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Explain your approach to optimizing a slow-running query on a table with billions of rows.

SQLmedium0.3 min readPremium

**Approach**: (1) Execution plan—identify full scans, shuffle size, sort. (2) Partition pruning—filter on partition key (e.g., dt) before scan. (3) Predicate pushdown—ensure filters reach storage layer. (4) Index/Clustering—B-tree on filter; Z-order on multi-column. (5) Reduce...

🤖 Analyze Your Answer
Frequency
Low
Asked at 1 company
Category
487
questions in SQL
Difficulty Split
130E|271M|86H
in this category
Total Bank
1,863
across 7 categories
Asked at these companies
Goldman Sachs
Key Concepts Tested
partition

Why This Question Matters

This medium-level SQL question appears frequently in data engineering interviews at companies like Goldman Sachs. While less common, it tests deeper understanding that distinguishes strong candidates. Mastering the underlying concepts (partition) will help you answer variations of this question confidently.

How to Approach This

Break this problem into components. Identify the core trade-offs involved, then walk the interviewer through your reasoning step by step. Demonstrate awareness of edge cases and production considerations - this is what separates good answers from great ones.

Expert Answer
69 words

Approach: (1) Execution plan—identify full scans, shuffle size, sort. (2) Partition pruning—filter on partition key (e.g., dt) before scan. (3) Predicate pushdown—ensure filters reach storage layer. (4) Index/Clustering—B-tree on filter; Z-order on multi-column. (5) Reduce scope—narrow columns; APPROX_COUNT_DISTINCT when exact not needed. (6) Incremental—process only new/changed data. (7) Materialized aggregation—pre-aggregate for frequent patterns. At scale: Unpruned scan of 1B rows = 100GB+ I/O; partitioned + columnar = 1% scan.

The complete answer continues with detailed implementation patterns, architectural trade-offs, and production-grade considerations covering performance optimization and real-world examples.

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According to DataEngPrep.tech, this is one of the most frequently asked SQL interview questions, reported at 1 company. DataEngPrep.tech maintains a curated database of 1,863+ real data engineering interview questions across 7 categories, verified by industry professionals.

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