Postgresql – Compound unique constraint with NULL values

check-constraintsconstraintpostgresqlunique-constraint

With the following table structure

CREATE TABLE favorite_comments(
   id SERIAL PRIMARY KEY NOT NULL,
   opinion_id BIGINT REFERENCES opinions(id),
   review_id BIGINT REFERENCES reviews(id),
   list_id INTEGER REFERENCES lists(id) NOT NULL
);

This are valid records (Review and opinion related to the same list):

id,  opinion_id,  review_id,  list_id
-------------------------------------
1,          333,       NULL,        4
2,         NULL,        444,        4

But i would like to prevent that same review or opinion can appear twice related to the same list:

id,  opinion_id,  review_id,  list_id
-------------------------------------
1,          333,       NULL,        4
2,         NULL,        444,        4
3,         NULL,        444,        4 <- WRONG

I would like to add a constraint that checks a conditional uniqueness, UNIQUE(opinion_id, list_id) OR UNIQUE(review_id, list_id), but since opinion_id,review_id can be NULL im having some trouble with this

Ive tried this but does not work

ALTER TABLE favorite_comments ADD CONSTRAINT comments_can_only_be_saved_once_per_list UNIQUE (opinion_id, review_id, list_id);

Best Answer

If you can identify a value that will never be stored in the referenced tables (e.g. a negative value), you can use a unique index that uses expressions:

create unique index 
  on favorite_comments( coalesce(opinion_id,-1), coalesce(review_id, -1), list_id );

the coalesce() will turn all null values in the same "real" value, forcing a unique constraint violation when you try to insert another NULL value.

The only drawback of a unique index compared to a unique constraint is, that the unique index can't be a target of a foreign key (while a unique constraint would be)

Asides

No use for mixed case identifiers without double quotes in PostgreSQL.

You might consider a serial column as primary key or an IDENTITY column in Postgres 10 or later. Related:

Auto increment table column

So:

CREATE TABLE my_table (
   my_table_id bigint GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY  -- for pg 10+
-- my_table_id bigserial PRIMARY KEY  -- for pg 9.6 or older
 , id_a int8 NOT NULL
 , id_b int8 NOT NULL
 , id_c int8
 , CONSTRAINT u_constraint UNIQUE (id_a, id_b, id_c)
);

If you don't expect more than 2 billion rows (> 2147483647) over the lifetime of your table (including waste and deleted rows), consider integer (4 bytes) instead of bigint (8 bytes).

Postgresql – Postgres multiple joins slow query, how to store default child record

You write:

Each customer can have multiple sites, but only one should be displayed in this list.

Yet, your query retrieves all rows. That would be a point to optimize. But you also do not define which site is to be picked.

Either way, it does not matter much here. Your EXPLAIN shows only 5026 rows for the site scan (5018 for the customer scan). So hardly any customer actually has more than one site. Did you ANALYZE your tables before running EXPLAIN?

From the numbers I see in your EXPLAIN, indexes will give you nothing for this query. Sequential table scans will be the fastest possible way. Half a second is rather slow for 5000 rows, though. Maybe your database needs some general performance tuning?

Maybe the query itself is faster, but "half a second" includes network transfer? EXPLAIN ANALYZE would tell us more.

If this query is your bottleneck, I would suggest you implement a materialized view.

After you provided more information I find that my diagnosis pretty much holds.

The query itself needs 27 ms. Not much of a problem there. "Half a second" was the kind of misunderstanding I had suspected. The slow part is the network transfer (plus ssh encoding / decoding, possibly rendering). You should only retrieve 100 rows, that would solve most of it, even if it means to execute the whole query every time.

If you go the route with a materialized view like I proposed you could add a serial number without gaps to the table plus index on it - by adding a column row_number() OVER (<your sort citeria here>) AS mv_id.

Then you can query:

SELECT *
FROM   materialized_view
WHERE  mv_id >= 2700
AND    mv_id <  2800;

This will perform very fast. LIMIT / OFFSET cannot compete, that needs to compute the whole table before it can sort and pick 100 rows.

pgAdmin timing

When you execute a query from the query tool, the message pane shows something like:

Total query runtime: 62 ms.

And the status line shows the same time. I quote pgAdmin help about that:

The status line will show how long the last query took to complete. If a dataset was returned, not only the elapsed time for server execution is displayed, but also the time to retrieve the data from the server to the Data Output page.

If you want to see the time on the server you need to use SQL EXPLAIN ANALYZE or the built in Shift + F7keyboard shortcut or Query -> Explain analyze. Then, at the bottom of the explain output you get something like this:

Total runtime: 0.269 ms

Best Answer

Related Solutions

PostgreSQL – Multi-Column Unique Constraint and NULL Values

Asides

Postgresql – Postgres multiple joins slow query, how to store default child record

pgAdmin timing

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