How to Test Queries in BigQuery

Galaxy Glossary

How do I test SQL queries in BigQuery without affecting data?

Run BigQuery DRY RUN or EXPLAIN to validate SQL logic, cost, and schema without changing data.

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Description

Why should I test queries before running them?

Testing prevents accidental data changes, controls costs, and confirms your SQL logic. BigQuery offers DRY RUN and EXPLAIN to catch errors early.

How do I perform a DRY RUN in BigQuery?

Add the --dry_run flag in the bq CLI or set dryRun=true in the REST API.BigQuery parses the query, estimates bytes processed, and returns immediately.

CLI example

bq query --use_legacy_sql=false --dry_run "SELECT * FROM `ecommerce.Orders` WHERE total_amount > 200"

What does EXPLAIN reveal?

Prefix your query with EXPLAIN to display the execution plan, stage details, and parallelism.This highlights expensive scans or joins.

EXPLAIN example

EXPLAIN SELECT o.id, c.name FROM `ecommerce.Orders` o JOIN `ecommerce.Customers` c ON c.id = o.customer_id;

Can I limit rows to test output structure?

Use LIMIT 0 or WHERE 1=0 to return an empty result set while still checking column names and types.

Structure-check example

SELECT * FROM `ecommerce.Products` WHERE 1=0;

How do I capture test results in a temporary table?

Wrap your query with CREATE TEMP TABLE to store a small sample for manual inspection.

Temp table example

CREATE TEMP TABLE temp_high_value_orders AS SELECT * FROM `ecommerce.Orders` WHERE total_amount > 1000 LIMIT 10;

Best practices for testing BigQuery SQL

Always start with DRY RUN, review scanned bytes, check quotas, and use EXPLAIN for performance tuning.Add row limits when exploring data.

How do I integrate testing in CI/CD?

Call the BigQuery REST API with dryRun=true in automated pipelines. Fail the build if the API returns errors or excessive bytes processed.

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Why How to Test Queries in BigQuery is important

How to Test Queries in BigQuery Example Usage


-- Verify a complex join without cost by dry run
bq query --use_legacy_sql=false --dry_run "\
SELECT c.name, SUM(oi.quantity * p.price) AS lifetime_value\
FROM `ecommerce.Customers` c\
JOIN `ecommerce.Orders` o ON o.customer_id = c.id\
JOIN `ecommerce.OrderItems` oi ON oi.order_id = o.id\
JOIN `ecommerce.Products` p ON p.id = oi.product_id\
GROUP BY c.name\
HAVING lifetime_value > 1000;\"

How to Test Queries in BigQuery Syntax


-- DRY RUN via bq CLI
bq query --use_legacy_sql=false --dry_run "SELECT * FROM `ecommerce.Customers` WHERE created_at >= '2023-01-01';"

-- EXPLAIN plan
EXPLAIN SELECT c.id, c.email, SUM(oi.quantity) AS items
FROM `ecommerce.Customers` c
JOIN `ecommerce.Orders` o ON o.customer_id = c.id
JOIN `ecommerce.OrderItems` oi ON oi.order_id = o.id
GROUP BY c.id, c.email;

-- LIMIT 0 for schema validation
SELECT * FROM `ecommerce.Products` LIMIT 0;

-- Temporary sample table
CREATE TEMP TABLE temp_big_spenders AS
SELECT customer_id, SUM(total_amount) AS total_spent
FROM `ecommerce.Orders`
GROUP BY customer_id
HAVING total_spent > 5000;

Common Mistakes

Ignoring DRY RUN results—developers often skip reviewing scanned bytes and hit unexpected costs. Always read the dry-run output and optimize filters or partitions.
Running full queries with LIMIT only—adding LIMIT after expensive joins still scans all data. Use DRY RUN or partition filters to truly reduce cost.

Frequently Asked Questions (FAQs)

Does DRY RUN count toward my BigQuery quota?

No, DRY RUN does not consume slots or incur charges; it only validates the query.

Can I EXPLAIN a DRY RUN?

Yes. Run EXPLAIN first; BigQuery implicitly performs a dry run during plan generation.

How do I test DML statements?

BigQuery cannot dry-run DML. Instead, wrap changes in a transaction on a cloned table or use CREATE TABLE AS SELECT with a LIMIT.

Want to learn about other SQL terms?

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