Postgresql – How to design a table/ unique index where uniqueness is temporal

postgresql

I have a table that records observed movies in a cinema:

CREATE TABLE cinema_movie_name (
  id integer DEFAULT nextval('cinema_movie_name_id_seq'::regclass) PRIMARY KEY,
  cinema_id integer NOT NULL REFERENCES cinema(id) ON DELETE CASCADE ON UPDATE RESTRICT,
  name text NOT NULL,
  movie_id integer REFERENCES movie(id) ON DELETE SET NULL ON UPDATE RESTRICT,
  created_at timestamp with time zone NOT NULL DEFAULT now()
);

Every time we see a new movie name in the cinema, we record it in this table and map it to movie_id in our database.

The current uniqueness is enforced on (cinema_id, name), i.e.

CREATE UNIQUE INDEX cinema_movie_name_cinema_id_name_idx ON cinema_movie_name(cinema_id, name);

The problem is that over a long time, there can be multiple different movies with the same name, i.e. after some arbitrary time duration (id, cinema_id, name, movie_id) becomes invalid.

Therefore, I need to create an index that enforces a unique constraint on (cinema_id, name) and created_at ±3 month.

How to create a unique index with a temporal column?

Best Answer

I would add a column is_current or something similar that indicates if the movie is a "current" one, then you can create a unique index on only the "current ones"

CREATE UNIQUE INDEX cinema_movie_name_cinema_id_name_idx 
    ON cinema_movie_name(cinema_id, name)
where is_current;

You can have a regular job that sets old movies to is_current = false

Related Solutions

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

Postgresql – Using an index for both uniqueness and fast lookup

You only need a single index, the unique one. Queries to lookup by username will use the unique index. There's no advantage to having a second index on the same column marked as non-unique.

You can verify that this by performing an EXPLAIN on the query. Connect to you database and compare the results of the following before and after you drop the index and confirm that they are the same:

EXPLAIN VERBOSE
SELECT *
FROM public.players
WHERE username = 'foobar'

In both cases it should show something like:

Index scan using using players_username_key on public.players (cost=...[truncated])
  Output: id, username, trophies ...

Best Answer

Related Solutions

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

pgAdmin timing

Postgresql – Using an index for both uniqueness and fast lookup

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