How does AWS Athena query Parquet files in S3?

AWS Athena is a serverless interactive query service that uses Presto/Trino under the hood. It reads Parquet files directly from S3 using the Parquet columnar reader.

Steps to query Parquet in Athena:

1. Define a Glue Data Catalog table pointing to the S3 prefix:

CREATE EXTERNAL TABLE events (
  user_id BIGINT,
  event_type STRING,
  revenue DOUBLE,
  event_date DATE
)
STORED AS PARQUET
LOCATION 's3://my-bucket/events/'
TBLPROPERTIES ("parquet.compress"="SNAPPY");

2. Query as normal SQL — Athena applies column pruning and predicate pushdown automatically.

Cost optimisation: Athena charges per byte scanned. Parquet compression + columnar reads can reduce costs by 90%+ compared to querying CSV for the same logical data.

Partition projection or partition-filtered queries (WHERE event_date BETWEEN ...) reduce both scan size and latency.

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