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https://issues.apache.org/jira/browse/HIVE-18885?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16390780#comment-16390780
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anishek commented on HIVE-18885:
--------------------------------

[~akolb] notfications are not created in the loop with the below statement 
{code}
        msdb.alterPartition(dbname, name, part.getValues(), part);
{code}

the above part of *HiveAlterHandler.alterTable*. Notifications are created at 
the end of all the operations so locks will be taken towards the end just 
before commit. as you said for a large query there may be lot of notifications 
that need to be created and that will take a lock(towards the end near commit) 
and lock dbNotification sequence till that transaction commits. 

For replication we depend on, given a timeline with transaction commits, time 
ordered, then notifications w.r.t to these transactions have to follow the same 
order in their sequence, this was achieved by the use of lock.


> Cascaded alter table + notifications = disaster
> -----------------------------------------------
>
>                 Key: HIVE-18885
>                 URL: https://issues.apache.org/jira/browse/HIVE-18885
>             Project: Hive
>          Issue Type: Bug
>          Components: Hive, Metastore
>    Affects Versions: 3.0.0
>            Reporter: Alexander Kolbasov
>            Priority: Major
>
> You can see the problem from looking at the code, but it actually created 
> severe problems for real life Hive user.
> When {{alter table}} has {{cascade}} option it does the following:
> {code:java}
>          msdb.openTransaction()
>           ...
>           List<Partition> parts = msdb.getPartitions(dbname, name, -1);
>           for (Partition part : parts) {
>             List<FieldSchema> oldCols = part.getSd().getCols();
>             part.getSd().setCols(newt.getSd().getCols());
>             String oldPartName = 
> Warehouse.makePartName(oldt.getPartitionKeys(), part.getValues());
>             updatePartColumnStatsForAlterColumns(msdb, part, oldPartName, 
> part.getValues(), oldCols, part);
>             msdb.alterPartition(dbname, name, part.getValues(), part);
>           }
>  {code}
> So it walks all partitions (and this may be huge list) and does some 
> non-trivial operations in one single uber-transaction.
> When DbNotificationListener is enabled, it adds an event for each partition, 
> all while
> holding a row lock on NOTIFICATION_SEQUENCE table. As a result, while this is 
> happening no other write DDL can proceed. This can sometimes cause DB lock 
> timeouts which cause HMS level operation retries which make things even worse.
> In one particular case this pretty much made HMS unusable.



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