PDE Maintaining and Automating Data Workloads Practice Question
You need to estimate the cost of a BigQuery query before running it. Which command or feature should you use?
⚠ Common exam trap
A common trap is confusing the EXPLAIN command (which shows the query plan) with the `--dry_run` flag (which estimates bytes processed and cost).
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
✓
Use the bq command with the --dry_run flag.
The `bq` command with the `--dry_run` flag allows you to estimate the amount of data a BigQuery query will process before actually executing it. This dry run does not read any data or incur charges; it simply returns the estimated bytes to be processed, which you can use to calculate the cost based on BigQuery's pricing model.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Check the BigQuery jobs list for similar queries.
Why it's wrong here
The jobs list shows historical bytes billed for past queries, not the bytes the new query will scan, so it cannot estimate its cost. It is tempting because reviewing similar past jobs is a reasonable heuristic, and it would be correct if the requirement were auditing historical spend rather than estimating a specific query.
- ✗
Use the BigQuery cache to estimate if the query is cached.
Why it's wrong here
The cache only indicates whether a query is cached, not the bytes it would scan on a cache miss, so it cannot produce a cost estimate. It is tempting because cached queries are free, and it would be correct if the question asked how to avoid charges for repeated identical queries.
- ✗
Run EXPLAIN on the query to see the query plan.
Why it's wrong here
EXPLAIN returns the query plan and slot usage, not a byte-based cost estimate, so it cannot predict the on-demand charge. It is tempting because EXPLAIN is the standard way to inspect query execution, and it would be correct if the goal were query performance tuning rather than cost estimation.
- ✓
Use the bq command with the --dry_run flag.
Why this is correct
The --dry_run flag validates the query and returns the bytes it would process without executing it, letting you estimate on-demand cost from that byte count. This satisfies the stem's requirement to price a query before running it.
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