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.
--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services