Optimize a slow query on sessions
Last updated: July 4, 2025
Quick Overview
A query on sessions is running slowly. Identify the bottleneck and optimize it.
Salesforce
July 4, 20258
10
535 solved
A query on sessions is running slowly. Identify the bottleneck and optimize it.
Salesforce asks this during the Technical Screen because data engineering skills are critical for the role. You should be comfortable with complex joins, window functions, CTEs, and performance optimization.
What the Interviewer Expects
- Solve complex analytical problems with elegant, readable SQL
- Optimize queries for large-scale datasets with partitioning and indexing
- Use recursive CTEs, lateral joins, and advanced window functions
- Design the data model alongside the query solution
- Discuss trade-offs between SQL and programmatic approaches (Python/pandas)
- Consider the operational aspects: query scheduling, incremental processing
Key Topics to Cover
How to Approach This
- Clarify the schema and expected output format before writing queries.
- Use CTEs (WITH clauses) to break complex queries into readable steps.
- Consider window functions (ROW_NUMBER, RANK, LAG, LEAD) for ranking and sequential analysis.
- Watch for NULLs, duplicates, and edge cases in JOINs and GROUP BY.
- For pandas, prefer vectorized operations over row-by-row iteration.
Possible Follow-up Questions
- What indexes would you create to support this query?
- Can you rewrite this without using subqueries?
- How would you optimize this query for a table with 100 million rows?
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Practice SQL ProblemsSample Answer
Problem Understanding
The task involves optimizing a slow SQL query on a table containing session data. The data likely includes fields such as session ID, user ID, session start and end timestamps, and session duration. T...
Approach
To build the query, follow these steps:
- Identify Key Columns: Determine the columns necessary for the analysis, likely including session ID, user ID, start and end timestamps.
- *Filter Data...
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