AlloyDB federated queries

As a data analyst, you can query data in AlloyDB for PostgreSQL from BigQuery using federated queries.

BigQuery AlloyDB federation lets BigQuery query data residing in AlloyDB in real time without copying or moving the data.

Before you begin

Query data

To send a federated query to AlloyDB from a GoogleSQL query, use the EXTERNAL_QUERY function.

Suppose that you store a customer table in BigQuery, while storing a sales table in AlloyDB, and want to join the two tables in a single query. The following example makes a federated query to an AlloyDB table named orders and joins the results with a BigQuery table named mydataset.customers.

The example query includes 3 parts:

  1. Run the external query SELECT customer_id, MIN(order_date) AS first_order_date FROM orders GROUP BY customer_id in the AlloyDB database to get the first order date for each customer through the EXTERNAL_QUERY function.

  2. Join the external query results table with the customers table in BigQuery by customer_id.

  3. Select customer information and first order date in the final result set.

SELECT c.customer_id, c.name, rq.first_order_date
FROM mydataset.customers AS c
LEFT OUTER JOIN EXTERNAL_QUERY(
  'us.connection_id',
  '''SELECT customer_id, MIN(order_date) AS first_order_date
  FROM orders
  GROUP BY customer_id''') AS rq ON rq.customer_id = c.customer_id
GROUP BY c.customer_id, c.name, rq.first_order_date;

View an AlloyDB table schema

You can use the EXTERNAL_QUERY function to query information_schema tables to access database metadata. For example, you can list all of the tables in the database or view the table schema. For more information, see PostgreSQL information_schema tables.

-- List all tables in a database.
SELECT * FROM EXTERNAL_QUERY("region.connection_id",
"select * from information_schema.tables;");

-- List all columns in a table.
SELECT * FROM EXTERNAL_QUERY("region.connection_id",
"select * from information_schema.columns where table_name='x';");

Track BigQuery federated queries

When you run a federated query against AlloyDB, BigQuery annotates the query with a comment similar to the following:

/* Federated query from BigQuery. Project ID: PROJECT_ID, BigQuery Job ID: JOB_ID. */

If you are monitoring logs for query usage, the following annotation can help you identify queries coming from BigQuery.

  1. Go to the Logs Explorer page.

    Go to the Logs Explorer

  2. In the Query tab, enter the following query:

    resource.type="alloydb.googleapis.com/Instance"
textPayload=~"Federated query from BigQuery"

  3. Click Run query.

    If there are records available for BigQuery federated queries, a list of records similar to the following appears in Query results.

    YYYY-MM-DD hh:mm:ss.millis UTC [3210064]: [4-1]
db=DATABASE, user=USER_ACCOUNT
STATEMENT: SELECT 1 FROM (SELECT FROM company_name_table) t;
/* Federated query from BigQuery.
Project ID: PROJECT_ID, BigQuery Job ID: JOB_ID
*/

YYYY-MM-DD hh:mm:ss.millis UTC [3210532]: [2-1]
db=DATABASE, user=USER_ACCOUNT
STATEMENT: SELECT "company_id", "company type_id" FROM
(SELECT FROM company_name_table) t;
/* Federated query from BigQuery.
Project ID: PROJECT_ID, BigQuery Job ID: JOB_ID
*/

    For more information about Cloud Logging, see Cloud Logging.

Troubleshooting

This section describes potential errors you might encounter when sending a federated query to AlloyDB and provides possible troubleshooting resolutions.

Issue: Failed to connect to the database server with this error: Invalid table-valued function EXTERNAL_QUERY Connect to PostgreSQL server failed: server closed the connection unexpectedly. This probably means the server terminated abnormally before or while processing the request.

Resolution: Ensure that you used valid credentials and followed all prerequisites while creating the connection to AlloyDB. Check if the service account that is automatically created when a connection to AlloyDB is created has the AlloyDB Client (roles/alloydb.client) role. For more information, see Grant access to the service account.

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