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BlinkDB User Guide

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6 June 2015


BlinkDB User Guide

BlinkDB supports a subset of SQL nearly identical to that implemented by Hive/Shark. This guide assumes you have some familiarity with Hive and Shark, and focuses on the extra functionality included in BlinkDB. Those who need a refresher can refer to the Hive Documentation and Shark Documentation

BlinkDB Query Language

BlinkDB is backwards compatible with Shark and supports all unmodified Shark queries. In addition to this, BlinkDB introduces a SAMPLEWITH operator that expects sampling ratio as a parameter and returns a statistical random sample of the original dataset with that sampling ratio. These samples can be created offline using the CTAS (Create Table as Select) operator. For instance, let us assume that we have a table called logs in our warehouse. A 1% random sample logs_sample of logs can be created as:

$ CREATE TABLE logs_sample AS SELECT * FROM logs SAMPLEWITH 0.01

NOTE: These samples can be created on any materialized view as well using the SAMPLEWITH operator. For instance, one can invoke the SAMPLEWITH operator on tables with WHERE clauses (SELECT * FROM logs WHERE src="hive" SAMPLEWITH 0.1), JOINS (SELECT * FROM logs_1 JOIN logs_2 ON (logs_1.id = logs_2.id) SAMPLEWITH 0.2), UNION ALL etc.

Any sample table can also be cached in memory using the shark.cache table property:

$ CREATE TABLE ... TBLPROPERTIES ("shark.cache" = "true") AS SELECT ...

Similar to Shark, BlinkDB supports a shortcut for caching a sample table in memory. Simply append _cached to the table name when using CREATE TABLE AS SELECT, and that table will be cached in memory. To disable this shortcut, see the Shark configuration options section. Below is an example:

$ CREATE TABLE logs_last_month_sample_cached AS
  SELECT * FROM logs WHERE time > date(...) SAMPLEWITH 0.01;

Once a sample is created, BlinkDB expects the user to specify the number of rows in the sample and the original dataset (which could be a native table or a materialized view) using the blinkdb.sample.size and blinkdb.dataset.size configuration operations. This allows BlinkDB to perform appropriate bias correction in calculating error bars and scale the output accordingly. The number of rows in the sample and original dataset can be found out by simply executing a COUNT(1) query on the sample and the original dataset.

$ set blinkdb.sample.size=13358102;
$ set blinkdb.dataset.size=1378291493;

Once this is done, users can execute any simple aggregation function (such as APPROX_AVG(), APPROX_COUNT or APPROX_SUM) on the sampled dataset. The returned answer would be an approximation of the original and will be augmented by appropriate error bars at 99% confidence, i.e. so say, that 99% of the time, the true answer should lie between Answer +/- Error. Below are few examples:

$ SELECT APPROX_COUNT(1) FROM logs_sample WHERE src="hive";
$ SELECT dt, APPROX_AVG(foo) FROM logs_sample GROUP BY dt;
$ SELECT dt, APPROX_SUM(bar) FROM logs_sample GROUP BY dt;

BlinkDB CLI

The easiest way to run BlinkDB is to start a BlinkDB Command Line Client (CLI) and begin executing queries. The BlinkDB CLI connects directly to the Hive Metastore, so it is compatible with existing Hive deployments. BlinkDB executables are available in the bin/ directory. To start the BlinkDB CLI, simply run:

$ ./bin/blinkdb                            # Start CLI for interactive session
$ ./bin/blinkdb -e "SELECT * FROM foo"     # Run a specific query and exit
$ ./bin/blinkdb -i queries.hql             # Run queries from a file
$ ./bin/blinkdb -H                         # Start CLI and print help

You can enter queries into the CLI directly, or use a flag to pass it a file. The BlinkDB CLI will only work correctly if the HIVE_HOME environment variable is set (see Configuration). Alternative versions of the CLI exist which print out more information: bin/blinkdb-withinfo and bin/blinkdb-withdebug.

Configuration Options

Configuration variables are environment vars that must be set for the BlinkDB driver and slaves to run correctly. These are specified in conf/blinkdb-env.sh. A few of the more important ones are described here:

HIVE_HOME     # Path to directory containing patched Hive jars
HIVE_CONF_DIR # Optional, a different path containing Hive configuration files 
SPARK_MEM     # How many much memory to allocate for slaves (e.g '1500m', '5g')

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