Production Query Pipeline: Radius Search with Hierarchical Covering

Hierarchical Covering
For a circular radius search, compute which quadtree cells the circle overlaps. Start with coarse cells that fully contain or partially overlap the circle. For partial overlaps, descend to finer cells. The result is a set of cells at varying levels that together cover the search area.
Cells fully inside the circle contribute all their points without further filtering. Cells partially overlapping need point-by-point distance checking. Cells fully outside contribute nothing. This hierarchical approach minimizes both over-fetching and false negatives.
Production Query Pipeline
Step 1: Compute cell covering for query region. Use library functions or custom algorithm to find minimal set of cells that cover the circle.
Step 2: Fetch points from each cell. For in-memory quadtrees, traverse the tree. For database-backed, query by cell ID ranges or prefixes.
Step 3: Filter candidates. Points from fully-contained cells pass automatically. Points from edge cells need distance calculation.
Step 4: Sort by distance if returning nearest neighbors. Apply business filters (open now, price range, etc.) before or after spatial filtering depending on selectivity.
Optimizing Cell Coverage
Too many small cells mean too many queries or lookups. Too few large cells mean too many false positives to filter. Target 10 to 50 cells for typical radius queries. Adjust cell resolution based on query radius: larger radius uses coarser cells.
✅ Best Practice: Separate spatial filtering from business filtering. Spatial index narrows candidates to hundreds. Business filters (ratings, availability) reduce to final results. Applying expensive filters to millions of points is slow; applying them to hundreds is fast.

💡 Key Takeaways

✓Hierarchical covering: cells at varying levels that together cover query region

✓Fully-contained cells skip distance check; partially-overlapping cells need filtering

✓Query pipeline: compute covering, fetch from cells, filter by distance, apply business rules

✓Target 10 to 50 cells per query; adjust resolution based on query radius

✓Separate spatial filtering from business filtering for performance

📌 Interview Tips

1Describe covering algorithm: descend into partially-overlapping cells, stop when cell is fully inside or outside circle

2Explain why fully-contained cells skip filtering: every point in the cell is guaranteed within radius

3For radius queries, recommend computing cell covering first, then fetching points from just those cells

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