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

On 10/15/24 13:50, Vijaykumar Jain wrote:
> 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.

Now you have changed the problem description.

To get a proper answer you will need to provide a more detailed
description of what you are doing with the following information:

1) Postgres version.

2) Definition of 'large'.

3) The command/process being used to create the partition.

4) The actual constraint definition.

5) The table definition.

> 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
> <mailto: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
> <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
> <https://github.com/cabecada>>
>
> --
> Adrian Klaver
> adrian(dot)klaver(at)aklaver(dot)com <mailto:adrian(dot)klaver(at)aklaver(dot)com>
>

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com