Tuesday, August 21, 2012

Performance tuning of hive queries

Hive performance optimization is a larger topic on its own and is very specific to the queries you are using. Infact each query in a query file needs separate performance tuning to get the most robust results.

I'll try to list a few approaches in general used for performance optimization

Limit the data flow down the queries
When you are on a hive query the volume of data that flows each level down is the factor that decides performance. So if you are executing a script that contains a sequence of hive QL, make sure that the data filtration happens on the first few stages rather than bringing unwanted data to bottom. This will give you significant performance numbers as the queries down the lane will have very less data to crunch on.

This is a common bottle neck when some existing SQL jobs are ported to hive, we just try to execute the same sequence of SQL steps in hive as well which becomes a bottle neck on the performance. Understand the requirement or the existing SQL script and design your hive job considering data flow

Use hive merge files
Hive queries are parsed into map only and map reduce job. In a hive script there will lots of hive queries. Assume one of your queries is parsed to a mapreduce job and the output files from the job are very small, say 10 mb. In such a case the subsequent query that consumes this data may generate more number of map tasks and would be inefficient. If you have more jobs on the same data set then all the jobs will get inefficient. In such scenarios if you enable merge files in hive, the first query would run a merge job at the end there by merging small files into  larger ones. This is controlled
using the following parameters

hive.merge.mapfiles=true (true by default in hive)

For more control over merge files you can tweak these properties as well
hive.merge.size.per.task (the max final size of a file after the merge task)
hive.merge.smallfiles.avgsize (the merge job is triggered only if the average output filesizes is less than the specified value)

The default values for the above properties are

When you enable merge an extra map only job is triggered, whether this job gets you an optimization or an over head is totally dependent on your use case or the queries.

Join Optimizations
Joins are very expensive.Avoid it if possible. If it is required try to use join optimizations as map joins, bucketed map joins etc

There is still more left on hive query performance optimization, take this post as the baby step. More tobe added on to this post and will be addded soon . :)

How to migrate a hive table from one hive instance to another or between hive databases

Hive has the EXPORT IMPORT feature since hive 0.8. With this feature you can export the metadata as well as the data for a corresponding table to a file in hdfs using the EXPORT command. The data is stored in json format. Data once exported this way could be imported back to another database or hive instance using the IMPORT command.

The syntax looks something like this:
EXPORT TABLE table_or_partition TO hdfs_path;
IMPORT [[EXTERNAL] TABLE table_or_partition] FROM hdfs_path [LOCATION [table_location]];

Some sample statements would look like:
EXPORT TABLE <table name> TO 'location in hdfs';

Use test_db;
IMPORT FROM 'location in hdfs';

Export Import can be appled on a partition basis as well:
EXPORT TABLE <table name> PARTITION (loc="USA") to 'location in hdfs';

The below import commands imports to an external table instead of a managed one
IMPORT EXTERNAL TABLE FROM 'location in hdfs' LOCATION ‘/location/of/external/table’;