Thread: [SQL] TRUNCATE TABLE corrupts pg_class.relfilenode = pg_attrdef.pg_attrdef

Hi all,

Testing with Postgresql 9.6rc1 (but also detected with prior versions):

It is normal that a TRUNCATE TABLE statement changes the table/sequence relation in pg_attrdef?

test1=> create table mytab ( pkey serial, name varchar(10) );
test1=> select a.adsrc from pg_class p join pg_attrdef a on (p.relfilenode = a.adrelid) where lower(p.relname) =
lower('mytab');               adsrc
 
------------------------------------- nextval('mytab_pkey_seq'::regclass)
(1 row)

test1=> truncate table mytab;
TRUNCATE TABLE
test1=> select a.adsrc from pg_class p join pg_attrdef a on (p.relfilenode = a.adrelid) where lower(p.relname) =
lower('mytab');adsrc
 
-------
(0 rows)

test1=> select adrelid, adsrc from pg_attrdef where adsrc like '%mytab%'; adrelid |                adsrc
---------+------------------------------------- 6904163 | nextval('mytab_pkey_seq'::regclass)
(1 row)


Are we mis-using the condition (p.relfilenode = a.adrelid) in the first SELECT?

How can we easily check if a table is defined with a SERIAL type?

Thanks!
Seb

We will test with 9.6.3 too.

Seb

On 05/22/2017 12:51 PM, Sebastien FLAESCH wrote:
> Hi all,
>
> Testing with Postgresql 9.6rc1 (but also detected with prior versions):
>
> It is normal that a TRUNCATE TABLE statement changes the table/sequence relation in pg_attrdef?
>
> test1=> create table mytab ( pkey serial, name varchar(10) );
> test1=> select a.adsrc from pg_class p join pg_attrdef a on (p.relfilenode = a.adrelid) where lower(p.relname) =
lower('mytab');
>                 adsrc
> -------------------------------------
>  nextval('mytab_pkey_seq'::regclass)
> (1 row)
>
> test1=> truncate table mytab;
> TRUNCATE TABLE
> test1=> select a.adsrc from pg_class p join pg_attrdef a on (p.relfilenode = a.adrelid) where lower(p.relname) =
lower('mytab');
>  adsrc
> -------
> (0 rows)
>
> test1=> select adrelid, adsrc from pg_attrdef where adsrc like '%mytab%';
>  adrelid |                adsrc
> ---------+-------------------------------------
>  6904163 | nextval('mytab_pkey_seq'::regclass)
> (1 row)
>
>
> Are we mis-using the condition (p.relfilenode = a.adrelid) in the first SELECT?
>
> How can we easily check if a table is defined with a SERIAL type?
>
> Thanks!
> Seb
>
>

Seems that the correct join condition should be:
    (pg_class.oid = pg_attrdef.adrelid)

I don't know why we have used pg_class.relfilenode...

So this should be the correct SELECT statement:

test1=> select a.adsrc from pg_class p join pg_attrdef a on (p.oid = a.adrelid) where lower(p.relname) =
lower('mytab');               adsrc
 
------------------------------------- nextval('mytab_pkey_seq'::regclass)
(1 row)


Can someone confirm?

Thanks!
Seb


On 05/22/2017 12:51 PM, Sebastien FLAESCH wrote:
> Hi all,
>
> Testing with Postgresql 9.6rc1 (but also detected with prior versions):
>
> It is normal that a TRUNCATE TABLE statement changes the table/sequence relation in pg_attrdef?
>
> test1=> create table mytab ( pkey serial, name varchar(10) );
> test1=> select a.adsrc from pg_class p join pg_attrdef a on (p.relfilenode = a.adrelid) where lower(p.relname) =
lower('mytab');
>                 adsrc
> -------------------------------------
>  nextval('mytab_pkey_seq'::regclass)
> (1 row)
>
> test1=> truncate table mytab;
> TRUNCATE TABLE
> test1=> select a.adsrc from pg_class p join pg_attrdef a on (p.relfilenode = a.adrelid) where lower(p.relname) =
lower('mytab');
>  adsrc
> -------
> (0 rows)
>
> test1=> select adrelid, adsrc from pg_attrdef where adsrc like '%mytab%';
>  adrelid |                adsrc
> ---------+-------------------------------------
>  6904163 | nextval('mytab_pkey_seq'::regclass)
> (1 row)
>
>
> Are we mis-using the condition (p.relfilenode = a.adrelid) in the first SELECT?
>
> How can we easily check if a table is defined with a SERIAL type?
>
> Thanks!
> Seb
>
>

Obviously pg_class.relfilenode can change when truncating tables:

test1=> select relfilenode from pg_class where relname = 'mytab'; relfilenode
-------------     6904188
(1 row)


test1=> truncate table mytab;


TRUNCATE TABLE
test1=> select relfilenode from pg_class where relname = 'mytab'; relfilenode
-------------     6904189
(1 row)


Seb

On 05/22/2017 01:07 PM, Sebastien FLAESCH wrote:
> Seems that the correct join condition should be:
>
>     (pg_class.oid = pg_attrdef.adrelid)
>
> I don't know why we have used pg_class.relfilenode...
>
> So this should be the correct SELECT statement:
>
> test1=> select a.adsrc from pg_class p join pg_attrdef a on (p.oid = a.adrelid) where lower(p.relname) =
lower('mytab');
>                 adsrc
> -------------------------------------
>  nextval('mytab_pkey_seq'::regclass)
> (1 row)
>
>
> Can someone confirm?
>
> Thanks!
> Seb
>
>
> On 05/22/2017 12:51 PM, Sebastien FLAESCH wrote:
>> Hi all,
>>
>> Testing with Postgresql 9.6rc1 (but also detected with prior versions):
>>
>> It is normal that a TRUNCATE TABLE statement changes the table/sequence relation in pg_attrdef?
>>
>> test1=> create table mytab ( pkey serial, name varchar(10) );
>> test1=> select a.adsrc from pg_class p join pg_attrdef a on (p.relfilenode = a.adrelid) where lower(p.relname) =
lower('mytab');
>>                 adsrc
>> -------------------------------------
>>  nextval('mytab_pkey_seq'::regclass)
>> (1 row)
>>
>> test1=> truncate table mytab;
>> TRUNCATE TABLE
>> test1=> select a.adsrc from pg_class p join pg_attrdef a on (p.relfilenode = a.adrelid) where lower(p.relname) =
lower('mytab');
>>  adsrc
>> -------
>> (0 rows)
>>
>> test1=> select adrelid, adsrc from pg_attrdef where adsrc like '%mytab%';
>>  adrelid |                adsrc
>> ---------+-------------------------------------
>>  6904163 | nextval('mytab_pkey_seq'::regclass)
>> (1 row)
>>
>>
>> Are we mis-using the condition (p.relfilenode = a.adrelid) in the first SELECT?
>>
>> How can we easily check if a table is defined with a SERIAL type?
>>
>> Thanks!
>> Seb
>>
>>
>
>
>

Sebastien FLAESCH <sf@4js.com> writes:
> It is normal that a TRUNCATE TABLE statement changes the table/sequence relation in pg_attrdef?

It is normal for TRUNCATE, as well as several other kinds of DDL, to
change relfilenode.

> test1=> select a.adsrc from pg_class p join pg_attrdef a on (p.relfilenode = a.adrelid) where lower(p.relname) =
lower('mytab');

This query is simply wrong; the join column should be p.oid.  See the
system catalog definitions,

https://www.postgresql.org/docs/current/static/catalogs.html
        regards, tom lane