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From "Jesus Camacho Rodriguez (Jira)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-23725) ValidTxnManager snapshot outdating causing partial reads in merge insert
Date Tue, 25 Aug 2020 15:01:04 GMT

    [ https://issues.apache.org/jira/browse/HIVE-23725?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17184112#comment-17184112
] 

Jesus Camacho Rodriguez commented on HIVE-23725:
------------------------------------------------

[~pvargacl], could you create a follow-up to address [~kgyrtkirk] comments? It also seems
the final version of the patch that landed is not the same one that I reviewed.

> ValidTxnManager snapshot outdating causing partial reads in merge insert
> ------------------------------------------------------------------------
>
>                 Key: HIVE-23725
>                 URL: https://issues.apache.org/jira/browse/HIVE-23725
>             Project: Hive
>          Issue Type: Bug
>            Reporter: Peter Varga
>            Assignee: Peter Varga
>            Priority: Major
>              Labels: pull-request-available
>          Time Spent: 5h 40m
>  Remaining Estimate: 0h
>
> When the ValidTxnManager invalidates the snapshot during merge insert and starts to read
committed transactions that were not committed when the query compilation happened, it can
cause partial read problems if the committed transaction created new partition in the source
or target table.
> The solution should be not only fix the snapshot but also recompile the query and acquire
the locks again.
> You could construct an example like this:
> 1. open and compile transaction 1 that merge inserts data from a partitioned source table
that has a few partition.
> 2. Open, run and commit transaction 2 that inserts data to an old and a new partition
to the source table.
> 3. Open, run and commit transaction 3 that inserts data to the target table of the merge
statement, that will retrigger a snapshot generation in transaction 1.
> 4. Run transaction 1, the snapshot will be regenerated, and it will read partial data
from transaction 2 breaking the ACID properties.
> Different setup.
> Switch the transaction order:
> 1. compile transaction 1 that inserts data to an old and a new partition of the source
table.
> 2. compile transaction 2 that insert data to the target table
> 2. compile transaction 3 that merge inserts data from the source table to the target
table
> 3. run and commit transaction 1
> 4. run and commit transaction 2
> 5. run transaction 3, since it cointains 1 and 2 in its snaphot the isValidTxnListState
will be triggered and we do a partial read of the transaction 1 for the same reasons.



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