id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
37344	Document that FETCH_PEERS does not apply to instances loaded by select_related()	Mykhailo Havelia		"**Description**

Combining `select_related()` with `FETCH_PEERS` silently disables peer batching one level below the join, turning an intended optimization into an N+1 and making the query count *worse* than not calling `select_related()` at all.

Given `Species -> Genus -> Family`:

{{{
# 3 queries: species, batch of genus, batch of family
for species in Species.objects.fetch_mode(FETCH_PEERS):
    species.genus.family.name

# 4 queries: the join, then ONE QUERY PER FAMILY
for species in Species.objects.select_related(""genus"").fetch_mode(FETCH_PEERS):
    species.genus.family.name
}}}

The second form emits:

{{{
SELECT ... FROM species INNER JOIN genus ON (species.genus_id = genus.id);
SELECT ... FROM family WHERE family.id = 1 LIMIT 21;
SELECT ... FROM family WHERE family.id = 2 LIMIT 21;
SELECT ... FROM family WHERE family.id = 3 LIMIT 21;
}}}

Was it intended to be this way? If so, I think it’s worth mentioning in the documentation."	Cleanup/optimization	new	Database layer (models, ORM)	6.1	Normal		fetch_mode, FETCH_PEERS, select_related, peers	Mykhailo Havelia	Unreviewed	0	0	0	0	0	0