Why does adding this field make my query run slow? - sql

Question

The query below runs in less than a second if I take off one field from the select list -- the last one, att. But having it there causes the query to take over a minute. Any thoughts? Note that the subquery inside the left join runs fast on its own.
select p.person_id, cfm.family_id, p.nick_name, p.last_name, s.lookup_value as group_role, gm.active, g.group_id, g.group_name,
g.active as group_active, tg.category, gc.cluster_name, coalesce(a.attendance,0) as att --this field is the culprit
from smgp_group g
join smgp_group_cluster gc on g.group_cluster_id = gc.group_cluster_id
join smgp_member gm on g.group_id = gm.group_id
join core_person p on gm.person_id = p.person_id
join core_family_member cfm on p.person_id = cfm.person_id
join core_lookup s on gm.role_luid = s.lookup_id
join #target_groups tg on g.group_id = tg.group_id
left join (
select at.person_id, goc.group_id, count(at.attended) as attendance from core_occurrence_attendance at
join core_occurrence o on at.occurrence_id = o.occurrence_id
join smgp_group_occurrence goc on o.occurrence_id = goc.occurrence_id
where goc.group_id in (select group_id from #target_groups)
and o.occurrence_start_time between #start_date and #end_date
group by at.person_id, goc.group_id
) a on p.person_id = a.person_id and g.group_id = a.group_id
where tg.category = 'adults' or (tg.category = 'kids' and s.lookup_value in ('Leader-Teacher','Assistant Leader'))

Answer 1

Try running the following -
SELECT p.persON_id, cfm.family_id, p.nick_name,
p.last_name, s.lookup_value as group_role,
gm.active, g.group_id, g.group_name,
g.active as group_active, tg.cATegory,
gc.cluster_name, a.ATtendance
INTO #tempDataset
FROM smgp_group g
JOIN smgp_group_cluster gc
ON g.group_cluster_id = gc.group_cluster_id
JOIN smgp_member gm
ON g.group_id = gm.group_id
JOIN core_persON p
ON gm.persON_id = p.persON_id
JOIN core_family_member cfm
ON p.persON_id = cfm.persON_id
JOIN core_lookup s
ON gm.role_luid = s.lookup_id
JOIN #target_groups tg
ON g.group_id = tg.group_id
LEFT JOIN (SELECT AT.persON_id, goc.group_id, count(AT.ATtended) as ATtendance FROM core_occurrence_ATtendance AT
JOIN core_occurrence o
ON AT.occurrence_id = o.occurrence_id
JOIN smgp_group_occurrence goc
ON o.occurrence_id = goc.occurrence_id
WHERE goc.group_id IN (SELECT group_id
FROM #target_groups)
AND o.occurrence_start_time BETWEEN #start_dATe AND #end_dATe
GROUP BY AT.persON_id, goc.group_id) a
ON p.persON_id = a.persON_id
AND g.group_id = a.group_id
WHERE tg.cATegory = 'adults' or (tg.cATegory = 'kids' AND s.lookup_value IN ('Leader-Teacher','Assistant Leader'))
SELECT persON_id,
family_id,
nick_name,
last_name,
group_role,
active,
group_id,
group_name,
group_active,
cATegory,
cluster_name,
coalesce(ATtendance,0) AS ATT
FROM #tempDataset
If that runs slowly, you can possibly add an index on the temp table, but I would try doing the above as a first step.

Answer 2

The query below runs in 3 seconds and resolved the problem. Thanks.
select at.person_id, goc.group_id, count(at.attended) as attendance
into #a
from core_occurrence_attendance at
join core_occurrence o on at.occurrence_id = o.occurrence_id
join smgp_group_occurrence goc on o.occurrence_id = goc.occurrence_id
where goc.group_id in (select group_id from #target_groups)
and o.occurrence_start_time between #start_date and #end_date
group by at.person_id, goc.group_id
select p.person_id, cfm.family_id, p.nick_name, p.last_name, s.lookup_value as group_role, gm.active, g.group_id, g.group_name,
g.active as group_active, tg.category, gc.cluster_name, coalesce(#a.attendance,0) as att--, --this field is the culprit
from smgp_group g
join smgp_group_cluster gc on g.group_cluster_id = gc.group_cluster_id
join smgp_member gm on g.group_id = gm.group_id
join core_person p on gm.person_id = p.person_id
join core_family_member cfm on p.person_id = cfm.person_id
join core_lookup s on gm.role_luid = s.lookup_id
join #target_groups tg on g.group_id = tg.group_id
left join #a on p.person_id = #a.person_id and g.group_id = #a.group_id
where tg.category = 'adults' or (tg.category = 'kids' and s.lookup_value in ('Leader-Teacher','Assistant Leader'))