array list See below

Working with enum fields

Enum8Field and Enum16Field provide support for working with ClickHouse enum columns. They accept strings or integers as values, and convert them to the matching Pythonic Enum member.

Python 3.4 and higher supports Enums natively. When using previous Python versions you need to install the enum34 library.

Example of a model with an enum field:

Gender = Enum('Gender', 'male female unspecified')

class Person(models.Model):

    first_name = fields.StringField()
    last_name = fields.StringField()
    birthday = fields.DateField()
    gender = fields.Enum32Field(Gender)

    engine = engines.MergeTree('birthday', ('first_name', 'last_name', 'birthday'))

suzy = Person(first_name='Suzy', last_name='Jones', gender=Gender.female)

Working with array fields

You can create array fields containing any data type, for example:

class SensorData(models.Model):

    date = fields.DateField()
    temperatures = fields.ArrayField(fields.Float32Field())
    humidity_levels = fields.ArrayField(fields.UInt8Field())

    engine = engines.MergeTree('date', ('date',))

data = SensorData(date=date.today(), temperatures=[25.5, 31.2, 28.7], humidity_levels=[41, 39, 66])

Table Engines

Each model must have an engine instance, used when creating the table in ClickHouse.

To define a MergeTree engine, supply the date column name and the names (or expressions) for the key columns:

engine = engines.MergeTree('EventDate', ('CounterID', 'EventDate'))

You may also provide a sampling expression:

engine = engines.MergeTree('EventDate', ('CounterID', 'EventDate'), sampling_expr='intHash32(UserID)')

A CollapsingMergeTree engine is defined in a similar manner, but requires also a sign column:

engine = engines.CollapsingMergeTree('EventDate', ('CounterID', 'EventDate'), 'Sign')

For a SummingMergeTree you can optionally specify the summing columns:

engine = engines.SummingMergeTree('EventDate', ('OrderID', 'EventDate', 'BannerID'),
                                  summing_cols=('Shows', 'Clicks', 'Cost'))

Data Replication

Any of the above engines can be converted to a replicated engine (e.g. ReplicatedMergeTree) by adding two parameters, replica_table_path and replica_name:

engine = engines.MergeTree('EventDate', ('CounterID', 'EventDate'),
                           replica_table_path='/clickhouse/tables/{layer}-{shard}/hits',
                           replica_name='{replica}')

Development

After cloning the project, run the following commands:

easy_install -U infi.projector
cd infi.clickhouse_orm
projector devenv build

To run the tests, ensure that the ClickHouse server is running on http://localhost:8123/ (this is the default), and run:

bin/nosetests