Does "ON UPDATE" for foreign keys require index?
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.
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.
--
�lvaro Herrera https://www.2ndQuadrant.com/
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