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Query reexecution provides a facility to re-run the query multiple times in case of an unfortunate event happens.


Introduced in Hive 3.0 (HIVE-17626)

ReExecition strategies

Overlay 

Enables to change the hive settings for all reexecutions which will be happening. It works by adding a configuration subtree as an overlay to the actual hive settings(reexec.overlay.*)

Example
set zzz=1;
set reexec.overlay.zzz=2;

set hive.query.reexecution.enabled=true;
set hive.query.reexecution.strategies=overlay;

create table t(a int);
insert into t values (1);
select assert_true(${hiveconf:zzz} > a) from t group by a;

Every hive setting which has a prefix of "reexec.overlay" will be set for all reexecutions.

A more real life example would be to disable join auto conversion for all reexecutions:

set reexec.overlay.hive.auto.convert.join=false;

Reoptimize

During query execution; the actual number passing rows in every operator is tracked. This information is reused during re-planning which could result in a better plan.

Situation in which this would be needed:

  • missing statististics
  • incorrect statistics
  • many joins

It's not that easy to craft queries which will lead to OOM situations; but to enable it:

set hive.query.reexecution.strategies=overlay,reoptimize;

Operator Matching

Operator level statistics are matched to the new plan using operator subtree matching this also enables to match the information to a query which have "similar" parts.

Configuration 


Configurationdefault
hive.query.reexecution.enabledtrueFeature enabler
hive.query.reexecution.strategiesoverlay,reoptimize

reexecution plugins; currently overlay and reoptimize is supported

hive.query.reexecution.stats.persist.scopequery

runtime statistics can be persisted:

  • query: - only used during the reexecution
  • hiveserver: persisted during the lifetime of the hiveserver
  • metastore: persisted in the metastore; and loaded on hiveserver startu[
hive.query.reexecution.max.count1

number of reexecution that may happen

hive.query.reexecution.always.collect.operator.statsfalse

Enable to gather runtime statistics on all queries.

hive.query.reexecution.stats.cache.batch.size-1If runtime stats are stored in metastore; the maximal batch size per round during load.
hive.query.reexecution.stats.cache.size100 000Size of the runtime statistics cache. Unit is: OperatorStat entry; a query plan consist ~100.
runtime.stats.clean.frequency3600sFrequency at which timer task runs to remove outdated runtime stat entries.
runtime.stats.max.age3daysStat entries which are older than this are removed.




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