Query Parquet files using serverless SQL pool in Azure Synapse Analytics
In this article, you'll learn how to write a query using serverless SQL pool that will read Parquet files.
OPENROWSET function enables you to read the content of parquet file by providing the URL to your file.
Read parquet file
The easiest way to see to the content of your
PARQUET file is to provide file URL to
OPENROWSET function and specify parquet
FORMAT. If the file is publicly available or if your Azure AD identity can access this file, you should be able to see the content of the file using the query like the one shown in the following example:
select top 10 * from openrowset( bulk 'https://pandemicdatalake.blob.core.windows.net/public/curated/covid-19/ecdc_cases/latest/ecdc_cases.parquet', format = 'parquet') as rows
Make sure that you can access this file. If your file is protected with SAS key or custom Azure identity, you would need to setup server level credential for sql login.
Ensure you are using a UTF-8 database collation (for example
Latin1_General_100_BIN2_UTF8) because string values in PARQUET files are encoded using UTF-8 encoding.
A mismatch between the text encoding in the PARQUET file and the collation may cause unexpected conversion errors.
You can easily change the default collation of the current database using the following T-SQL statement:
alter database current collate Latin1_General_100_BIN2_UTF8'
If you use the
Latin1_General_100_BIN2_UTF8 collation you will get an additional performance boost compared to the other collations. The
Latin1_General_100_BIN2_UTF8 collation is compatible with parquet string sorting rules. The SQL pool is able to eliminate some parts of the parquet files that will not contain data needed in the queries (file/column-segment pruning). If you use other collations, all data from the parquet files will be loaded into Synapse SQL and the filtering is happening within the SQL process. The
Latin1_General_100_BIN2_UTF8 collation has additional performance optimization that works only for parquet and CosmosDB. The downside is that you lose fine-grained comparison rules like case insensitivity.
Data source usage
Previous example uses full path to the file. As an alternative, you can create an external data source with the location that points to the root folder of the storage, and use that data source and the relative path to the file in
create external data source covid with ( location = 'https://pandemicdatalake.blob.core.windows.net/public/curated/covid-19/ecdc_cases' ); go select top 10 * from openrowset( bulk 'latest/ecdc_cases.parquet', data_source = 'covid', format = 'parquet' ) as rows
If a data source is protected with SAS key or custom identity you can configure data source with database scoped credential.
Explicitly specify schema
OPENROWSET enables you to explicitly specify what columns you want to read from the file using
select top 10 * from openrowset( bulk 'latest/ecdc_cases.parquet', data_source = 'covid', format = 'parquet' ) with ( date_rep date, cases int, geo_id varchar(6) ) as rows
Make sure that you are explicilty specifying some UTF-8 collation (for example
Latin1_General_100_BIN2_UTF8) for all string columns in
WITH clause or set some UTF-8 collation at database level.
Mismatch between text encoding in the file and string column collation might cause unexpected conversion errors.
You can easily change default collation of the current database using the following T-SQL statement:
alter database current collate Latin1_General_100_BIN2_UTF8
You can easily set collation on the colum types using the following definition:
geo_id varchar(6) collate Latin1_General_100_BIN2_UTF8
In the following sections you can see how to query various types of PARQUET files.
Your first step is to create a database with a datasource that references NYC Yellow Taxi storage account. Then initialize the objects by executing setup script on that database. This setup script will create the data sources, database scoped credentials, and external file formats that are used in these samples.
NYC Yellow Taxi dataset is used in this sample. You can query Parquet files the same way you read CSV files. The only difference is that the
FILEFORMAT parameter should be set to
PARQUET. Examples in this article show the specifics of reading Parquet files.
Query set of parquet files
You can specify only the columns of interest when you query Parquet files.
SELECT YEAR(tpepPickupDateTime), passengerCount, COUNT(*) AS cnt FROM OPENROWSET( BULK 'puYear=2018/puMonth=*/*.snappy.parquet', DATA_SOURCE = 'YellowTaxi', FORMAT='PARQUET' ) WITH ( tpepPickupDateTime DATETIME2, passengerCount INT ) AS nyc GROUP BY passengerCount, YEAR(tpepPickupDateTime) ORDER BY YEAR(tpepPickupDateTime), passengerCount;
Automatic schema inference
You don't need to use the OPENROWSET WITH clause when reading Parquet files. Column names and data types are automatically read from Parquet files.
The sample below shows the automatic schema inference capabilities for Parquet files. It returns the number of rows in September 2018 without specifying a schema.
You don't have to specify columns in the OPENROWSET WITH clause when reading Parquet files. In that case, serverless SQL pool query service will utilize metadata in the Parquet file and bind columns by name.
SELECT TOP 10 * FROM OPENROWSET( BULK 'puYear=2018/puMonth=9/*.snappy.parquet', DATA_SOURCE = 'YellowTaxi', FORMAT='PARQUET' ) AS nyc
Query partitioned data
The data set provided in this sample is divided (partitioned) into separate subfolders. You can target specific partitions using the filepath function. This example shows fare amounts by year, month, and payment_type for the first three months of 2017.
The serverless SQL pool query is compatible with Hive/Hadoop partitioning scheme.
SELECT YEAR(tpepPickupDateTime), passengerCount, COUNT(*) AS cnt FROM OPENROWSET( BULK 'puYear=*/puMonth=*/*.snappy.parquet', DATA_SOURCE = 'YellowTaxi', FORMAT='PARQUET' ) nyc WHERE nyc.filepath(1) = 2017 AND nyc.filepath(2) IN (1, 2, 3) AND tpepPickupDateTime BETWEEN CAST('1/1/2017' AS datetime) AND CAST('3/31/2017' AS datetime) GROUP BY passengerCount, YEAR(tpepPickupDateTime) ORDER BY YEAR(tpepPickupDateTime), passengerCount;
For Parquet type mapping to SQL native type check type mapping for Parquet.
Advance to the next article to learn how to Query Parquet nested types.
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