Re: Index seems "lost" after consecutive deletes

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.

>
>
> 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!!!

>
> 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!

Edson
>
> David J.
>