Data Engineering Optimizing Query Performance 3

Question 1

Data skew in a distributed join causes which symptom?

Accepted Answer

A few tasks run far longer than the rest

Answer

Skew concentrates rows on a few keys, overloading the tasks that process them.

Question 2

Which technique mitigates skew on a hot join key?

Accepted Answer

Salting the key to spread rows across partitions

Answer

Salting appends a random component to distribute a hot key over more partitions.

Question 3

A covering index improves a query because it:

Accepted Answer

Contains all columns the query needs, avoiding table lookups

Answer

When the index includes every referenced column, the engine answers from the index alone.

Question 4

SELECT * in an analytical query on columnar storage is discouraged mainly because it:

Accepted Answer

Forces reading all columns, increasing I/O

Answer

Selecting every column negates columnar's ability to skip unused columns.

Question 5

Materializing an expensive, frequently-run aggregation into a summary table is best described as:

Accepted Answer

A materialized view / precomputation tradeoff

Answer

Precomputing results trades storage and refresh cost for faster reads.

Data Engineering Practice Test