how to know if the sql will run a seq scan

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.

/*
postgres=# \d t
Table "public.t"
Column | Type | Collation | Nullable | Default
--------+---------+-----------+----------+---------
col1 | integer | | |

postgres=# insert into t select 0 from generate_series(1, 1000000) x;
INSERT 0 1000000

-- this does a full seq scan as new constraint
postgres=# alter table t add constraint col1c check ( col1 < 2 );
ALTER TABLE
-- this will not since the table has valid constraint to make it think only
worry about changed data ?
postgres=# insert into t values (3);
ERROR: new row for relation "t" violates check constraint "col1c"
DETAIL: Failing row contains (3).

-- the below setup making use of not valid and validate constraint still
runs a seq scan but does not block writes
postgres=# alter table t add constraint col1c_not_neg check ( col1 > -1 )
not valid; ALTER TABLE
postgres=# select relname,seq_scan,last_seq_scan, age(last_seq_scan,
current_timestamp), seq_tup_read from pg_stat_user_tables where relname =
't';
-[ RECORD 1 ]-+------------------------------
relname | t
seq_scan | 7
last_seq_scan | 2024-10-15 19:34:46.837628+00
age | -00:06:46.030264
seq_tup_read | 4000000

postgres=# alter table t validate constraint col1c_not_neg;
ALTER TABLE
postgres=# select relname,seq_scan,last_seq_scan, age(last_seq_scan,
current_timestamp), seq_tup_read from pg_stat_user_tables where relname =
't';
-[ RECORD 1 ]-+------------------------------
relname | t
seq_scan | 8
last_seq_scan | 2024-10-15 19:41:50.931282+00
age | -00:00:01.85388
seq_tup_read | 5000000

postgres=# -- now i dont want this seq scan, so i update the pg_constraint
(ok we dont do this but i want to trace seq scans)

postgres=# alter table t drop constraint col1c_not_neg;
ALTER TABLE
postgres=# select relname,seq_scan,last_seq_scan, age(last_seq_scan,
current_timestamp), seq_tup_read from pg_stat_user_tables where relname =
't';
-[ RECORD 1 ]-+------------------------------
relname | t
seq_scan | 8
last_seq_scan | 2024-10-15 19:41:50.931282+00
age | -00:00:21.980611
seq_tup_read | 5000000

postgres=# alter table t add constraint col1c_not_neg check ( col1 > -1 )
not valid; ALTER TABLE
postgres=# select oid from pg_constraint where conrelid =
't'::regclass::oid and convalidated = 'f'; -[ RECORD
1 ]
oid | 16410

-- i save a seq scan in validate constraint because i know my data. (like
in attaching partitions etc) by updating the catalog directly

postgres=# update pg_constraint set convalidated = 't' where conrelid =
't'::regclass::oid and convalidated = 'f' and oid = 16410;
UPDATE 1

postgres=# select relname,seq_scan,last_seq_scan, age(last_seq_scan,
current_timestamp), seq_tup_read from pg_stat_user_tables where relname =
't';
-[ RECORD 1 ]-+------------------------------
relname | t
seq_scan | 8
last_seq_scan | 2024-10-15 19:41:50.931282+00
age | -00:05:14.066944
seq_tup_read | 5000000

but how do i log this seq scan here for this sql.
*/

if this does not make sense, pls ignore. not critical.
--
Thanks,
Vijay

Open to work
Resume - Vijaykumar Jain <https://github.com/cabecada>