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Interview/Databricks

Delta Lake interview questions

Interactive Databricks interview questions on Delta Lake. Same topic as /learn/databricks/delta. Delta is ACID on object storage: Parquet files plus a transaction log. Time travel, OPTIMIZE, and MERGE all come from that log.

Lesson · Simulation

beginner
Delta Lake

What is Delta Lake?

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Production scenario

Delta Lake

BI reads stale or duplicate Parquet

Symptoms

  • Someone listed the folder and read *.parquet
  • Concurrent MERGE in progress
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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 Delta Lake?

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Short: Parquet files plus a transaction log (_delta_log) that gives ACID on object storage.

Detailed: Every write commits JSON (and checkpoints). Time travel is 'read version N'. OPTIMIZE and MERGE use the log to know which files are current.

Common mistake: Delta as a file extension you rename to .delta.

Follow-up: What happens if you read the Parquet files and ignore the log?

Lesson · Simulation

intermediate

Why can two jobs writing the same Delta table not corrupt it the way raw Parquet can?

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Short: Optimistic concurrency: each commit names the files added/removed. Conflicts abort.

Detailed: Concurrent overwrites of the same partition can still fail — you retry or isolate. The log is the source of truth, not 'whoever finished last silently'.

Common mistake: Writing Parquet next to Delta and expecting ACID.

Follow-up: How do you time-travel to yesterday 8am?

Lesson · Simulation

senior

Readers see partial data during a write. What's broken?

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Short: They are not reading through the Delta log — or the writer isn't Delta.

Detailed: Spark parquet. format on a Delta path, a broken symlink, or copying files in S3. DESCRIBE HISTORY should exist. If not, it's not a Delta table.

Senior: VACUUM deletes unreferenced files. Time travel older than retention dies. Never VACUUM 0 hours in prod.

Common mistake: Restarting the cluster to 'clear the partial write'.

Follow-up: When do you VACUUM and what's the footgun?

Lesson · Simulation