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Catalyst optimizer interview questions

Interactive Spark interview questions on Catalyst optimizer. Same topic as /learn/spark/catalyst. Catalyst rewrites your query four times. Exchange in the physical plan is the shuffle you will pay for.

Lesson · Simulation

beginner
Catalyst optimizer

What is Catalyst?

Answer it out loud, then reveal. Play steps through like the simulators.

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Production scenario

Catalyst optimizer

Filter on date still scans the whole lake

Symptoms

  • Table not partitioned by date
  • Or filter wrapped in a UDF
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All questions on this page

Indexed as FAQ. Open any item if you prefer a list to Play.

beginner

What is Catalyst?

Catalyst optimizer · tap to open the answer

Short: Spark SQL's optimizer: analysis → logical optimization → physical planning.

Detailed: It resolves names, pushes filters, reorders joins, then picks physical operators (broadcast vs SMJ). DataFrame/SQL get Catalyst; raw RDDs do not.

Common mistake: Catalyst as a storage format.

Follow-up: At which phase do unresolved attributes fail?

Lesson · Simulation

intermediate

How do you read explain('formatted') in an interview?

Catalyst optimizer · tap to open the answer

Short: Bottom is the scan. Look for PushedFilters, PartitionFilters, Exchange, and the join type.

Detailed: If the filter isn't in PushedFilters, you're scanning extra files. Exchange means shuffle. BroadcastHashJoin vs SortMergeJoin is the money line.

Common mistake: Pasting explain() without pointing at an operator.

Follow-up: What does AdaptiveSparkPlan mean?

Lesson · Simulation

senior

Catalyst picked SMJ. Stats were wrong. What's your move?

Catalyst optimizer · tap to open the answer

Short: ANALYZE TABLE / fix stats, add a broadcast hint if you know the size, or enable AQE join conversion.

Detailed: Don't hint forever as a substitute for stats. Hints rot. Measure with actual size after filters — AQE sees runtime sizes.

Senior: When you've measured the build side after filters and documented why stats cannot see it (UDF, JDBC, etc.).

Common mistake: Broadcast hint on a table that will grow 20×.

Follow-up: When is a hint the right senior answer?

Lesson · Simulation