athena-cost-guard

Estimate what an AWS Athena query will scan and cost — before you run it — and block queries that blow your budget.

Available on PyPI: pip install athena-cost-guard

Athena bills by the volume of data scanned from S3 (~$5/TB). Unlike BigQuery, it has no built-in dry-run, so it's easy to fire off one unpartitioned SELECT * and scan a terabyte by accident. athena-cost-guard gives you the pre-flight check Athena is missing.

from athena_cost_guard import estimate

est = estimate(
    "SELECT id FROM billing.line_items WHERE dt = '2026-08'",
    region="us-east-1",
)
print(est.summary())
# tables:            billing.line_items
# partitions matched: 1
# bytes scanned:     ≤ 4.2 GB
# estimated cost:    ≤ $0.0210  (@ $5.00/TB)

Guardrail mode — refuse to run anything over budget:

from athena_cost_guard import cost_guard, BudgetExceeded

@cost_guard(max_usd=1.00, region="us-east-1")
def run(sql):
    return athena.start_query_execution(QueryString=sql, ...)

try:
    run("SELECT * FROM billing.line_items")   # unpartitioned full scan
except BudgetExceeded as e:
    print(e)            # estimated cost $52.31 exceeds budget $1.00 (scanning 10.5 TB)
    print(e.estimate)   # the full Estimate for logging

How it works

1. Parse the SQL with sqlglot (Athena dialect) to find the tables, referenced columns, and WHERE predicates on partition keys.
2. Prune partitions by calling Glue GetPartitions with a pushdown expression built from those predicates — so you only pay attention to the partitions the query would actually touch.
3. Size the surviving partitions by summing their S3 object bytes.
4. Price the total using Athena's real billing rules (10 MB per-query minimum, rounded up to the nearest 10 MB, at your region's $/TB rate).

Install

pip install athena-cost-guard

# for column-aware (Tier-2) estimates, add the parquet extra:
pip install "athena-cost-guard[parquet]"

Note on the [parquet] extra: the base install is intentionally dependency-light (sqlglot + boto3) and covers Tier-1 estimates and @cost_guard. Column-aware Tier-2 estimates (sample_columns=True) read Parquet footers via pyarrow, which only ships with the [parquet] extra. Without it, calling estimate(..., sample_columns=True) raises an ImportError pointing you to pip install "athena-cost-guard[parquet]" — the core estimator keeps working.

Requires AWS credentials with glue:GetTable, glue:GetPartitions, and s3:ListBucket on the relevant tables/buckets (standard boto3 resolution: env vars, shared config, or instance role).

Column-aware estimates (Tier-2)

By default (sample_columns=False) you get the Tier-1 upper bound: every column in the matched partitions is assumed read. But Athena on Parquet only scans the columns a query references, so pass sample_columns=True for a tight, column-aware figure:

est = estimate(
    "SELECT servicename, SUM(billedcost) FROM billing.line_items WHERE dt = '2026-08' GROUP BY 1",
    region="us-east-1",
    sample_columns=True,      # needs the [parquet] extra
)
print(est.summary())
# bytes scanned:     ~317.4 MB        <- was ≤ 16.1 GB as a Tier-1 upper bound
# estimated cost:    ~$0.0016
# column-aware:      2% of bytes referenced  (from 8 sampled Parquet footer(s))

How it works: it reads the footers of up to sample_size (default 8) of the largest Parquet files in the matched partitions — via ranged S3 GETs, never downloading whole files — sums the compressed size of the columns the query references, and scales the byte total by that fraction. The result is an estimate (~), not an upper bound (≤). Falls back to the Tier-1 upper bound for SELECT *, non-Parquet data, or unreadable footers (with a warning — never silently wrong).

Proven against Athena's own numbers

On a real wide-table query selecting a handful of columns out of many, the column-aware estimate tracked Athena's actual DataScannedInBytes closely:

Within ~8%, and on the conservative (slightly-over) side — the safe direction for a budget guard.

Accuracy: read this

Roadmap

• 0.2 — ✅ Tier-2 column-aware estimates via Parquet footer sampling.
• 0.3 — CLI (athena-cost-guard "SELECT ..."), time-window partition pruning (literal date bounds), partition-projection support, Iceberg awareness.

Changelog

0.2.1

• Docs only: refreshed README with the [parquet]-extra note and the Athena ground-truth validation table. No code changes.

0.2.0

• Tier-2 column-aware estimates (sample_columns=True): reads Parquet footers (ranged S3 GETs) and scales the byte total by the fraction of referenced columns, turning the upper bound into a tight ~ estimate.
• New optional [parquet] extra (pyarrow); the core stays sqlglot + boto3.
• Validated against Athena DataScannedInBytes: ≤ 16.1 GB upper bound → ~317 MB estimate → 295 MB actual.

0.1.0

• Initial release: Tier-1 upper-bound estimate() (SQL parse → Glue partition pruning → S3 sizing → Athena pricing) and the @cost_guard budget decorator.

Development

pip install -e ".[dev]"
pytest            # parser & pricing tests need no AWS; estimate tests use fakes

License

MIT