Does "ON UPDATE" for foreign keys require index?

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Does "ON UPDATE" for foreign keys require index?

rihad

Hi. Say I have column A.b_id which references B.id (which is a primary key) and as such it is declared as a foreign key constraint. A.b_id has no index because it doesn't need one. What happens when table B's rows are modified (but never deleted)? Will PG still have have to scan A fo find A.b_id to do nothing with it? )) B.id itself is never modified, it's just a normal serial value typically used for id's.


The docs are a bit ambiguous:

Since a DELETE of a row from the referenced table or an UPDATE of a referenced column will require a scan of the referencing table for rows matching the old value, it is often a good idea to index the referencing columns too. Because this is not always needed, and there are many choices available on how to index, declaration of a foreign key constraint does not automatically create an index on the referencing columns.



https://www.postgresql.org/docs/9.6/ddl-constraints.html

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Re: Does "ON UPDATE" for foreign keys require index?

Alvaro Herrera-9
On 2019-Apr-25, rihad wrote:

> Hi. Say I have column A.b_id which references B.id (which is a primary key)
> and as such it is declared as a foreign key constraint. A.b_id has no index
> because it doesn't need one. What happens when table B's rows are modified
> (but never deleted)? Will PG still have have to scan A fo find A.b_id to do
> nothing with it? )) B.id itself is never modified, it's just a normal serial
> value typically used for id's.

We have an optimization that if you update a row in a transaction just
once, and the column is not modified, then it won't need to scan the
referencing table.  However, if you make two updates in a transaction,
the optimization isn't smart enough to detect that the FK isn't
invalidated, so a scan will occur.  Therefore, if A is large [enough
that you care about a seqscan on it] and you expect to be doing more
than one UPDATE of B in the same transaction, then this could be
noticeable.

I suggest you run some tests, just to be sure.

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Álvaro Herrera                https://www.2ndQuadrant.com/
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