Optimizing BigQuery: Master EXPLAIN Plans

Google Cloud’s BigQuery offers a robust, managed service for big data analytics within the Google Cloud ecosystem. It’s designed to support rapid SQL queries across vast datasets, making it a top choice for data analysis and business intelligence endeavors. Widely used by data professionals and analysts alike, BigQuery’s seamless integration with other Google Cloud services enhances its utility for comprehensive data analysis solutions.

BigQuery stands out as a dynamic data warehousing tool, enabling Google to swiftly handle petabytes of data. To leverage BigQuery effectively, it’s crucial to optimize query performance. This guide delves into how to use EXPLAIN plans to refine and enhance the efficiency of your queries in BigQuery.

Query Processing in BigQuery

BigQuery’s query processing entails a sequence of actions to analyze and report data via SQL. Here’s an overview:

Accessing BigQuery can be through its web console, command line interface, API, or client libraries.

Selection of databases and tables is straightforward, allowing for easy interaction with existing structures or the creation of new ones.

SQL queries are crafted using standard SQL syntax. For instance:

SELECT name, age FROM mydataset.mytable WHERE age > 30;

Following query composition, execution and result analysis come next.

BigQuery also supports advanced functionalities, robust security measures, and techniques for optimizing performance and managing costs.

1 BigQuery’s Underlying Architecture

Designed for high efficiency in large-scale data operations, BigQuery’s architecture separates storage and processing. It leverages columnar storage in Google Cloud Storage, compressing data significantly. Queries are processed using Google’s Dremel technology, allowing for rapid operations on extensive datasets. BigQuery dynamically scales and utilizes Google’s vast network for optimal performance, emphasizing a columnar data format for quick query responses. Data partitioning and clustering are strategies to manage large tables effectively, reducing the data volume scanned per query.

Furthermore, BigQuery employs Google Cloud’s robust security measures, offering easy integration with other Google Cloud services and external data sources.

2 Managing Resources with Slots

BigQuery utilizes “slots” – allocations of CPU and RAM for processing queries. These slots enable parallel processing, enhancing query speed. Users choose between On-Demand and Flat-Rate capacities, catering to different workloads and query volumes. Custom slot pools allow for tailored resource allocation to specific projects or teams, with some plans offering elastic slots for handling peak loads, ensuring sustained performance during high demand periods.

Understanding and leveraging slots and resource management effectively is key to maximizing BigQuery’s capabilities while managing costs.

Meaning and Utilization of EXPLAIN Plans

EXPLAIN plans are instrumental in BigQuery for dissecting SQL query execution strategies. They reveal the execution plan, identifying potential performance bottlenecks and optimization opportunities.

1 The Essence of EXPLAIN Plans

An EXPLAIN plan outlines the operational steps and phases of a query, indicating resource usage and potential efficiency gains. It’s a roadmap for understanding and refining query execution.

2 Interpreting EXPLAIN Plans

Begin by prefixing your query with EXPLAIN, such as:

EXPLAIN SELECT * FROM mydataset.mytable;

This reveals the query’s operational phases and steps, highlighting data volume at each stage and pinpointing resource-intensive operations. The plan may offer optimization suggestions, guiding performance improvements.

3 Key Elements of EXPLAIN Plans

EXPLAIN plans break down into phases and steps, detailing operations like reading, joining, and filtering. They show how queries are parallelized and provide resource usage estimates, aiding in performance and cost evaluation.

4 Stages in Execution Plans

The execution stages in an EXPLAIN plan, from planning and optimization to execution, play critical roles in query performance. They encompass operations like filtering, joining, sorting, and aggregation, each impacting overall efficiency.

Performance Analysis

Analyzing and optimizing query performance in BigQuery is vital for speed and cost-effectiveness. This involves reviewing the EXPLAIN plan, restructuring queries, minimizing data scanning, optimizing table and column selection for joins, and managing groupings and sort operations efficiently.

1 Plan Analysis in Performance Evaluation

In performance analysis, dissecting the execution plan helps identify the query’s operational aspects and performance-influencing factors. It involves assessing each phase and step for data volume and resource usage, guiding optimization efforts.

