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

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.*)

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