Prevent consecutive duplicate values without a trigger - sql

Question

Within a group, I'd like to prevent INSERTs of consecutive duplicate values, where "consecutive" is defined by a simple ORDER BY clause.
Imagine a set of experiments which is regularly sampling values from a sensor. We only want to insert a value if it is new for that experiment.
Note that older values are allowed to be duplicates. So this is allowed:
id experiment value
1 A 10
2 A 20
3 A 10
but this is not:
id experiment value
1 A 10
2 A 10
I know how to find the previous value per experiment:
SELECT
*,
lag(sample_value) OVER experiment_and_id
FROM new_samples
WINDOW experiment_and_id AS (
PARTITION BY experiment
ORDER BY id
);
From the docs I know that CHECK constraints are not allowed to use other rows in their checking:
PostgreSQL does not support CHECK constraints that reference table data other than the new or updated row being checked. While a CHECK constraint that violates this rule may appear to work in simple tests, it cannot guarantee that the database will not reach a state in which the constraint condition is false (due to subsequent changes of the other row(s) involved). This would cause a database dump and reload to fail. The reload could fail even when the complete database state is consistent with the constraint, due to rows not being loaded in an order that will satisfy the constraint. If possible, use UNIQUE, EXCLUDE, or FOREIGN KEY constraints to express cross-row and cross-table restrictions.
If what you desire is a one-time check against other rows at row insertion, rather than a continuously-maintained consistency guarantee, a custom trigger can be used to implement that. (This approach avoids the dump/reload problem because pg_dump does not reinstall triggers until after reloading data, so that the check will not be enforced during a dump/reload.)
The EXCLUDE constraint looks promising, but is primarily for cases where the test is not equality. And I'm not sure if I can include window functions in there.
So I'm left with a custom trigger but this seems like a bit of a hack for what seems like a fairly common use case.
Can anyone improve on using a trigger?
Ideally, I'd like to be able to just say:
INSERT ....
ON CONFLICT DO NOTHING
and have Postgres deal with the rest!
Minimum working example
BEGIN;
CREATE TABLE new_samples (
id INT GENERATED ALWAYS AS IDENTITY,
experiment VARCHAR,
sample_value INT
);
INSERT INTO new_samples(experiment, sample_value)
VALUES
('A', 1),
-- This is fine because they are for different groups
('B', 1),
-- This is fine because the value has changed
('A', 2),
-- This is fine because it's different to the previous value in
-- experiment A.
('A', 1),
-- Two is not allowed here because it's the same as the value
-- before it, within this experiment.
('A', 1);
SELECT
*,
lag(sample_value) OVER experiment_and_id
FROM new_samples
WINDOW experiment_and_id AS (
PARTITION BY experiment
ORDER BY id
);
ROLLBACK;

Answer 1

If the samples will not change, then the restriction cited in the docs will not be relevant to your use case.
You can create a function to accomplish this:
create or replace function check_new_sample(_experiment text, _sample_value int)
returns boolean as
$$
select _sample_value != first_value(sample_value)
over (partition by experiment
order by id desc)
from new_samples
where experiment = _experiment;
$$ language sql;
alter table new_samples add constraint new_samples_ck_repeat
check (check_new_sample(experiment, sample_value));
Example inserts:
insert into new_samples (experiment, sample_value) values ('A', 1);
INSERT 0 1
insert into new_samples (experiment, sample_value) values ('B', 1);
INSERT 0 1
insert into new_samples (experiment, sample_value) values ('A', 2);
INSERT 0 1
insert into new_samples (experiment, sample_value) values ('A', 1);
INSERT 0 1
insert into new_samples (experiment, sample_value) values ('A', 1);
ERROR: new row for relation "new_samples" violates check constraint "new_samples_ck_repeat"
DETAIL: Failing row contains (5, A, 1).