Using the ORC File Format with Impala Tables
Impala can read ORC data files as an experimental feature since Impala 3.1.
To enable the feature, set --enable_orc_scanner to true when starting the cluster.
|File Type||Format||Compression Codecs||Impala Can CREATE?||Impala Can INSERT?|
|ORC||Structured||gzip, Snappy, LZO, LZ4; currently gzip by default||Experimental support in Impala 3.1.0 and higher.||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.|
Creating ORC Tables and Loading Data
If you do not have an existing data file to use, begin by creating one in the appropriate format.
To create an ORC table:
In the impala-shell interpreter, issue a command similar to:
CREATE TABLE orc_table (column_specs) STORED AS ORC;
Because Impala can query some kinds of tables that it cannot currently write to, after creating tables of certain file formats, you might use the Hive shell to load the data. See How Impala Works with Hadoop File Formats for details. After loading data into a table through Hive or other mechanism outside of Impala, issue a REFRESH table_name statement the next time you connect to the Impala node, before querying the table, to make Impala recognize the new data.
For example, here is how you might create some ORC tables in Impala (by specifying the columns explicitly, or cloning the structure of another table), load data through Hive, and query them through Impala:
$ impala-shell -i localhost [localhost:21000] default> CREATE TABLE orc_table (x INT) STORED AS ORC; [localhost:21000] default> CREATE TABLE orc_clone LIKE some_other_table STORED AS ORC; [localhost:21000] default> quit; $ hive hive> INSERT INTO TABLE orc_table SELECT x FROM some_other_table; 3 Rows loaded to orc_table Time taken: 4.169 seconds hive> quit; $ impala-shell -i localhost [localhost:21000] default> SELECT * FROM orc_table; Fetched 0 row(s) in 0.11s [localhost:21000] default> -- Make Impala recognize the data loaded through Hive; [localhost:21000] default> REFRESH orc_table; [localhost:21000] default> SELECT * FROM orc_table; +---+ | x | +---+ | 1 | | 2 | | 3 | +---+ Fetched 3 row(s) in 0.11s
Enabling Compression for ORC Tables
ORC tables are in zlib (Deflate in Impala) compression in default. You may want to use Snappy or LZO compression on existing tables for different balance between compression ratio and decompression speed. In Hive-1.1.0, the supported compressions for ORC tables are NONE, ZLIB, SNAPPY and LZO. For example, to enable Snappy compression, you would specify the following additional settings when loading data through the Hive shell:
hive> SET hive.exec.compress.output=true; hive> SET orc.compress=SNAPPY; hive> INSERT OVERWRITE TABLE new_table SELECT * FROM old_table;
If you are converting partitioned tables, you must complete additional steps. In such a case, specify additional settings similar to the following:
hive> CREATE TABLE new_table (your_cols) PARTITIONED BY (partition_cols) STORED AS new_format; hive> SET hive.exec.dynamic.partition.mode=nonstrict; hive> SET hive.exec.dynamic.partition=true; hive> INSERT OVERWRITE TABLE new_table PARTITION(comma_separated_partition_cols) SELECT * FROM old_table;
Remember that Hive does not require that you specify a source format for it. Consider the case of converting a table with two partition columns called year and month to a Snappy compressed ORC table. Combining the components outlined previously to complete this table conversion, you would specify settings similar to the following:
hive> CREATE TABLE tbl_orc (int_col INT, string_col STRING) STORED AS ORC; hive> SET hive.exec.compress.output=true; hive> SET orc.compress=SNAPPY; hive> SET hive.exec.dynamic.partition.mode=nonstrict; hive> SET hive.exec.dynamic.partition=true; hive> INSERT OVERWRITE TABLE tbl_orc SELECT * FROM tbl;
To complete a similar process for a table that includes partitions, you would specify settings similar to the following:
hive> CREATE TABLE tbl_orc (int_col INT, string_col STRING) PARTITIONED BY (year INT) STORED AS ORC; hive> SET hive.exec.compress.output=true; hive> SET orc.compress=SNAPPY; hive> SET hive.exec.dynamic.partition.mode=nonstrict; hive> SET hive.exec.dynamic.partition=true; hive> INSERT OVERWRITE TABLE tbl_orc PARTITION(year) SELECT * FROM tbl;
Query Performance for Impala ORC Tables
In general, expect query performance with ORC tables to be faster than with tables using text data, but slower than with Parquet tables since there're bunch of optimizations for Parquet. See Using the Parquet File Format with Impala Tables for information about using the Parquet file format for high-performance analytic queries.
In CDH 5.8 / Impala 2.6 and higher, Impala queries are optimized for files stored in Amazon S3. For Impala tables that use the file formats Parquet, ORC, RCFile, SequenceFile, Avro, and uncompressed text, the setting fs.s3a.block.size in the core-site.xml configuration file determines how Impala divides the I/O work of reading the data files. This configuration setting is specified in bytes. By default, this value is 33554432 (32 MB), meaning that Impala parallelizes S3 read operations on the files as if they were made up of 32 MB blocks. For example, if your S3 queries primarily access Parquet files written by MapReduce or Hive, increase fs.s3a.block.size to 134217728 (128 MB) to match the row group size of those files. If most S3 queries involve Parquet files written by Impala, increase fs.s3a.block.size to 268435456 (256 MB) to match the row group size produced by Impala.
Data Type Considerations for ORC Tables
The ORC format defines a set of data types whose names differ from the names of the corresponding Impala data types. If you are preparing ORC files using other Hadoop components such as Pig or MapReduce, you might need to work with the type names defined by ORC. The following figure lists the ORC-defined types and the equivalent types in Impala.
BINARY -> STRING BOOLEAN -> BOOLEAN DOUBLE -> DOUBLE FLOAT -> FLOAT TINYINT -> TINYINT SMALLINT -> SMALLINT INT -> INT BIGINT -> BIGINT TIMESTAMP -> TIMESTAMP DATE (not supported)
Complex types are currently not supported on ORC. However, queries materializing only scalar type columns are allowed:
$ hive hive> CREATE TABLE orc_nested_table (id INT, a ARRAY<INT>) STORED AS ORC; hive> INSERT INTO TABLE orc_nested_table SELECT 1, ARRAY(1,2,3); OK Time taken: 2.629 seconds hive> quit; $ impala-shell -i localhost [localhost:21000] default> INVALIDATE METADATA orc_nested_table; [localhost:21000] default> SELECT 1 FROM orc_nested_table t, t.a; ERROR: NotImplementedException: Scan of table 't' in format 'ORC' is not supported because the table has a column 'a' with a complex type 'ARRAY<INT>'. Complex types are supported for these file formats: PARQUET. [localhost:21000] default> SELECT COUNT(*) FROM orc_nested_table; +----------+ | count(*) | +----------+ | 1 | +----------+ Fetched 1 row(s) in 0.12s [localhost:21000] default> SELECT id FROM orc_nested_table; +----+ | id | +----+ | 1 | +----+ Fetched 1 row(s) in 0.12s