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File formats & storage · Formats & Table Formats

What is Delta Lake?

Mediumformat-05
Delta LakeACIDParquettime travellakehouse

Question

What is Delta Lake, and what does it add on top of Parquet files?

Solution

Delta Lake is a table format (and transaction log) that turns a directory of Parquet files into an ACID table with versioning. The data is still Parquet; the magic is the _delta_log that records commits.

Problem first. A bare Parquet folder has no safe concurrent writers, no reliable UPDATE/DELETE, and no easy time travel. Readers can see half-written jobs. Delta adds warehouse-like table semantics on cheap object storage.

/table/
  part-000.parquet
  part-001.parquet
  _delta_log/
    000000.json   <- commits: add/remove files, schema, etc.
    000001.json

What Delta gives you

  • ACID transactions (atomic commits of file adds/removes)
  • MERGE / upserts, deletes, updates
  • Time travel (VERSION AS OF / timestamp)
  • Schema enforcement and evolution controls
  • Optimize / Z-Order / vacuum tooling (engine-dependent)

Interview tip: "Delta = Parquet data files + transaction log." Stress that the format, not just Databricks, defines the table; Spark, Trino, and others can read Delta depending on the stack.

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