infer_storage_schema plugin

This plug-in infers schema of external data, and returns it as CSL schema string. The string can be used when creating external tables. The plugin is invoked with the evaluate operator.

Syntax

evaluate infer_storage_schema( Options )

Arguments

A single Options argument is a constant value of type dynamic that holds a property bag specifying properties of the request:

Name Required Description
StorageContainers Yes List of storage connection strings that represent prefix URI for stored data artifacts
DataFormat Yes One of supported data formats.
FileExtension No Only scan files ending with this file extension. It's not required, but specifying it may speed up the process (or eliminate data reading issues)
FileNamePrefix No Only scan files starting with this prefix. It's not required, but specifying it may speed up the process
Mode No Schema inference strategy, one of: any, last, all. Infer data schema from any (first found) file, from last written file, or from all files respectively. The default value is last.

Returns

The infer_storage_schema plugin returns a single result table containing a single row/column holding CSL schema string.

Note

  • Storage container URI secret keys must have the permissions for List in addition to Read.
  • Schema inference strategy 'all' is a very "expensive" operation, as it implies reading from all artifacts found and merging their schema.
  • Some returned types may not be the actual ones as a result of wrong type guess (or, as a result of schema merge process). This is why you should review the result carefully before creating an external table.

Example

let options = dynamic({
  'StorageContainers': [
    h@'https://storageaccount.blob.core.chinacloudapi.cn/MovileEvents;secretKey'
  ],
  'FileExtension': '.parquet',
  'FileNamePrefix': 'part-',
  'DataFormat': 'parquet'
});
evaluate infer_storage_schema(options)

Result

CslSchema
app_id:string, user_id:long, event_time:datetime, country:string, city:string, device_type:string, device_vendor:string, ad_network:string, campaign:string, site_id:string, event_type:string, event_name:string, organic:string, days_from_install:int, revenue:real

Use the returned schema in external table definition:

.create external table MobileEvents(
    app_id:string, user_id:long, event_time:datetime, country:string, city:string, device_type:string, device_vendor:string, ad_network:string, campaign:string, site_id:string, event_type:string, event_name:string, organic:string, days_from_install:int, revenue:real
)
kind=blob
partition by (dt:datetime = bin(event_time, 1d), app:string = app_id)
pathformat = ('app=' app '/dt=' datetime_pattern('yyyyMMdd', dt))
dataformat = parquet
(
    h@'https://storageaccount.blob.core.chinacloudapi.cn/MovileEvents;secretKey'
)