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From Young-Seok Kim <>
Subject Re: Delete transactions
Date Sat, 25 Jun 2016 00:09:57 GMT
Please see inline below.


On Fri, Jun 24, 2016 at 4:35 PM, Mike Carey <> wrote:

> So to clarify, record-level consistency (and primary/secondary index
> consistency) is guaranteed and will work "correctly" in all cases if a
> record R is updated (or deleted) by T2 after being targeted (by primary
> key) for deletion by T1.

Yes, agreed on.

> The only semantic issue is that there is a (hopefully very, very small)
> window of time between when T1 sees R in a secondary index and when it
> acquires for the lock on R's primary key - during which T2 could change R
> in a way that makes it no longer query-compliant.

Here, let me clarify the above sentence: "when it acquires for the lock on
R's primary key" means that the lock is acquired and released by T1 since
the lock was an instant shared-mode(read) lock. So, T2 can change R after
acquiring an exclusive lock and consequently the R is not qualified for the
query predicate anymore.

> (However, at its time of being observed - which happened under
> read-committed - it was a correct candidate for deletion.  So this is kind
> of "expected" but admittedly kind of weird.  It seems like this could maybe
> be fixed in the future via a mechanism similar to the index-only branch's
> way of handling locks?)

This expected but undesired situation can be avoided by introducing a
version number which will be stored as a field (which is not exposed to
users) in each entry of the primary index and the secondary indexes such
that the version number can be used to verify that the record searched
during the search phase is the same record to be deleted during the delete
phase. If the verification is succeeded, the delete will be performed.
Otherwise, it will not.

> On 6/24/16 10:59 AM, Young-Seok Kim wrote:
>> This is somewhat expected issue by having read-committed isolation level
>> based on strict 2PL locking protocol.
>> The strict 2PL guarantees that all acquired exclusive locks by a
>> transaction can be released after the transaction is committed.
>> But, read lock doesn't follow this.
>> So, as you described in the email, a record read by a transaction, T1
>> during search can be modified by another transaction T2 before the record
>> is deleted by T1.  This is a possible situation under the read-committed
>> isolation level.
>> However, there is no inconsistency between a primary index and secondary
>> indexes in the way that the modified record by T2 is deleted by T1 from
>> the
>> primary index and the corresponding secondary index entry may not be
>> deleted by T1. This is because when T1 starts deleting process through the
>> job pipeline, an exclusive lock for the record is first acquired and then
>> the delete operations in primary and secondary indexes are performed. So,
>> either case1) the record should exist with the identical primary key for
>> the record to be deleted by T1 (since the search will deliver the primary
>> key, not the complete record) or case2) the record will not be deleted by
>> T1 if the record with the primary key does not exist.
>> For case1), once a record is deleted from the primary index, all rest of
>> secondary indexes in the job pipeline correctly find and delete the
>> corresponding secondary index entries.
>> For case2), I need to check the behavior whether the job pipeline throws
>> an
>> exception due to trying to delete the non-existing record and stops
>> proceeding the job by aborting the job, or the exception is just swallowed
>> and the job proceeds for the next record.
>> Best,
>> Young-Seok
>> On Fri, Jun 24, 2016 at 10:14 AM, abdullah alamoudi <>
>> wrote:
>> Hi everyone,
>>> I think we have a problem related to the deletes transaction
>>> behavior:here
>>> is the problem:
>>> Our delete starts by searching the tree to identify delete tuples based
>>> on
>>> the delete statement conditional clause. It follows that by inserting
>>> delete tuples in primary index, followed by updating secondary indexes,
>>> followed by a commit on the PK
>>> The problem happens if after searching the tree and identifying the
>>> records
>>> to be deleted, one of those records was updated. This will cause the
>>> record
>>> to be deleted in the primary index even though it might not meet the
>>> conditional clause. Moreover, the new entries in the secondary indexes
>>> will
>>> remain without their record in the primary index.
>>> In order to fix this, we need to do one of the following:
>>> 1. lock the records when we do the search to identify the records to be
>>> deleted
>>> OR
>>> 2. when performing the delete, we double check that the record we're
>>> deleting is the same as the record we find when we do the actual delete
>>> A better way would be to perform the delete as we do the search since
>>> there
>>> is no need to do the whole search, materialize then perform the delete.
>>> There is a change I got something wrong. Did I? Thoughts?

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