I have a table in postgresql named 'views', containing information about users viewing a classified ad.
CREATE TABLE views (
view_id uuid DEFAULT random_gen_uuid() NOT NULL,
user_id uuid NOT NULL,
ad_id uuid NOT NULL,
timestamp timestamp with time zone DEFAULT 'NOW()' NOT NULL
);
I want to be able to insert a row for a specific user/ad ONLY when there is no other row 'younger' than 5 minutes. So I want to check if there already is a row with the user ID and the ad ID and where the timestamp is less than 5 minutes old. If so, I want to do something like INSERT... ON CONFLICT DO NOTHING.
Is this possible to do with a UNIQUE constraint? Or do I need a CHECK constraint, or do I have to do a separate query first every time I insert this?
You have to do a lookup first, but you can do the lookup and the insert in one statement using something like this:
with invars (user_id, ad_id) as (
values (?, ?) -- Pass your two ids in
)
insert into views (user_id, ad_id)
select user_id, ad_id
from invars i
where not exists (select 1
from views
where (user_id, ad_id) = (i.user_id, i.ad_id)
and "timestamp" >= now() - interval '5 minutes');
Related
I have an array with some ids on the front-end and I'm writing a query to check if I can find any user with a user_id inside the array, the table is very simple:
CREATE TABLE user_accounts (
user_id UUID NOT NULL REFERENCES users (id) ON DELETE CASCADE,
name TEXT,
connector_type connector_type_enum NOT NULL,
created_at TIMESTAMPTZ DEFAULT NOW() NOT NULL,
updated_at TIMESTAMPTZ DEFAULT NOW() NOT NULL
);
The array on the front-end is something like ["328hdgyas78g2", "3adhdayas7453"]
and here's the query:
SELECT *
FROM user_accounts
WHERE user_id IN $1
of course there's a syntax error, I have tried the following:
SELECT *
FROM user_accounts
WHERE user_id = ANY(ARRAY["328hdgyas78g2", "3adhdayas7453"])
but I get the following error:
ERROR: column "328hdgyas78g2" does not exist
How can I fix the query?
That's because the " is used to reference a column or table name. To reference a string you use the single '
You have a couple of options there
WHERE user_id = ANY(ARRAY['328hdgyas78g2', '3adhdayas7453'])
or
WHERE user_id = ANY('{328hdgyas78g2,3adhdayas7453}')
Also, good reference is this
Given the following table:
CREATE TABLE event_partitioned (
customer_id varchar(50) NOT NULL,
user_id varchar(50) NOT NULL,
event_id varchar(50) NOT NULL,
comment varchar(50) NOT NULL,
event_timestamp timestamp with time zone DEFAULT NOW()
)
PARTITION BY RANGE (event_timestamp);
And partitioning by calendar week [one example]:
CREATE TABLE event_partitioned_2020_51 PARTITION OF event_partitioned
FOR VALUES FROM ('2020-12-14') TO ('2020-12-20');
And the unique constraint [event_timestamp necessary since the partition key]:
ALTER TABLE event_partitioned
ADD UNIQUE (customer_id, user_id, event_id, event_timestamp);
I would like to update if customer_id, user_id, event_id exist, otherwise insert:
INSERT INTO event_partitioned (customer_id, user_id, event_id)
VALUES ('9', '99', '999')
ON CONFLICT (customer_id, user_id, event_id, event_timestamp) DO UPDATE
SET comment = 'I got updated';
But I cannot add a unique constraint only for customer_id, user_id, event_id, hence event_timestamp as well.
So this will insert duplicates of customer_id, user_id, event_id. Even so with adding now() as a fourth value, unless now() precisely matches what's already in event_timestamp.
Is there a way that ON CONFLICT could be less 'granular' here and update if now() falls in the week of the partition, rather than precisely on '2020-12-14 09:13:04.543256' for example?
Basically I am trying to avoid duplication of customer_id, user_id, event_id, at least within a week, but still benefit from partitioning by week (so that data retrieval can be narrowed to a date range and not scan the entire partitioned table).
I don't think you can do this with on conflict in a partitioned table. You can, however, express the logic with CTEs:
with
data as ( -- data
select '9' as customer_id, '99' as user_id, '999' as event_id
),
ins as ( -- insert if not exists
insert into event_partitioned (customer_id, user_id, event_id)
select * from data d
where not exists (
select 1
from event_partitioned ep
where
ep.customer_id = d.customer_id
and ep.user_id = d.user_id
and ep.event_id = d.event_id
)
returning *
)
update event_partitioned ep -- update if insert did not happen
set comment = 'I got updated'
from data d
where
ep.customer_id = d.customer_id
and ep.user_id = d.user_id
and ep.event_id = d.event_id
and not exists (select 1 from ins)
#GMB's answer is great and works well. Since enforcing a unique constrain on a partitioned table (parent table) partitioned by time range is usually not that useful, why now just have a unique constraint/index placed on the partition itself?
