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Using a clickhouse-local database

clickhouse-local is a CLI with an embedded version of ClickHouse. It gives users the power of ClickHouse without having to install a server. In this guide, we will learn how to use a clickhouse-local database from chDB.

Setup​

Let's first create a virtual environment:

python -m venv .venv
source .venv/bin/activate

And now we'll install chDB. Make sure you have version 2.0.2 or higher:

pip install "chdb>=2.0.2"

And now we're going to install ipython:

pip install ipython

We're going to use ipython to run the commands in the rest of the guide, which you can launch by running:

ipython

Installing clickhouse-local​

Downloading and installing clickhouse-local is the same as downloading and installing ClickHouse. We can do this by running the following command:

curl https://clickhouse.com/ | sh

To launch clickhouse-local with the data being persisted to a directory, we need to pass in a --path:

./clickhouse -m --path demo.chdb

Ingesting data into clickhouse-local​

The default database only stores data in memory, so we'll need to create a named database to make sure any data we ingest is persisted to disk.

CREATE DATABASE foo;

Let's create a table and insert some random numbers:

CREATE TABLE foo.randomNumbers
ORDER BY number AS
SELECT rand() AS number
FROM numbers(10_000_000);

Let's write a query to see what data we've got:

SELECT quantilesExact(0, 0.5, 0.75, 0.99)(number) AS quants
FROM foo.randomNumbers

┌─quants────────────────────────────────┐
│ [69,2147776478,3221525118,4252096960] │
└───────────────────────────────────────┘

Once you've done that, make sure you exit; from the CLI as only one process can hold a lock on this directory. If we don't do that, we'll get the following error when we try to connect to the database from chDB:

ChdbError: Code: 76. DB::Exception: Cannot lock file demo.chdb/status. Another server instance in same directory is already running. (CANNOT_OPEN_FILE)

Connecting to a clickhouse-local database​

Go back to the ipython shell and import the session module from chDB:

from chdb import session as chs

Initialize a session pointing to demo..chdb:

sess = chs.Session("demo.chdb")

We can then run the same query that returns the quantiles of numbers:

sess.query("""
SELECT quantilesExact(0, 0.5, 0.75, 0.99)(number) AS quants
FROM foo.randomNumbers
""", "Vertical")

Row 1:
──────
quants: [0,9976599,2147776478,4209286886]

We can also insert data into this database from chDB:

sess.query("""
INSERT INTO foo.randomNumbers
SELECT rand() AS number FROM numbers(10_000_000)
""")

Row 1:
──────
quants: [0,9976599,2147776478,4209286886]

We can then re-run the quantiles query from chDB or clickhouse-local.