Strict mode of hive performance tuning (4)

Homo sapiens 2021-01-22 18:06:44
strict mode hive performance tuning


Hive Provides a strict pattern , It can prevent users from executing “ High-risk ” Query for .

By setting properties hive.mapred.mode The value is the default non-strict mode nonstrict . Turning strict mode on requires modification hive.mapred.mode The value is strict, Turning strict mode on can be disabled 3 Types of queries .

<property>
<name>hive.mapred.mode</name>
<value>strict</value>
<description>
The mode in which the Hive operations are being performed.
In strict mode, some risky queries are not allowed to run. They include:
Cartesian Product.
No partition being picked up for a query.
Comparing bigints and strings.
Comparing bigints and doubles.
Orderby without limit.
</description>
</property>

<1> For partitioned tables , Users are not allowed to scan all partitions , Unless where The statement contains a partition field filter condition to limit the scope , Otherwise, execution is not allowed . The reason for this restriction is , Partitioned tables typically have very large data sets , And the Numbers are growing fast . Queries that are not partitioned can consume unacceptably large resources to process this table .

<2> For the order by Query of statement , Be required to use limit sentence . because order by In order to perform the sorting process all the resulting data is distributed to the same Reducer Intermediate processing , Force the user to add this LIMIT Statements prevent Reducer Extra execution for a long time .

<3> Queries that restrict cartesian products . Users who know a lot about relational databases might expect to execute JOIN Not used when querying ON Statement instead USES where sentence , This allows the relational database's execution optimizer to efficiently WHERE Convert the statement to that ON sentence . Unfortunately ,Hive This optimization is not performed , therefore , If the watch is big enough , Then the query will get out of control .

That's all for this sharing , Those who benefit from it should pay attention to it ٩(๑>◡<๑)۶

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