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Select API Quickstart Guide

Traditional retrieval of objects is always as whole entities, i.e GetObject for a 5 GiB object, will always return 5 GiB of data. S3 Select API allows us to retrieve a subset of data by using simple SQL expressions. By using Select API to retrieve only the data needed by the application, drastic performance improvements can be achieved.

You can use the Select API to query objects with following features:

CSV, JSON and Parquet - Objects must be in CSV, JSON, or Parquet format.
UTF-8 is the only encoding type the Select API supports.
GZIP or BZIP2 - CSV and JSON files can be compressed using GZIP or BZIP2. The Select API supports columnar compression for Parquet using GZIP, Snappy, LZ4. Whole object compression is not supported for Parquet objects.
Server-side encryption - The Select API supports querying objects that are protected with server-side encryption.

Type inference and automatic conversion of values is performed based on the context when the value is un-typed (such as when reading CSV data). If present, the CAST function overrides automatic conversion.

1. Prerequisites

Install MinIO Server from here.
Familiarity with AWS S3 API.
Familiarity with Python and installing dependencies.

2. Install boto3

Install aws-sdk-python from AWS SDK for Python official docs here

3. Example

As an example, let us take a gzip compressed CSV file. Without S3 Select, we would need to download, decompress and process the entire CSV to get the data you needed. With Select API, can use a simple SQL expression to return only the data from the CSV you’re interested in, instead of retrieving the entire object. Following Python example shows how to retrieve the first column Location from an object containing data in CSV format.

Please replace endpoint_url,aws_access_key_id, aws_secret_access_key, Bucket and Key with your local setup in this select.py file.

#!/usr/bin/env/env python3
import boto3

s3 = boto3.client('s3',
                  endpoint_url='http://localhost:9000',
                  aws_access_key_id='minio',
                  aws_secret_access_key='minio123',
                  region_name='us-east-1')

r = s3.select_object_content(
    Bucket='mycsvbucket',
    Key='sampledata/TotalPopulation.csv.gz',
    ExpressionType='SQL',
    Expression="select * from s3object s where s.Location like '%United States%'",
    InputSerialization={
        'CSV': {
            "FileHeaderInfo": "USE",
        },
        'CompressionType': 'GZIP',
    },
    OutputSerialization={'CSV': {}},
)

for event in r['Payload']:
    if 'Records' in event:
        records = event['Records']['Payload'].decode('utf-8')
        print(records)
    elif 'Stats' in event:
        statsDetails = event['Stats']['Details']
        print("Stats details bytesScanned: ")
        print(statsDetails['BytesScanned'])
        print("Stats details bytesProcessed: ")
        print(statsDetails['BytesProcessed'])

4. Run the Program

Upload a sample dataset to MinIO using the following commands.

$ curl "https://esa.un.org/unpd/wpp/DVD/Files/1_Indicators%20(Standard)/CSV_FILES/WPP2017_TotalPopulationBySex.csv" > TotalPopulation.csv
$ mc mb myminio/mycsvbucket
$ gzip TotalPopulation.csv
$ mc cp TotalPopulation.csv.gz myminio/mycsvbucket/sampledata/

Now let us proceed to run our select example to query for Location which matches United States.

$ python3 select.py
840,United States of America,2,Medium,1950,1950.5,79233.218,79571.179,158804.395

840,United States of America,2,Medium,1951,1951.5,80178.933,80726.116,160905.035

840,United States of America,2,Medium,1952,1952.5,81305.206,82019.632,163324.851

840,United States of America,2,Medium,1953,1953.5,82565.875,83422.307,165988.190
....
....
....

Stats details bytesScanned:
6758866
Stats details bytesProcessed:
25786743

For a more detailed SELECT SQL reference, please see here

5. Explore Further

6. Implementation Status

Full AWS S3 SELECT SQL syntax is supported.
All operators are supported.
All aggregation, conditional, type-conversion and string functions are supported.
JSON path expressions such as FROM S3Object[*].path are not yet evaluated.
Large numbers (outside of the signed 64-bit range) are not yet supported.
The Date functions DATE_ADD, DATE_DIFF, EXTRACT and UTCNOW along with type conversion using CAST to the TIMESTAMP data type are currently supported.
AWS S3's reserved keywords list is not yet respected.
CSV input fields (even quoted) cannot contain newlines even if RecordDelimiter is something else.

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