Sankar Hariappan commented on HIVE-18864:

Attached 01.patch with changes in logic to build ValidWriteIdList.
 # First find writeIdHwm based on txnHwm. This could be the writeId allocated 
by txnHwm itself or max(writeId) allocated by txnId<txnHwm.
 # Now, get all txns with write Ids <= writeIdHwm.
 ## If any txnId >txnHwm, then move them to open/invalid list.
 ## If any txnId is marked open/aborted in ValidtxnList, then move them also to 
open/invalid list.

Request [~ekoifman] to take a look!

> ValidWriteIdList snapshot seems incorrect if obtained after allocating 
> writeId by current transaction.
> ------------------------------------------------------------------------------------------------------
>                 Key: HIVE-18864
>                 URL: https://issues.apache.org/jira/browse/HIVE-18864
>             Project: Hive
>          Issue Type: Sub-task
>          Components: Transactions
>    Affects Versions: 3.0.0
>            Reporter: Sankar Hariappan
>            Assignee: Sankar Hariappan
>            Priority: Major
>              Labels: ACID
>             Fix For: 3.0.0
>         Attachments: HIVE-18864.01.patch
> For multi-statement txns, it is possible that write on a table happens after 
> a read. Let's see the below scenario.
>  # Committed txn=9 writes on table T1 with writeId=5.
>  # Open txn=10. ValidTxnList(open:null, txn_HWM=10),
>  # Read table T1 from txn=10. ValidWriteIdList(open:null, write_HWM=5).
>  # Open txn=11, writes on table T1 with writeid=6.
>  # Read table T1 from txn=10. ValidWriteIdList(open:null, write_HWM=5).
>  # Write table T1 from txn=10 with writeId=7.
>  # Read table T1 from txn=10. {color:#d04437}*ValidWriteIdList(open:null, 
> write_HWM=7)*. – This read will able to see rows added by txn=11 which is 
> still open.{color}
> {color:#d04437}So, it is needed to rebuild the open/aborted list of 
> ValidWriteIdList based on txn_HWM. Any writeId allocated by txnId > txn_HWM 
> should be marked as open. In this example, *ValidWriteIdList(open:6, 
> write_HWM=7)* should be generated.{color}
> {color:#333333}cc{color} [~ekoifman], [~thejas]

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