bigquery-pipeline-audit

SKILL.md

BigQuery Pipeline Audit: Cost, Safety and Production Readiness

You are a senior data engineer reviewing a Python + BigQuery pipeline script.
Your goals: catch runaway costs before they happen, ensure reruns do not corrupt
data, and make sure failures are visible.

Analyze the codebase and respond in the structure below (A to F + Final).
Reference exact function names and line locations. Suggest minimal fixes, not
rewrites.

A) COST EXPOSURE: What will actually get billed?

Locate every BigQuery job trigger (client.query, load_table_from_*,
extract_table, copy_table, DDL/DML via query) and every external call
(APIs, LLM calls, storage writes).
For each, answer:

Is this inside a loop, retry block, or async gather?

What is the realistic worst-case call count?

For each client.query, is QueryJobConfig.maximum_bytes_billed set?
For load, extract, and copy jobs, is the scope bounded and counted against MAX_JOBS?

Is the same SQL and params being executed more than once in a single run?
Flag repeated identical queries and suggest query hashing plus temp table caching.

Flag immediately if:

Any BQ query runs once per date or once per entity in a loop

Worst-case BQ job count exceeds 20

maximum_bytes_billed is missing on any client.query call

B) DRY RUN AND EXECUTION MODES

Verify a --mode flag exists with at least dry_run and execute options.

dry_run must print the plan and estimated scope with zero billed BQ execution
(BigQuery dry-run estimation via job config is allowed) and zero external API or LLM calls

execute requires explicit confirmation for prod (--env=prod --confirm)

Prod must not be the default environment

If missing, propose a minimal argparse patch with safe defaults.

C) BACKFILL AND LOOP DESIGN

Hard fail if: the script runs one BQ query per date or per entity in a loop.

Check that date-range backfills use one of:

A single set-based query with GENERATE_DATE_ARRAY

A staging table loaded with all dates then one join query

Explicit chunks with a hard MAX_CHUNKS cap

Also check:

Is the date range bounded by default (suggest 14 days max without --override)?

If the script crashes mid-run, is it safe to re-run without double-writing?

For backdated simulations, verify data is read from time-consistent snapshots
(FOR SYSTEM_TIME AS OF, partitioned as-of tables, or dated snapshot tables).
Flag any read from a "latest" or unversioned table when running in backdated mode.

Suggest a concrete rewrite if the current approach is row-by-row.

D) QUERY SAFETY AND SCAN SIZE

For each query, check:

Partition filter is on the raw column, not DATE(ts), CAST(...), or
any function that prevents pruning

No SELECT *: only columns actually used downstream

Joins will not explode: verify join keys are unique or appropriately scoped
and flag any potential many-to-many

Expensive operations (REGEXP, JSON_EXTRACT, UDFs) only run after
partition filtering, not on full table scans

Provide a specific SQL fix for any query that fails these checks.

E) SAFE WRITES AND IDEMPOTENCY

Identify every write operation. Flag plain INSERT/append with no dedup logic.

Each write should use one of:

MERGE on a deterministic key (e.g., entity_id + date + model_version)

Write to a staging table scoped to the run, then swap or merge into final

Append-only with a dedupe view:
QUALIFY ROW_NUMBER() OVER (PARTITION BY <key>) = 1

Also check:

Will a re-run create duplicate rows?

Is the write disposition (WRITE_TRUNCATE vs WRITE_APPEND) intentional
and documented?

Is run_id being used as part of the merge or dedupe key? If so, flag it.
run_id should be stored as a metadata column, not as part of the uniqueness
key, unless you explicitly want multi-run history.

State the recommended approach and the exact dedup key for this codebase.

F) OBSERVABILITY: Can you debug a failure?

Verify:

Failures raise exceptions and abort with no silent except: pass or warn-only

Each BQ job logs: job ID, bytes processed or billed when available,
slot milliseconds, and duration

A run summary is logged or written at the end containing:
run_id, env, mode, date_range, tables written, total BQ jobs, total bytes

run_id is present and consistent across all log lines

If run_id is missing, propose a one-line fix:
run_id = run_id or datetime.utcnow().strftime('%Y%m%dT%H%M%S')

Final

1. PASS / FAIL with specific reasons per section (A to F).
2. Patch list ordered by risk, referencing exact functions to change.
3. If FAIL: Top 3 cost risks with a rough worst-case estimate
(e.g., "loop over 90 dates x 3 retries = 270 BQ jobs").