parquet

Syntax

extractParquetSchema(fileName)

Details

The method returns the schema of the Parquet file, with two columns: column names and data types.

Parameters

• fileName: a STRING scalar indicating the Parquet file name.

Examples

parquet::extractParquetSchema("userdata1.parquet")

Syntax

loadParquet(fileName,[schema],[column],[rowGroupStart],[rowGroupNum])

Details

The method imports a Parquet file to a DolphinDB in-memory table. Regarding data type conversion, refer to "Data Type Mappings".

Parameters

• fileName: A STRING scalar indicating the Parquet file name.
• schema (optional): A table with the column names and their data types. Specify the parameter to modify the data types of the columns generated by the system.
• column (optional): A vector of integers indicating the column index to be imported. All columns will be read if it is not specified.
• rowGroupStart (optional): A non-negative integer indicating the index of the row group from which the data import starts. The file will be read from the beginning if it is not specified.
• rowGroupNum (optional): An integer indicating the number of row groups to be read. The file will be read from the beginning if it is not specified.

Examples

parquet::loadParquet("userdata1.parquet")

Syntax

loadParquetEx(dbHandle,tableName,partitionColumns,fileName,[schema],[column],[rowGroupStart],[rowGroupNum],[tranform])

Details

The method loads a Parquet file to a DolphinDB partitioned table and returns a table object with metadata of the table.

• If dbHandle is specified and is not an empty string "": load the file to a DFS database.
• If dbHandle is an empty string "" or unspecified: load the file to a partitioned in-memory table.

Regarding data type conversion, refer to "Data Type Mappings".

Parameters

• dbHandle: A database handle.
• tableName: A string indicating the table name.
• partitionColumns: A STRING scalar or vector indicating the partitioning column(s). For a composite partition, it is a vector.
• fileName: a STRING scalar indicating the Parquet file name.
• schema (optional): A table with the column names and their data types. Specify the parameter to modify the data types of the columns generated by the system.
• column (optional): A vector of integers indicating the column index to be imported. All columns will be read if it is not specified.
• rowGroupStart (optional): A non-negative integer indicating the index of the row group from which the data import starts. The file will be read from the beginning if it is not specified.
• rowGroupNum (optional): An integer indicating the number of row groups to be read. The file will be read from the beginning if it is not specified.
• tranform (optional): A unary function which takes a table as input. If it is specified, a partitioned table must be created before loading the file. The method will first apply the specified function to the data, and then save the result to the partitioned table.

Examples

• Import to a partitioned DFS table

db = database("dfs://rangedb", RANGE, 0 500 1000)
parquet::loadParquetEx(db,`tb,`id,"userdata1.parquet")

• Import to a partitioned in-memory table

db = database("", RANGE, 0 500 1000)
parquet::loadParquetEx(db,`tb,`id,"userdata1.parquet")

• Specify the parameter transform to transform the default data type (e.g. 20200101) to a specific type (e.g. DATE)

dbPath="dfs://DolphinDBdatabase"
db=database(dbPath,VALUE,2020.01.01..2020.01.30)
dataFilePath="level.parquet"
schemaTB=parquet::extractParquetSchema(dataFilePath)
update schemaTB set type="DATE" where name="date"
tb=table(1:0,schemaTB.name,schemaTB.type)
tb1=db.createPartitionedTable(tb,`tb1,`date);
def i2d(mutable t){
    return t.replaceColumn!(`date,datetimeParse(t.date),"yyyy.MM.dd"))
}
t = parquet::loadParquetEx(db,`tb1,`date,dataFilePath,datasetName,,,,i2d)

Syntax

parquetDS(fileName,[schema])

Details

The method creates data sources based on the input file name. The number of tables is the same as the number of row groups.

Parameters

• fileName: A STRING scalar indicating the Parquet file name.
• schema (optional): A table with the column names and their data types. Specify the parameter to modify the data types of the columns generated by the system.

Examples

ds = parquet::parquetDS("userdata1.parquet")
size ds;
// Output: 1
ds[0];
// Output: DataSource< loadParquet("userdata1.parquet",,,0,1) >

Syntax

saveParquet(table, fileName, [compression])

Details

The method exports a DolphinDB table to a Parquet file.

Parameters

• table: The table to be exported.
• fileName: A STRING scalar indicating the Parquet file name to save the table
• compression (optional): A STRING scalar indicating the compression mode. It supports “snappy”, “gzip”, “zstd” and the default is no compression.

Examples

parquet::saveParquet(tb, "userdata1.parquet")

Syntax

setReadThreadNum(num)

Details

The method enables the concurrency level for reading Parquet files and the maximum number of threads to use.

Note: The actual concurrency is influenced by worker due to the plugin's internal use of the ploop function for column-wise parallel reading.

Parameters

• num: Maximum number of read threads.
  • 1 (default): no additional threads are created, and the Parquet file is read in the current thread.
  • Greater than 1: The task of reading the Parquet file is divided into num parts, allowing up to num threads.
  • 0: Each column read will be treated as a separate task.

Examples

parquet::setReadThreadNum(0)

Syntax

getReadThreadNum()

Details

The method retrieves the maximum number of read threads for the Parquet plugin.

Examples

parquet::getReadThreadNum()

When a Parquet file is imported to DolphinDB, the data types are converted based on the LogicalType as annotated in the file. If the LogicalType or ConvertedType is not defined, the conversion will be performed based on the physical types.

Note:

• Conversion of the Parquet repeated fields is not supported.
• DECIMAL can be used to convert data of the following physical types: INT32, INT64 and FIXED_LEN_BYTE_ARRAY.
• DolphinDB does not support unsigned data types. Therefore, in case of UINT_64 overflow when loading a Parquet file, the data will be converted to NULL values in DolphinDB.

When exporting data from DolphinDB to a Parquet file, the system will convert the data types to Parquet types based on the given table schema.