delta-ops

name	delta-ops
description	Delta Lake 4.2 operations on Spark 4.1 in this stack. Load when working with Delta tables specifically — covers OPTIMIZE, VACUUM, time travel, UniForm interop with Iceberg, and when to choose Delta over Iceberg here.

Delta Lake operations

Delta Lake 4.2.0 is wired by default in this stack. The JARs (delta-spark_2.13-4.2.0.jar, delta-storage-4.2.0.jar) ship via ./lakehouse setup, and config/spark/spark-defaults.conf.example enables both Iceberg and Delta extensions plus registers spark_catalog as the DeltaCatalog. You don't need per-session config — just write Delta.

# Sanity check the extensions on a fresh Connect session
spark.sql("SHOW CATALOGS").show()
# expect: iceberg, spark_catalog

Delta tables live under the default spark_catalog (not iceberg). Address them as spark_catalog.<schema>.<table>, or path-based with delta.\s3a://warehouse/path`. Use a spark_catalog.bronze.*` namespace pattern if you want to mirror the medallion layout used for Iceberg.

When to choose Delta in this stack

• Downstream consumer is a Databricks workspace (terraform-databricks/ target).
• A demo specifically shows Delta features (DML on streaming targets, Change Data Feed).
• Otherwise, prefer Iceberg — it's the default and Unity Catalog OSS surfaces it natively.

Reading and writing

# Write
(df.write.format("delta")
   .mode("overwrite")
   .saveAsTable("spark_catalog.silver.orders"))

# Read
spark.read.format("delta").table("spark_catalog.silver.orders")

# Path-based (no catalog)
spark.read.format("delta").load("s3a://warehouse/delta/orders")

OPTIMIZE (compaction)

OPTIMIZE spark_catalog.silver.orders;
OPTIMIZE spark_catalog.silver.orders WHERE event_date >= '2026-05-01';
OPTIMIZE spark_catalog.silver.orders ZORDER BY (customer_id);   -- multi-dim clustering

Z-ordering helps point queries on the listed columns. Cost: rewrite. Don't z-order on a high-cardinality column you don't query against.

VACUUM (file cleanup)

VACUUM spark_catalog.silver.orders RETAIN 168 HOURS;   -- 7 days, the minimum safe default

Lower retention than 7 days is possible (spark.databricks.delta.retentionDurationCheck.enabled = false) but don't — concurrent readers may be using files newer than the new horizon.

Time travel

SELECT * FROM spark_catalog.silver.orders VERSION AS OF 42;
SELECT * FROM spark_catalog.silver.orders TIMESTAMP AS OF '2026-05-01 00:00:00';

-- Inspect history
DESCRIBE HISTORY spark_catalog.silver.orders;

MERGE INTO

MERGE INTO spark_catalog.silver.orders t
USING staging s ON t.order_id = s.order_id
WHEN MATCHED AND s.event_ts > t.event_ts THEN UPDATE SET *
WHEN NOT MATCHED THEN INSERT *;

MERGE is the canonical Delta upsert. Iceberg has the same syntax — most code is portable.

UniForm (Delta ↔ Iceberg interop)

Delta tables can expose an Iceberg metadata layer for read-only access by Iceberg readers (DuckDB, Trino):

CREATE TABLE spark_catalog.silver.orders (...)
USING delta
TBLPROPERTIES (
  'delta.universalFormat.enabledFormats' = 'iceberg',
  'delta.enableIcebergCompatV2' = 'true'
);

The table is still Delta-canonical for writes; Iceberg readers see a synthesized snapshot. Useful when your write path is Delta-native but your downstream is Iceberg-only.

Change Data Feed (CDF)

Enable on table creation or via ALTER:

ALTER TABLE spark_catalog.silver.orders SET TBLPROPERTIES (delta.enableChangeDataFeed = true);

Read changes:

SELECT * FROM table_changes('spark_catalog.silver.orders', 100, 105);   -- versions 100..105

CDF emits per-row change records (_change_type ∈ insert/update_preimage/update_postimage/delete). Use for incremental downstream pipelines without re-reading the full table.

Delta vs Iceberg cheat sheet (this stack)

Need	Choose
Multi-engine read (DuckDB/Trino/etc.) via UC OSS REST	Iceberg
Hand-off to Databricks	Delta
Time travel + schema evolution	Either (both support it)
Change Data Feed	Delta (Iceberg has incremental reads, different API)
Hidden partitioning	Iceberg
Liquid clustering	Delta (CLUSTER BY)
Default for this stack's medallion path	Iceberg

Common pitfalls

• Catalog name is spark_catalog, not delta. Our spark-defaults.conf registers DeltaCatalog on Spark's default catalog name (spark_catalog). If you see code that addresses tables as delta.silver.orders, it's from a different setup — either rename to spark_catalog.silver.orders or use path-based writes.
• Mixing Iceberg and Delta extensions — the order of spark.sql.extensions matters when both are listed. Iceberg first, Delta second, comma-separated.
• ALTER TABLE doesn't auto-OPTIMIZE — schema changes don't compact. Run OPTIMIZE explicitly after large evolution events.