In your case, event_partitioned_2020_51 can have a unique constraint:
ALTER TABLE event_partitioned_2020_51
ADD UNIQUE (customer_id, user_id, event_id, event_timestamp);
And subsequent query can just use
INSERT ... INTO event_partitioned_2020_51 ON CONFLICT (customer_id, user_id, event_id, event_timestamp)
as long as this its the partition intended, which is usually the case.
I have a demo table
CREATE TABLE items (
id SERIAL primary key,
user_id integer,
name character varying,
created timestamp with time zone default now()
);
And I want a single query to run and first insert data, then return primary key using returning id and then update the same table with the returned id.
INSERT INTO items (name) values ('pen') RETURNING id as idd
update items set user_id=(select idd) where id=(select idd)
but the above command doesn't work and throws syntax error.
Any help will be appriciated.
You can do that right within the INSERT statement:
INSERT INTO items
(name, user_id)
values
('pen', currval(pg_get_serial_sequence('items','id')));
Online example
You can try this way also :
create temp table insert_item as
with insert_item_cte as (
INSERT INTO items (name)
values ('pen') returning id
)
select id from insert_item_cte;
update items set user_id = items.id
from insert_item ii
where ii.id = items.id;
Online Demo
I have two tables, connected in E/R by a is-relation. One representing the "mother table"
CREATE TABLE PERSONS(
id SERIAL NOT NULL,
name character varying NOT NULL,
address character varying NOT NULL,
day_of_creation timestamp NOT NULL DEFAULT current_timestamp,
PRIMARY KEY (id)
)
the other representing the "child table"
CREATE TABLE EMPLOYEES (
id integer NOT NULL,
store character varying NOT NULL,
paychecksize integer NOT NULL,
FOREIGN KEY (id)
REFERENCES PERSONS(id),
PRIMARY KEY (id)
)
Now those two tables are joined in a view
CREATE VIEW EMPLOYEES_VIEW AS
SELECT
P.id,name,address,store,paychecksize,day_of_creation
FROM
PERSONS AS P
JOIN
EMPLOYEES AS E ON P.id = E.id
I want to write either a rule or a trigger to enable a db user to make an insert on that view, sparing him the nasty details of the splitted columns into different tables.
But I also want to make it convenient, as the id is a SERIAL and the day_of_creation has a default value there is no actual need that a user has to provide those, therefore a statement like
INSERT INTO EMPLOYEES_VIEW (name, address, store, paychecksize)
VALUES ("bob", "top secret", "drugstore", 42)
should be enough to result in
PERSONS
id|name|address |day_of_creation
-------------------------------
1 |bob |top secret| 2013-08-13 15:32:42
EMPLOYEES
id| store |paychecksize
---------------------
1 |drugstore|42
A basic rule would be easy as
CREATE RULE EMPLOYEE_VIEW_INSERT AS ON INSERT TO EMPLOYEE_VIEW
DO INSTED (
INSERT INTO PERSONS
VALUES (NEW.id,NEW.name,NEW.address,NEW.day_of_creation),
INSERT INTO EMPLOYEES
VALUES (NEW.id,NEW.store,NEW.paychecksize)
)
should be sufficient. But this will not be convenient as a user will have to provide the id and timestamp, even though it actually is not necessary.
How can I rewrite/extend that code base to match my criteria of convenience?
Something like:
CREATE RULE EMPLOYEE_VIEW_INSERT AS ON INSERT TO EMPLOYEES_VIEW
DO INSTEAD
(
INSERT INTO PERSONS (id, name, address, day_of_creation)
VALUES (default,NEW.name,NEW.address,default);
INSERT INTO EMPLOYEES (id, store, paychecksize)
VALUES (currval('persons_id_seq'),NEW.store,NEW.paychecksize)
);
That way the default values for persons.id and persons.day_of_creation will be the default values. Another option would have been to simply remove those columns from the insert:
INSERT INTO PERSONS (name, address)
VALUES (NEW.name,NEW.address);
Once the rule is defined, the following insert should work:
insert into employees_view (name, address, store, paychecksize)
values ('Arthur Dent', 'Some Street', 'Some Store', 42);
Btw: with a current Postgres version an instead of trigger is the preferred way to make a view updateable.
I'm using PostgreSQL 9.0 and I have a table with just an artificial key (auto-incrementing sequence) and another unique key. (Yes, there is a reason for this table. :)) I want to look up an ID by the other key or, if it doesn't exist, insert it:
SELECT id
FROM mytable
WHERE other_key = 'SOMETHING'
Then, if no match:
INSERT INTO mytable (other_key)
VALUES ('SOMETHING')
RETURNING id
The question: is it possible to save a round-trip to the DB by doing both of these in one statement? I can insert the row if it doesn't exist like this:
INSERT INTO mytable (other_key)
SELECT 'SOMETHING'
WHERE NOT EXISTS (SELECT * FROM mytable WHERE other_key = 'SOMETHING')
RETURNING id
... but that doesn't give the ID of an existing row. Any ideas? There is a unique constraint on other_key, if that helps.
Have you tried to union it?
