Interview Q2 - 𝗗𝗲𝗹𝘁𝗮 𝗟𝗮𝗸𝗲 𝗽𝗲𝗿𝗳𝗼𝗿𝗺𝗮𝗻𝗰𝗲 𝗶𝘀𝘀𝘂𝗲𝘀

𝗠𝗼𝘀𝘁 𝗗𝗲𝗹𝘁𝗮 𝗟𝗮𝗸𝗲 𝗽𝗲𝗿𝗳𝗼𝗿𝗺𝗮𝗻𝗰𝗲 𝗶𝘀𝘀𝘂𝗲𝘀 𝗰𝗼𝗺𝗲 𝗱𝗼𝘄𝗻 𝘁𝗼 𝟰 𝘁𝗵𝗶𝗻𝗴𝘀.

𝗔𝗻𝗱 𝗺𝗼𝘀𝘁 𝗲𝗻𝗴𝗶𝗻𝗲𝗲𝗿𝘀 𝘂𝘀𝗲 𝗼𝗻𝗹𝘆 𝟭 𝗼𝗿 𝟮 𝗼𝗳 𝘁𝗵𝗲𝗺 𝗰𝗼𝗿𝗿𝗲𝗰𝘁𝗹𝘆.
𝗛𝗲𝗿𝗲'𝘀 𝘁𝗵𝗲 𝗳𝘂𝗹𝗹 𝗯𝗿𝗲𝗮𝗸𝗱𝗼𝘄𝗻 𝗼𝗳 𝗩𝗔𝗖𝗨𝗨𝗠 𝘃𝘀 𝗢𝗣𝗧𝗜𝗠𝗜𝗭𝗘 𝘃𝘀 𝗭-𝗢𝗥𝗗𝗘𝗥 𝘃𝘀 𝗟𝗜𝗤𝗨𝗜𝗗 𝗖𝗟𝗨𝗦𝗧𝗘𝗥𝗜𝗡𝗚 👇

─────────────────────
𝗪𝗵𝗮𝘁 𝗲𝗮𝗰𝗵 𝗼𝗻𝗲 𝗱𝗼𝗲𝘀
─────────────────────

𝗩𝗔𝗖𝗨𝗨𝗠 → Storage cleanup
Removes obsolete files no longer referenced by the Delta transaction log.
𝗢𝗣𝗧𝗜𝗠𝗜𝗭𝗘 → File compaction
Compacts many small files into larger files for efficient reads.
𝗢𝗣𝗧𝗜𝗠𝗜𝗭𝗘 → Query pruning
Co-locates related column values in the same files to improve data skipping.
𝗟𝗜𝗤𝗨𝗜𝗗 𝗖𝗟𝗨𝗦𝗧𝗘𝗥𝗜𝗡𝗚 → Adaptive data layout
Automatically reorganizes data layout dynamically without rewriting entire tables.

─────────────────────
𝗗𝗲𝗰𝗶𝘀𝗶𝗼𝗻 𝗠𝗮𝗽
─────────────────────

Storage Growth → VACUUM
Too Many Files → OPTIMIZE
Slow Queries → Z-ORDER
Dynamic Workloads / Changing Queries → Liquid Clustering

─────────────────────
𝗞𝗲𝘆 𝗱𝗶𝗳𝗳𝗲𝗿𝗲𝗻𝗰𝗲𝘀 𝘁𝗼 𝗸𝗻𝗼𝘄
─────────────────────

Query Performance Hierarchy:
Liquid Clustering > Z-ORDER > OPTIMIZE > VACUUM
(VACUUM has no direct query performance impact)

Compute Cost:
VACUUM = Low | OPTIMIZE = Medium | Z-ORDER = High | Liquid Clustering = Moderate & incremental

Maintenance:
Highest → Z-ORDER (must rerun after large data loads)
Lowest → Liquid Clustering (minimal manual maintenance)

Example:
If your query is:
SELECT * FROM sales WHERE country='US' AND product_id=123
→ ZORDER(country, product_id) works best.
Note: ZORDER works best with ≤ 3 columns.

Before OPTIMIZE: 100M rows across 50,000 small files
After OPTIMIZE: 100M rows across 200 large files
Query performance improves dramatically.

─────────────────────
𝗟𝗶𝗺𝗶𝘁𝗮𝘁𝗶𝗼𝗻𝘀 𝘁𝗼 𝗸𝗲𝗲𝗽 𝗶𝗻 𝗺𝗶𝗻𝗱
─────────────────────

⚠️ VACUUM → Cannot recover deleted data after retention period
⚠️ OPTIMIZE → Does not improve predicate filtering
⚠️ Z-ORDER → Limited number of columns recommended
⚠️ LIQUID CLUSTERING → Requires newer Delta Lake versions

VACUUM retention default = 7 days
VACUUM table RETAIN 168 HOURS

These are not competing techniques. They solve different problems.
A well-maintained Delta table uses all four.

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