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Keeping data compressed in Hive tables has, in some cases, been known to give better performance that than uncompressed storage; both , in terms of disk usage and query performance.
You can import text files compressed with Gzip or Bzip2 directly into a table stored as TextFile. The compression will be detected automatically and the file will be decompressed on-the-fly during query execution. For example:
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CREATE TABLE raw (line STRING)
ROW FORMAT DELIMITED FIELDS TERMINATED BY '\t' LINES TERMINATED BY '\n';
LOAD DATA LOCAL INPATH '/tmp/weblogs/20090603-access.log.gz' INTO TABLE raw;
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The table 'raw' is stored as a TextFile, which is the default storage. However, in this case Hadoop will not be able to split your file into chunks/blocks and run multiple maps in parallel. This can cause under-utilization underutilization of your cluster's 'mapping' power.
The recommended practice is to insert data into another table, which is stored as a SequenceFile. A SequenceFile can be split by Hadoop and distributed across map jobs (is this statement correct?)whereas a GZIP file cannot be. For example:
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CREATE TABLE raw (line STRING) ROW FORMAT DELIMITED FIELDS TERMINATED BY '\t' LINES TERMINATED BY '\n'; CREATE TABLE raw_sequence (line STRING) ROW FORMAT DELIMITED FIELDS TERMINATED BY '\t' LINES TERMINATED BY '\n' STORED AS SEQUENCEFILE; LOAD DATA LOCAL INPATH '/tmp/weblogs/20090603-access.log.gz' INTO TABLE raw; SET hive.exec.compress.output=true; SET io.seqfile.compression.type=BLOCK; -- NONE/RECORD/BLOCK (see below) INSERT OVERWRITE TABLE raw_sequence SELECT LINE FROM raw; INSERT OVERWRITE TABLE raw_sequence SELECT * FROM raw; -- The previous line did not work for me, but this does. |
The value for io.seqfile.compression.type determines how the compression is performed. If you set it to RECORD you will get as many output files as the number of map/reduce jobs. If you set it to BLOCK, you will get as many output files as there were input files. There is a tradeoff involved here – large number of output files => more parellel map jobs => lower compression ratio.
aRecord compresses each value individually while BLOCK buffers up 1MB (default) before doing compression.
LZO Compression
See LZO Compression for information about using LZO with Hive.