Postgresql – Will FK ON UPDATE CASCADE work

constraintforeign keypostgresql

Intuitively, adding the option ON UPDATE CASCADE to a foreign key constraint will have the effect of updating all referencing columns with the updated value of the key. Would that really work?

Example lifted from manual:

CREATE TABLE products (
    product_no integer PRIMARY KEY,
    name text,
    price numeric
);

CREATE TABLE orders (
    order_id integer PRIMARY KEY,
    shipping_address text,
    ...
);

CREATE TABLE order_items (
    product_no integer REFERENCES products ON DELETE RESTRICT,
    ## HERE'S THE LINE THAT I MODIFIED FROM THE MANUAL, IT USED TO SAY 'ON DELETE CASCADE'
    order_id integer REFERENCES orders ON UPDATE CASCADE,
    quantity integer,
    PRIMARY KEY (product_no, order_id)
);

I am using Postgres 9.1.

Best Answer

Possible options for the ON DELETE and ON UPDATE clauses are explained in the CREATE TABLE page of the manual.

The default is NO ACTION, so if you have any foreign keys defined without explicitly specifying what action to be performed on updates of the referenced primary or unique key, it is the same as if ON UPDATE NO ACTION was specified:

NO ACTION

Produce an error indicating that the deletion or update would create a foreign key constraint violation. If the constraint is deferred, this error will be produced at constraint check time if there still exist any referencing rows. This is the default action.

If you have specified ON UPDATE CASCADE on a foreign key, the referencing rows will be updated accordingly when a referenced row is updated:

CASCADE

Delete any rows referencing the deleted row, or update the value of the referencing column to the new value of the referenced column, respectively.

Primary keys are usually not updated (and for good reasons) but there are situations where it can be useful. Plus, the foreign key may be referencing not a primary key but a (rarely updated) unique key.

Related Solutions

Sql-server – How to create a relationship to a non-primary key in SQL Server

Create a unique index or unique constraint on UserName then you can reference it in a FK constraint fine.

Your statement that

Sql Server doesn't allow me to create a relationship on a non primary key column

is incorrect. SQL Server only cares that the column(s) participating in the FK relationship have a unique index defined.

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

Sql-server – How to create a relationship to a non-primary key in SQL Server

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

pgAdmin timing

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