How Impala Works with Hadoop File Formats
Impala supports several familiar file formats used in Apache Hadoop. Impala can load and query data files produced by other Hadoop components such as Spark, and data files produced by Impala can be used by other components also. The following sections discuss the procedures, limitations, and performance considerations for using each file format with Impala.
The file format used for an Impala table has significant performance consequences. Some file formats include compression support that affects the size of data on the disk and, consequently, the amount of I/O and CPU resources required to deserialize data. The amounts of I/O and CPU resources required can be a limiting factor in query performance since querying often begins with moving and decompressing data. To reduce the potential impact of this part of the process, data is often compressed. By compressing data, a smaller total number of bytes are transferred from disk to memory. This reduces the amount of time taken to transfer the data, but a tradeoff occurs when the CPU decompresses the content.
For the file formats that Impala cannot write to, create the table from within Impala whenever possible and insert data using another component such as Hive or Spark. See the table below for specific file formats.
The following table lists the file formats that Impala supports.
|File Type||Format||Compression Codecs||Impala Can CREATE?||Impala Can INSERT?|
|Parquet||Structured||Snappy, gzip; currently Snappy by default||Yes.||Yes: CREATE TABLE, INSERT, LOAD DATA, and query.|
|ORC||Structured||gzip, Snappy, LZO, LZ4; currently gzip by default||
The ORC support is an experimental feature since CDH 6.1 / Impala 3.1 & Impala 2.12.
To disable it, set ‑‑enable_orc_scanner to false when starting the cluster.
|No. Import data by using LOAD DATA on data files already in the right format, or use INSERT in Hive followed by REFRESH table_name in Impala.|
|Text||Unstructured||LZO, gzip, bzip2, Snappy||Yes. For CREATE TABLE with no STORED AS clause, the default file format is uncompressed text, with values separated by ASCII 0x01 characters (typically represented as Ctrl-A).||Yes: CREATE TABLE, INSERT, LOAD DATA, and query. If LZO compression is used, you must create the table and load data in Hive. If other kinds of compression are used, you must load data through LOAD DATA, Hive, or manually in HDFS.|
|Avro||Structured||Snappy, gzip, deflate||Yes, in Impala 1.4.0 and higher. In lower versions, create the table using Hive.||No. Import data by using LOAD DATA on data files already in the right format, or use INSERT in Hive followed by REFRESH table_name in Impala.|
|RCFile||Structured||Snappy, gzip, deflate, bzip2||Yes.||No. Import data by using LOAD DATA on data files already in the right format, or use INSERT in Hive followed by REFRESH table_name in Impala.|
|SequenceFile||Structured||Snappy, gzip, deflate, bzip2||Yes.|
Impala supports the following compression codecs:
Recommended for its effective balance between compression ratio and decompression speed. Snappy compression is very fast, but gzip provides greater space savings. Supported for text, RC, Sequence, and Avro files in Impala 2.0 and higher.
Recommended when achieving the highest level of compression (and therefore greatest disk-space savings) is desired. Supported for text, RC, Sequence and Avro files in Impala 2.0 and higher.
Not supported for text files.
Supported for text, RC, and Sequence files in Impala 2.0 and higher.
For text files only. Impala can query LZO-compressed text tables, but currently cannot create them or insert data into them. You need to perform these operations in Hive.
Choosing the File Format for a Table
Different file formats and compression codecs work better for different data sets. Choosing the proper format for your data can yield performance improvements. Use the following considerations to decide which combination of file format and compression to use for a particular table:
- If you are working with existing files that are already in a supported file format, use the same format for the Impala table if performance is acceptable. If the original format does not yield acceptable query performance or resource usage, consider creating a new Impala table with different file format or compression characteristics, and doing a one-time conversion by rewriting the data to the new table.
- Text files are convenient to produce through many different tools, and are human-readable for ease of verification and debugging. Those characteristics are why text is the default
format for an Impala CREATE TABLE statement. However, when performance and resource usage are the primary considerations, use one of the structured file formats that
include metadata and built-in compression.
A typical workflow might involve bringing data into an Impala table by copying CSV or TSV files into the appropriate data directory, and then using the INSERT ... SELECT syntax to rewrite the data into a table using a different, more compact file format.