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From: David Rowley <dgrowleyml@gmail.com>
Date: Mon, 19 Aug 2024 23:03:32 +1200
Message-ID: <CAApHDvq3mwhsiB69eXt6OJJL5=7UnbE74gWvWVJN57yEhrSkkg@mail.gmail.com>
Subject: Re: Insert query performance
To: sud <suds1434@gmail.com>
Cc: pgsql-general <pgsql-general@lists.postgresql.org>
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On Mon, 19 Aug 2024 at 19:48, sud <suds1434@gmail.com> wrote:
> In a version 15.4 postgres database, Is it possible that , if we have two=
 big range partition tables with foreign key relationships between them, in=
sert into the child table can cause slowness if we don't have foreign key i=
ndex present in the child table? Basically it need to make sure the new row=
 already added to parent partition table or not.

Having an index on the referencing columns is only useful for DELETEs
and UPDATEs affecting the foreign key column(s).  For INSERTs to the
referencing table, technically having indexes there would only slow
down inserts due to the additional overhead of having to maintain the
index, however, the overhead of having the index might be fairly
minuscule when compared to performing a CASCADE UPDATE or DELETE to
the referencing table when the DDL is performed on the referenced
table.

> And if there is any possible way(example query tracing etc) to get the un=
derlying system queries which gets triggered as part of the main insert que=
ry? For example in above scenario, postgres must be executing some query to=
 check if the incoming  row to the child table already exists in the parent=
 table or not?

EXPLAIN ANALYZE will list the time it took to execute the foreign key
trigger in the "Trigger for constraint" section.

David