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

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.klaver@aklaver.com