Re: Index seems "lost" after consecutive deletes

Em 14/06/2016 10:32, David G. Johnston escreveu:
> On Tue, Jun 14, 2016 at 12:51 AM, Edson Richter
> <edsonrichter(at)hotmail(dot)com <mailto:edsonrichter(at)hotmail(dot)com>>wrote:
>
> Em 14/06/2016 01:33, David G. Johnston escreveu:
>> On Monday, June 13, 2016, Edson Richter <edsonrichter(at)hotmail(dot)com
>> <mailto:edsonrichter(at)hotmail(dot)com>> wrote:
>>
>> Em 13/06/2016 23:36, Edson Richter escreveu:
>>> Em 13/06/2016 23:18, rob stone escreveu:
>>>> On Mon, 2016-06-13 at 22:41 -0300, Edson Richter wrote:
>>>>> Em 13/06/2016 22:33, Edson Richter escreveu:
>>>>>> I've a table "A" with 4,000,000 records.
>>>>>>
>>>>>> I've decided to delete records from oldest to newest but I can't
>>>>>> delete records that have references in tables "B", "C" or "D".
>>>>>>
>>>>>>
>>>>>> so, I've
>>>>>>
>>>>>>
>>>>>> with qry as (
>>>>>>
>>>>>> select A.id
>>>>>>
>>>>>> from A
>>>>>>
>>>>>> where not exists (select 1 from B where B.a_id = A.id)
>>>>>>
>>>>>> and not exists (select 1 from C where C.a_id = A.id)
>>>>>>
>>>>>> and not exists (select 1 from D where D.a_id = A.id)
>>>>>>
>>>>>> and A.creation_date < (now()::date - interval '12 month')
>>>>>>
>>>>>> order by A.id DESC
>>>>>>
>>>>>> limit 2000
>>>>>>
>>>>>> )
>>>>>>
>>>>>> delete from A where id in (select id from qry);
>>>>>>
>>>>>>
>>>>>> All three referenced tables have indexes (B.a_id; C.a_id; D.a_id)
>>>>>> in
>>>>>> order to make query faster.
>>>>>>
>>>>>> So for first 2 million rows it worked really well, taking about 1
>>>>>> minute to delete each group of 2000 records.
>>>>>>
>>>>>> Then, after a while I just started to get errors like:
>>>>>>
>>>>>>
>>>>>> ​​
>>>>>> ERROR: update or delete in "A" violates foreign key
>>>>>> ​​
>>>>>> "fk_C_A" in
>>>>>> "C".
>>>>>>
>>>>>> DETAIL: Key (id)=(3240124) is still referenced by table "C".
>>>>>>
>>>>>>
>>>>>> Seems to me that indexes got lost in the path - the query is
>>>>>> really
>>>>>> specific and no "C" referenced records can be in my deletion.
>>>>>>
>>>>>> Has anyone faced a behavior like this?
>>>>>>
>>>>>> Am I doing something wrong?
>>>>>>
>>>>>>
>>>>> Of course:
>>>>> Version string PostgreSQL 9.4.8 on x86_64-unknown-linux-gnu,
>>>>> compiled
>>>>> by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-4), 64-bit
>>>>> Oracle Linux 7 x64 with all updates. Running on EXT4 file system.
>>>>> Computer is Dell R420 with mirrored disks, 80GB of RAM (database has
>>>>> <
>>>>> 40GB in total).
>>>>>
>>>>> Sorry for not putting the info in the first e-mail.
>>>>>
>>>>> Edson
>>>>>
>>>>>
>>>> What does:-
>>>>
>>>> SELECT COUNT(*) FROM C WHERE C.a_id = 3240124;
>>>>
>>>> return?
>>>>
>>>> Is it a many-to-one or a one-to-one relationship?
>>>>
>>>>
>>>
>>> SELECT COUNT(*) FROM C WHERE C.a_id = 3240124;
>>> count
>>> -------
>>> 1
>>> (1 registro)
>>>
>>>
>>> A.id is primary key of A table. Each table has its own
>>> primary key.
>>>
>>> Relationship to others table is 1-N, being N = {0,1}
>>>
>>>
>>> A.id -> B.a_id (being B.a_id unique but not enforced by
>>> unique key)
>>>
>>> A.id -> C.a_id (being C.a_id unique but not enforced by
>>> unique key)
>>>
>>> A.id -> D.a_id (being D.a_id unique but not enforced by
>>> unique key)
>>>
>>>
>>> Regards,
>>>
>>> Edson
>>
>> Just in case, I've run:
>>
>> - vacuum full analyze verbose;
>> - reindex index ix_c_a_id;
>>
>> Result I get same error. So, I'm inclined to discard that
>> this is a index error.
>>
>>
>> Interesting:
>>
>> with qry as (select A.id
>> from A
>> where creatingdate < (now()::date - interval '12 month')
>> and not exists (select 1 from B where B.a_id = A.id)
>> and not exists (select 1 from C where C.a_id = A.id)
>> and not exists (select 1 from D where D.a_id = A.id)
>> order by A.id limit 2000)
>>
>> select * from qry where id = 3240124;
>>
>>
>> Why do you assume 3240124 is within the first 2000 qualified
>> records that the CTE is limited to checking?
>
> Because this is the key causing the error on delete.
>
>
> ​Wasn't the point though I do suspect your expectations are acceptable
> in this instance. If you truly want to see if qry contains 3240124
> you should lose the LIMIT 2000.​
>

