Re: Index seems "lost" after consecutive deletes

[Edson Richter <edsonrichter@xxxxxxxxxxx>]

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