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From Sergey Shelukhin <>
Subject Re: sql mapjoin very slow
Date Fri, 28 Aug 2015 01:56:46 GMT
Is the small-side table large, does it have a lot of rows for the same keys, or does it have
a lot of skew?
Are there lots of misses (where there’d be no value in the small table for the large table

If you have enough memory you can try increasing initial size and decreasing load factor.
Although without low-level debugging it’s hard to tell if the issue is not obvious (I.e
the above).
If there’s no obvious problem you might consider not using map join.

From: "<>" <<>>
Reply-To: user <<>>
Date: Thursday, August 27, 2015 at 18:51
To: user <<>>
Subject: Re: Re: sql mapjoin very slow

I use MR.
My mapjoin config as showed in follow picture:


From: Sergey Shelukhin<>
Date: 2015-08-28 09:21
To: user<>
Subject: Re: sql mapjoin very slow
Are you using MR and Tez? You could try optimized hash table in case of Tez, although it’s
supposed to improve memory, not necessarily perf.

Can you also share characteristics of the query and data? It is surprising to see so much
time for HashMap.get.

From: "<>" <<>>
Reply-To: user <<>>
Date: Thursday, August 27, 2015 at 18:03
To: user <<>>
Subject: sql mapjoin very slow

When I enable mapjoin ,I see Mapjoin task run very slow. My envrioment is hadoop 2.3.0 hive

My attach is  one map hive log and this map's xprof log.

In map xprof log ,I see
Compiled + native Method
92.3% 643527 + 0 java.util.HashMap.get
2.8% 19856 + 0 java.util.HashMap.put
1.2% 8623 + 0 org.apache.hadoop.hive.ql.exec.persistence.HashMapWrapper$GetAdaptor.setFromRow
0.1% 953 + 0 org.apache.hadoop.hive.ql.exec.ExprNodeColumnEvaluator._evaluate
0.1% 576 + 0 org.apache.hadoop.hive.serde2.objectinspector.ObjectInspectorUtils.copyToStandardObject

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