Create and distribute tables in Azure Cosmos DB for PostgreSQL

Artikkeli
08/14/2024

APPLIES TO: Azure Cosmos DB for PostgreSQL (powered by the Citus database extension to PostgreSQL)

In this example, we'll use Azure Cosmos DB for PostgreSQL distributed tables to store and query events recorded from GitHub open source contributors.

Prerequisites

To follow this quickstart, you'll first need to:

Create a cluster in the Azure portal.
Connect to the cluster with psql to run SQL commands.

Create tables

Once you've connected via psql, let's create our table. Copy and paste the following commands into the psql terminal window, and hit enter to run:

CREATE TABLE github_users
(
	user_id bigint,
	url text,
	login text,
	avatar_url text,
	gravatar_id text,
	display_login text
);

CREATE TABLE github_events
(
	event_id bigint,
	event_type text,
	event_public boolean,
	repo_id bigint,
	payload jsonb,
	repo jsonb,
	user_id bigint,
	org jsonb,
	created_at timestamp
);

CREATE INDEX event_type_index ON github_events (event_type);
CREATE INDEX payload_index ON github_events USING GIN (payload jsonb_path_ops);

Notice the GIN index on payload in github_events. The index allows fast querying in the JSONB column. Since Citus is a PostgreSQL extension, Azure Cosmos DB for PostgreSQL supports advanced PostgreSQL features like the JSONB datatype for storing semi-structured data.

Distribute tables

create_distributed_table() is the magic function that Azure Cosmos DB for PostgreSQL provides to distribute tables and use resources across multiple machines. The function decomposes tables into shards, which can be spread across nodes for increased storage and compute performance.

Note

In real applications, when your workload fits in 64 vCores, 256GB RAM and 2TB storage, you can use a single-node cluster. In this case, distributing tables is optional. Later, you can distribute tables as needed using create_distributed_table_concurrently.

Let's distribute the tables:

SELECT create_distributed_table('github_users', 'user_id');
SELECT create_distributed_table('github_events', 'user_id');

Important

Distributing tables or using schema-based sharding is necessary to take advantage of Azure Cosmos DB for PostgreSQL performance features. If you don't distribute tables or schemas then worker nodes can't help run queries involving their data.

Load data into distributed tables

We're ready to fill the tables with sample data. For this quickstart, we'll use a dataset previously captured from the GitHub API.

We're going to use the pg_azure_storage extension, to load the data directly from a public container in Azure Blob Storage. First we need to create the extension in our database:

SELECT * FROM create_extension('azure_storage');

Run the following commands to have the database fetch the example CSV files and load them into the database tables.

-- download users and store in table

COPY github_users FROM 'https://pgquickstart.blob.core.windows.net/github/users.csv.gz';

-- download events and store in table

COPY github_events FROM 'https://pgquickstart.blob.core.windows.net/github/events.csv.gz';

Notice how the extension recognized that the URLs provided to the copy command are from Azure Blob Storage, the files we pointed were gzip compressed and that was also automatically handled for us.

We can review details of our distributed tables, including their sizes, with the citus_tables view:

SELECT * FROM citus_tables;

  table_name   | citus_table_type | distribution_column | colocation_id | table_size | shard_count | table_owner | access_method 
---------------+------------------+---------------------+---------------+------------+-------------+-------------+---------------
 github_events | distributed      | user_id             |             1 | 388 MB     |          32 | citus       | heap
 github_users  | distributed      | user_id             |             1 | 39 MB      |          32 | citus       | heap
(2 rows)

Next steps

Now we have distributed tables and loaded them with data. Next, let's try running queries across the distributed tables.

Run distributed queries >

Jaa