Re: how to know if the sql will run a seq scan

Sorry top posting, coz Gmail app on phone.

Yeah, my point was for example we have a large table and we are attaching a
table as a partition. Now it will scan the whole table to validate the
constraint and that will create all sorts of problems.
I understand the benefit of not valid constraint and then validating
constraint to reduce blocking.
But yeah monitoring locks for the statement should give me good enough hint
of what will happen.

Thanks for your reply. It helps.

On Wed, Oct 16, 2024, 1:54 AM Adrian Klaver <adrian(dot)klaver(at)aklaver(dot)com>
wrote:

> On 10/15/24 12:50, Vijaykumar Jain wrote:
> >
> > Hi,
> >
> > tl;dr
> > I am trying to learn what sql can result in a full seq scan.
> >
> > Basically there is a lot of info on the internet of what ddl change may
> > take an access exclusive lock while running a seq scan and hold for long.
> > And for some cases we can make use of
> > "not valid" constraint and then run a validate constraint as work
> > arounds to avoid long exclusive locks etc.
> > but how do we check the same. i mean for dmls there is a explain/
> > auto_explain.
> >
> > but for DDLs, how do we check the same.
> > i tried to isolate my setup and use pg_stat_user_tables and monitor the
> > same, which helped, but it is not useful as it does not link me to what
> > process/command invoked the seq scan.
> >
> > am i clear in my question ?
> >
> > if yes,
> > how do i log an alter table that may or may not do a seq scan, that may
> > or may not rewrite the table file on disk etc.
> > its a useless question, i am just playing with it for building
> > knowledge, no requirement as such.
>
> Look at the docs:
>
> https://www.postgresql.org/docs/current/sql-altertable.html
>
> "Scanning a large table to verify a new foreign key or check constraint
> can take a long time, and other updates to the table are locked out
> until the ALTER TABLE ADD CONSTRAINT command is committed. The main
> purpose of the NOT VALID constraint option is to reduce the impact of
> adding a constraint on concurrent updates. With NOT VALID, the ADD
> CONSTRAINT command does not scan the table and can be committed
> immediately. After that, a VALIDATE CONSTRAINT command can be issued to
> verify that existing rows satisfy the constraint. The validation step
> does not need to lock out concurrent updates, since it knows that other
> transactions will be enforcing the constraint for rows that they insert
> or update; only pre-existing rows need to be checked. Hence, validation
> acquires only a SHARE UPDATE EXCLUSIVE lock on the table being altered.
> (If the constraint is a foreign key then a ROW SHARE lock is also
> required on the table referenced by the constraint.) In addition to
> improving concurrency, it can be useful to use NOT VALID and VALIDATE
> CONSTRAINT in cases where the table is known to contain pre-existing
> violations. Once the constraint is in place, no new violations can be
> inserted, and the existing problems can be corrected at leisure until
> VALIDATE CONSTRAINT finally succeeds."
>
>
> > --
> > Thanks,
> > Vijay
> >
> > Open to work
> > Resume - Vijaykumar Jain <https://github.com/cabecada>
>
> --
> Adrian Klaver
> adrian(dot)klaver(at)aklaver(dot)com
>
>