2 Slot Efficiency in Performance Evaluation

Efficient slot usage is paramount in BigQuery for optimal query performance. Managing slots effectively balances performance with cost, ensuring queries are processed swiftly without unnecessary expenditure.

3 Optimization Strategies in Performance Analysis

Effective optimization strategies include minimizing data scanning, optimizing join operations, and managing groupings and aggregations wisely. These strategies are essential for enhancing query performance and managing costs.

Advanced Optimization Techniques

To further refine BigQuery performance, advanced techniques focus on analyzing query structures, adopting efficient data modeling practices, and simplifying query complexity.

1 Analyzing Query Structures

A thorough examination of the query structure is essential for identifying performance bottlenecks and areas for improvement. This includes assessing JOIN operations, WHERE conditions, and the overall query execution plan.

2 Data Modeling Techniques

Effective data modeling is crucial for optimizing query performance in BigQuery. Strategies such as denormalization, partitioning, clustering, and the use of materialized views and summary tables can significantly impact query efficiency.

3 Simplifying Query Complexity

Reducing the complexity of queries can lead to better performance and lower costs. This involves breaking down large queries, optimizing JOINs, applying effective filtering early in the query process, and leveraging partitioning and clustering to focus on relevant data segments.

Practical Applications and Case Studies

Implementing EXPLAIN plans in real-world scenarios provides invaluable insights into query optimization. Here are some practical applications:

Complex Queries on Large Datasets: In cases where complex queries on vast datasets run slower than expected, an EXPLAIN plan can highlight resource-intensive steps, guiding optimizations such as restructuring JOIN operations or improving filtering strategies.

Data Model Optimization: When seeking to enhance data model efficiency, an EXPLAIN plan can reveal which tables and columns are most resource-intensive, suggesting areas for partitioning or clustering to improve performance.

Reducing Query Costs: To lower query expenses, EXPLAIN plans identify steps that process large amounts of data, indicating where more precise filtering or partitioning could reduce costs.

Enhancing Parallelism: For queries that need faster processing, EXPLAIN plans can show the extent of parallelism and suggest modifications to increase slot usage or improve parallel processing efficiency.

Optimizing Slow Queries: For queries that run slower than desired, EXPLAIN plans can pinpoint bottlenecks, suggesting areas for simplification or optimization, such as reducing subqueries or refining calculations.

Example EXPLAIN Plan Analysis

Consider the query: EXPLAIN SELECT COUNT(*) FROM sales JOIN dates ON sales.date_id = dates.date_id WHERE dates.year = 2020;

This query’s EXPLAIN plan may reveal a large JOIN operation. Optimizing this could involve rearranging the JOIN order, using more specific WHERE conditions, or implementing partitioning and clustering for efficiency.

Key Observations and Solutions:

JOIN operations heavily influencing performance can be optimized by adjusting the JOIN order and using precise filtering.

Excessive data scanning can be mitigated by applying more specific WHERE conditions and leveraging partitioning and clustering.

Complex aggregations and groupings on large datasets can be optimized by using pre-calculated results or simplifying operations.

Insufficient parallelism indicated by the EXPLAIN plan can be addressed by optimizing slot distribution and query structure for better parallel processing.

High query costs can be reduced by refining the query to process less data, employing efficient filtering, and using BigQuery’s cost estimation tools for better budget management.



Utilizing EXPLAIN plans is instrumental in optimizing query performance in BigQuery, offering a detailed blueprint for executing SQL queries. Through careful analysis and continuous optimization of queries, it’s possible to achieve cost-effective and high-performance data analytics. Regularly reviewing and adjusting query strategies in response to evolving data sets ensures sustained efficiency and cost control in your BigQuery operations.

For personalized guidance or to explore more about optimizing your BigQuery queries, feel free to reach out to the experts at Oredata.  Connect with us to unlock the full potential of your data with Google Cloud’s BigQuery.

Author: Mümin İrican, Cloud DevOps Engineer at Oredata 

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