DA0-002 Data Concepts and Environments Practice Question
A DBA wants to improve query performance on a large table that is frequently filtered on two columns: department_id and hire_date. The table has millions of rows. Which index strategy would be most effective?
⚠ Common exam trap
Many candidates assume two separate single-column indexes are equivalent to a composite index, but they fail to realize that the database cannot efficiently combine them for range predicates without a costly index merge operation.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Create a composite B-tree index on (department_id, hire_date)
A composite B-tree index on (department_id, hire_date) is most effective because it allows the database to satisfy equality and range predicates on both columns in a single index scan. B-tree indexes are optimized for high-cardinality columns and support efficient multi-column filtering when the leading column matches the query's equality condition, followed by the range condition on hire_date.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Create a composite B-tree index on (department_id, hire_date)
Why this is correct
Composite index on both columns in the filter order can be used for both conditions.
- ✗
Create a bitmap index on hire_date
Why it's wrong here
Bitmap indexes are for low cardinality; hire_date is high cardinality.
- ✗
Create a hash index on department_id only
Why it's wrong here
Hash index doesn't support range queries on hire_date.
- ✗
Create two separate B-tree indexes, one on each column
Why it's wrong here
Separate indexes may not be used together efficiently; the database may use only one index.
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