Edit - this requires Postgres 9.1:
create table mytable (id serial primary key, other_key varchar not null unique);
WITH new_row AS (
INSERT INTO mytable (other_key)
SELECT 'SOMETHING'
WHERE NOT EXISTS (SELECT * FROM mytable WHERE other_key = 'SOMETHING')
RETURNING *
)
SELECT * FROM new_row
UNION
SELECT * FROM mytable WHERE other_key = 'SOMETHING';
results in:
id | other_key
----+-----------
1 | SOMETHING
(1 row)
No, there is no special SQL syntax that allows you to do select or insert. You can do what Ilia mentions and create a sproc, which means it will not do a round trip fromt he client to server, but it will still result in two queries (three actually, if you count the sproc itself).
using 9.5 i successfully tried this
based on Denis de Bernardy's answer
only 1 parameter
no union
no stored procedure
atomic, thus no concurrency problems (i think...)
The Query:
WITH neworexisting AS (
INSERT INTO mytable(other_key) VALUES('hello 2')
ON CONFLICT(other_key) DO UPDATE SET existed=true -- need some update to return sth
RETURNING *
)
SELECT * FROM neworexisting
first call:
id|other_key|created |existed|
--|---------|-------------------|-------|
6|hello 1 |2019-09-11 11:39:29|false |
second call:
id|other_key|created |existed|
--|---------|-------------------|-------|
6|hello 1 |2019-09-11 11:39:29|true |
First create your table ;-)
CREATE TABLE mytable (
id serial NOT NULL,
other_key text NOT NULL,
created timestamptz NOT NULL DEFAULT now(),
existed bool NOT NULL DEFAULT false,
CONSTRAINT mytable_pk PRIMARY KEY (id),
CONSTRAINT mytable_uniq UNIQUE (other_key) --needed for on conflict
);
you can use a stored procedure
IF (SELECT id FROM mytable WHERE other_key = 'SOMETHING' LIMIT 1) < 0 THEN
INSERT INTO mytable (other_key) VALUES ('SOMETHING')
END IF
I have an alternative to Denis answer, that I think is less database-intensive, although a bit more complex:
create table mytable (id serial primary key, other_key varchar not null unique);
WITH table_sel AS (
SELECT id
FROM mytable
WHERE other_key = 'test'
UNION
SELECT NULL AS id
ORDER BY id NULLS LAST
LIMIT 1
), table_ins AS (
INSERT INTO mytable (id, other_key)
SELECT
COALESCE(id, NEXTVAL('mytable_id_seq'::REGCLASS)),
'test'
FROM table_sel
ON CONFLICT (id) DO NOTHING
RETURNING id
)
SELECT * FROM table_ins
UNION ALL
SELECT * FROM table_sel
WHERE id IS NOT NULL;
In table_sel CTE I'm looking for the right row. If I don't find it, I assure that table_sel returns at least one row, with a union with a SELECT NULL.
In table_ins CTE I try to insert the same row I was looking for earlier. COALESCE(id, NEXTVAL('mytable_id_seq'::REGCLASS)) is saying: id could be defined, if so, use it; whereas if id is null, increment the sequence on id and use this new value to insert a row. The ON CONFLICT clause assure
that if id is already in mytable I don't insert anything.
At the end I put everything together with a UNION between table_ins and table_sel, so that I'm sure to take my sweet id value and execute both CTE.
This query needs to search for the value other_key only once, and is a "search this value" not a "check if this value not exists in the table", that is very heavy; in Denis alternative you use other_key in both types of searches. In my query you "check if a value not exists" only on id that is a integer primary key, that, for construction, is fast.
Minor tweak a decade late to Denis's excellent answer:
-- Create the table with a unique constraint
CREATE TABLE mytable (
id serial PRIMARY KEY
, other_key varchar NOT NULL UNIQUE
);
WITH new_row AS (
-- Only insert when we don't find anything, avoiding a table lock if
-- possible.
INSERT INTO mytable ( other_key )
SELECT 'SOMETHING'
WHERE NOT EXISTS (
SELECT *
FROM mytable
WHERE other_key = 'SOMETHING'
)
RETURNING *
)
(
-- This comes first in the UNION ALL since it'll almost certainly be
-- in the query cache. Marginally slower for the insert case, but also
-- marginally faster for the much more common read-only case.
SELECT *
FROM mytable
WHERE other_key = 'SOMETHING'
-- Don't check for duplicates to be removed
UNION ALL
-- If we reach this point in iteration, we needed to do the INSERT and
-- lock after all.
SELECT *
FROM new_row
) LIMIT 1 -- Just return whatever comes first in the results and allow
-- the query engine to cut processing short for the INSERT
-- calculation.
;
The UNION ALL tells the planner it doesn't have to collect results for de-duplication. The LIMIT 1 at the end allows the planner to short-circuit further processing/iteration once it knows there's an answer available.
NOTE: There is a race condition present here and in the original answer. If the entry does not already exist, the INSERT will fail with a unique constraint violation. The error can be suppressed with ON CONFLICT DO NOTHING, but the query will return an empty set instead of the new row. This is a difficult problem because getting that info from another transaction would violate the I in ACID.