var_substitution.xml

/tags/release-0.1-rc2/hive/external/docs/xdocs/language_manual/var_substitution.xml

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<document>

  <properties>
    <title>Hadoop Hive- Variable Substitution</title>
    <author email="hive-user@hadoop.apache.org">Hadoop Hive Documentation Team</author>
  </properties>

  <body>
<h3>Hive Variable Substitution</h3>
<section name="Introduction" href="Introduction">

<p>Hive is used for both interactive queries as well as part. The hive variable substitution mechanism was 
designed to avoid some of the code that was getting baked into the scripting language ontop of hive. For example:</p>

<source><![CDATA[$ a=b
$ hive -e " describe $a "
]]></source>

<p>
are becoming common place. This is frustrating as hive becomes closely coupled with scripting languages. The hive
startup time of a couple seconds is non-trivial when doing thousands of manipulations multiple hive -e invocations.</p>

<p>
Hive Variables combine the set capability you know and love with some limited yet powerful (evil laugh) substitution 
ability. For example:</p>

<source><![CDATA[$ bin/hive -hiveconf a=b -e 'set a; set hiveconf:a; \
create table if not exists b (col int); describe ${hiveconf:a}'
]]></source>

<p>Results in:</p>
<source><![CDATA[Hive history file=/tmp/edward/hive_job_log_edward_201011240906_1463048967.txt
a=b
hiveconf:a=b
OK
Time taken: 5.913 seconds
OK
col	int	
Time taken: 0.754 seconds
]]></source>

</section>

<section name="Using variables" href="using_variables">

<p>There are three namespaces for variables hiveconf,system, and env. hiveconf variables are set as normal:</p>

<source><![CDATA[set x=myvalue
]]></source>

<p>However they are retrieved using</p>

<source><![CDATA[${hiveconf:x}
]]></source>
 
<p>Annotated examples of usage from the test case ql/src/test/queries/clientpositive/set_processor_namespaces.q</p>

<source><![CDATA[set zzz=5;
--  sets zzz=5
set zzz;

set system:xxx=5;
set system:xxx;
-- sets a system property xxx to 5

set system:yyy=${system:xxx};
set system:yyy;
-- sets yyy with value of xxx

set go=${hiveconf:zzz};
set go;
-- sets go base on value on zzz

set hive.variable.substitute=false;
set raw=${hiveconf:zzz};
set raw;
-- disable substitution set a value to the literal

set hive.variable.substitute=true;

EXPLAIN SELECT * FROM src where key=${hiveconf:zzz};
SELECT * FROM src where key=${hiveconf:zzz};
--use a variable in a query

set a=1;
set b=a;
set c=${hiveconf:${hiveconf:b}};
set c;
--uses nested variables. 


set jar=../lib/derby.jar;

add file ${hiveconf:jar};
list file;
delete file ${hiveconf:jar};
list file;
]]></source>
</section>

<section name="Disabling" href="disable">
  <p>Variable substitution is on by default. If this causes an issue with an already existing script disable it.</p>

<source><![CDATA[set hive.variable.substitute=false;
]]></source>

</section>

</body>
</document>