Running the risk to deviate the focus, if records are ordered in the
query, limiting them will always produce same result.

>
>
>>
>>
>> Total query runtime: 2.2 secs
>> 0 rows retrieved.
>>
>>
>> Why delete causes error, but querying don't?
>>
>>
>> Given the error message this answer seems self-evident...
>
> Not self-evident to me: delete says I'm trying to delete the
> record with id = 3240124 and I can't. But the select says this
> record with id = 3240124 is not there!!!
>
>
> ​This error:
> ​ERROR: update or delete in "A" violates foreign key "fk_C_A" in
> ​ ​
> "C".
> is impossible to encounter when executing a pure select...

Yes, but is also impossible to get this error if the record is not in
the subquery results. That's why I've executed the query filtering
id=3240124.
If this record is not in the subquery, why does the "delete..." is
trying to remove it?

>
>>
>> Would it be a bug when using delete ... where id in (subquery)?
>>
>> I'm unsure regarding the solution but I suspect the problem is
>> that between selecting the A row and deleting it another
>> concurrent process added a record to C that, if you were to
>> re-run the select would cause the row from A to be skipped. But
>> the single query doesn't have that option so it ends up failing.
>>
>> There is a FOR UPDATE clause you can add to the select but I
>> don't think that works here since table C is the one being
>> altered and at the time of the query there is nothing to lock.
>>
>> I'm doubting this is a bug, just poor concurrency understanding.
>> Sorry I cannot enlighten further at the moment.
>
> There is not other process adding/updating records - and even
> there is, the ID would be so high (because it is a sequence) that
> cannot be the 3240124 (this record has been generated more than 2
> years ago - that's why my query has the "creationdate" filter - I
> don't touch records created within last 12 months).
>
> Also, I've tried the same with "for update": same error!
>
>
> ​Have you confirmed that
> ​
> "fk_C_A" is referencing the columns you think it is?

Yes, first thing. All references are maintained by a automated system.
If the relation is not there, then it will be automatically created.
But your question raised another interesting line of investigation: if
there is any other cascading foreign keys pointing to A table.
Until now, I've been concentrated in the related tables, but would be
possible that another FK is cascading, which in turn would have another
cascade that is causing the error.

>
> ​What's the history of this machine? Did you pass through 9.3
> (especially early releases) on your way to 9.4?

Started with 9.0, then 9.1, 9.2, 9.3 and now 9.4.
Nevertheless, for every migration I've used a "dump" and "restore" to
avoid the "upgrade" caveats.
For example, when migrating from 9.3 to 9.4, I've used "9.4" pg_dump to
create the dump, and then "9.4" pg_restore to restore it in the new cluster.

Edson

>
> David J.
>