Slow download speeds compared to Statement Execution API

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Dal 10/3/2025.

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Descrizione

Context

I've been exploring different ways of getting large amounts of data (100GB+) out of Unity Catalog and into external ray clusters for distributed ml model training and while assessing databricks-sql-python, noticed the download speeds are significantly slower than using the statement execution API. In the actual external ray cluster, the difference is 10x, however I was able to also replicate this to a lesser extent in a databricks notebook.

Replication

The first two approaches both lead to a ~45MB/s download speed on an i3.4xlarge

Using databricks-sql-python directly
from databricks.sql import connect

with connect(
    server_hostname="same-host",
    http_path="same-http-path",
    access_token="token",
    use_cloud_fetch=True,
) as connection:
    cursor = connection.cursor()
    cursor.execute(
        "SELECT * from foo.bar.baz"
    )
    print(cursor.fetchall_arrow())
Using databricks-sql-python + ray.data.read_sql

reference: https://docs.ray.io/en/latest/data/api/doc/ray.data.read_sql.html#ray.data.read_sql

from databricks.sql import connect
from ray.data import read_sql

import ray

ray.init(num_cpu=16)

def connection_factory():
    return connect(
        server_hostname="same-host",
        http_path="same-http-path",
        access_token="token"
        use_cloud_fetch=True,
    )

ray_dataset = read_sql(
    sql="SELECT * from foo.bar.baz",
    connection_factory=connection_factory,
    override_num_blocks=1,
    ray_remote_args={"num_cpus": 16},
)
print(f"Ray dataset count: {ray_dataset.count()}")

However when I use ray.data.read_databricks_tables, I can reach download speeds of ~150MB/s on the same machine.

Using ray.data.read_databricks_tables
import os
import ray

from ray.data import read_databricks_tables


ray.init(num_cpus=16)


os.environ["DATABRICKS_TOKEN"] = "token"
os.environ["DATABRICKS_HOST"] = "same-host"
ray_dataset = read_databricks_tables(
    warehouse_id="same-id-in-http-path-above",
    catalog="foo",
    schema="bar",
    query="SELECT * from baz",
)
print(f"Ray dataset size: {ray_dataset.size_bytes()}")
print(f"Ray dataset count: {ray_dataset.count()}")
print(
    f"Ray dataset summed: {ray_dataset.sum('some_column')}"
)

Potential Cause

I suspect this is because the statement execution api allows you to make separate parallel requests to retrieve different "chunks" of data vs how the sql connector adopts a cursor based approach where you can only retrieve data sequentially.

Ask

Are there any plans on supporting a similar chunking pattern for databricks-sql-python and in lieu of that, is there currently any way to reach download speed parity with the statement execution api?
databricks-sql-python is great because it does not have the 100GB limit of the statement execution api but the slow download speed is a major blocker for use in ml applications requiring the transfer of large data, which to be fair may not the use case that databricks-sql-python has been designed for.

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