Databricks Unity Catalog Tutorial: Diagnose Catalog USE Schema Dependency Failure

This tutorial guides you through diagnosing a Catalog USE Schema dependency failure within Databricks Unity Catalog using Databricks SQL. We’ll focus specifically on understanding and resolving permission issues that prevent access to data within a catalog and schema.

Scenario

A data engineer has created a schema named “staging” within a catalog named “my_catalog.” Users are experiencing errors when attempting to query tables within the “staging” schema, despite having appropriate permissions on the catalog. We need to determine the root cause and verify the correct permissions are in place.

Script 1: Initial Setup & Verification

This script creates the necessary objects in the Unity Catalog and verifies that the catalog and schema exist, as well as the basic permission settings.

CREATE CATALOG IF NOT EXISTS my_catalog;
CREATE SCHEMA IF NOT EXISTS my_catalog.staging
  WITH (
    comment = 'Staging schema for data preparation',
    visibility = 'PUBLIC'
  );

-- Grant SELECT permission to the 'data_users' group on the 'my_catalog.staging' schema.
GRANT SELECT ON SCHEMA my_catalog.staging TO GROUP data_users;

-- Verify the schema exists
SHOW SCHEMAS IN my_catalog;

-- Verify permissions for the 'data_users' group on the 'my_catalog.staging' schema
SHOW GRANTS FOR GROUP data_users IN my_catalog.staging;

Explanation:

• `CREATE CATALOG IF NOT EXISTS my_catalog;` creates the catalog if it doesn’t already exist.
• `CREATE SCHEMA IF NOT EXISTS my_catalog.staging WITH ( … )` creates the staging schema with a comment and sets its visibility to public.
• `GRANT SELECT ON SCHEMA my_catalog.staging TO GROUP data_users;` grants the ‘data_users’ group SELECT privileges on the schema. This is the key step that could be causing the issue.
• `SHOW SCHEMAS IN my_catalog;` checks the schema exists.
• `SHOW GRANTS FOR GROUP data_users IN my_catalog.staging;` verifies the grant was applied.

This script establishes the foundation for our diagnostic process. It’s crucial to confirm the schema and grant exist before attempting to query it.

[
  {
    "schema_name": "staging",
    "catalog_name": "my_catalog"
  }
]

Script 2: Simulating the Failure – Incorrect Permissions

This script attempts to query the “staging” schema, simulating a scenario where permissions are not correctly configured, leading to an access error.

-- Create a sample table in the staging schema
CREATE TABLE IF NOT EXISTS my_catalog.staging.sample_table (
  id INT,
  value STRING
);

-- Insert some sample data
INSERT INTO my_catalog.staging.sample_table (id, value) VALUES
(1, 'data1'),
(2, 'data2');

-- Attempt to query the sample table – this will fail if permissions are not set correctly
SELECT  FROM my_catalog.staging.sample_table;

Explanation:

• `CREATE TABLE IF NOT EXISTS …` creates a sample table within the staging schema.
• `INSERT INTO …` populates the table with some test data.
• `SELECT FROM …` attempts to query the table. This query will fail if the ‘data_users’ group doesn’t have SELECT privileges on the ‘my_catalog.staging’ schema.

By executing this script, you should encounter an error indicating that the ‘data_users’ group does not have permission to access the “staging” schema. This validates the diagnostic process.

ERROR:  permission denied to table my_catalog.staging.sample_table
...

Script 3: Verifying and Correcting Permissions

This script demonstrates how to verify the root cause and correct the permissions issue.

-- Verify the permission error (from Script 2) still occurs.
SELECT  FROM my_catalog.staging.sample_table;

-- Grant SELECT permission to the 'data_users' group on the 'my_catalog.staging' schema. This step is already done from Script 1, ensure this step still works as expected.
GRANT SELECT ON SCHEMA my_catalog.staging TO GROUP data_users;

-- Retry the query
SELECT  FROM my_catalog.staging.sample_table;

Explanation:

• The first `SELECT` statement re-enacts the permission error.
• `GRANT SELECT ON SCHEMA …` re-establishes the required permission if it was accidentally revoked (simulated by the problem scenario).
• The second `SELECT` statement, after re-granting permission, should execute successfully.

This final script confirms that the original permission issue was indeed the cause of the access failure and demonstrates the correct remediation step.

[
  {
    "id": 1,
    "value": "data1"
  },
  {
    "id": 2,
    "value": "data2"
  }
]

Conclusion

This tutorial demonstrated a practical approach to diagnosing Catalog USE Schema dependency failure within Databricks Unity Catalog using Databricks SQL. By systematically creating objects, simulating failures, and verifying permissions, you can effectively troubleshoot and resolve access issues, ensuring data security and compliance within your Unity Catalog environment. Remember to always verify grant settings and schema visibility to prevent future access problems.