Count Joins from Multiple Tables - sql

Question

For reference, I am using Postgres 9.2.23.
I have several tables where one table (user_group) is related to some other tables (eg: posts, group_invites, and some more other ones). There is, also a groups table, but it doesn't hold any data that I need for the purposes of these queries.
Table user_group:
fk_user_group_id, fk_user_id, fk_group_id, fk_invite_id user_status, ...
Table message:
pk_message_id, fk_user_id, fk_group_id, child_message_id, ...
Table group_prospective_user:
pk_prospective_user_id, fk_group_id, ...
I want to get some statistics for each of the related tables for a list of specified group ids if the user is a member of the group.
Right now I do this with one query for each related table, eg:
select
"public"."user_group"."fk_group_id" as "groupId",
count(case
when (
"public"."message"."child_message_id" is null
and "public"."message"."pk_message_id" is not null
) then "public"."message"."pk_message_id"
end) as "numDiscussions",
count("public"."message"."pk_message_id") as "numDiscussionPosts"
from "public"."user_group"
left outer join "public"."message"
on "public"."message"."fk_group_id" = "public"."user_group"."fk_group_id"
where (
"public"."user_group"."fk_group_id" in (
1, 11, 23, 530, 1070
)
and "public"."user_group"."role" in (
'ADMINISTRATOR', 'MODERATOR', 'MEMBER'
)
and "public"."user_group"."fk_user_id" = 17517
)
group by "public"."user_group"."fk_group_id"
And for invites:
select
"public"."user_group"."fk_group_id" as "groupId",
count(case
when "public"."prospective_user"."status" = 1 then "public"."prospective_user"."pk_prospective_user_id"
end) as "numInviteesExternal"
from "public"."user_group"
left outer join "public"."prospective_user"
on "public"."prospective_user"."fk_group_id" = "public"."user_group"."fk_group_id"
where (
"public"."user_group"."fk_group_id" in (
1, 11, 23, 530, 6176
)
and "public"."user_group"."role" in (
'ADMINISTRATOR', 'MODERATOR', 'MEMBER'
)
and "public"."user_group"."fk_user_id" = 17517
)
group by "public"."user_group"."fk_group_id"
The query to count the number of group invites is very similar to the above query. Just the count when and join on change.
Each of the queries to these tables has the same related logic for checking the groups to which the current user is an active member. Is there efficient way to merge multiple similar queries like this into a single query?
I tried using multiple LEFT JOINs with select count distinct, but that ran into performance issues on groups with both lots of messages, and lots of invites. Is there a way to easily/efficiently do this with, say, a subquery?

Answer 1

The answer from user #Parfait was the most scalable solution I could find. I based my queries on this tutorial: https://www.sqlteam.com/articles/using-derived-tables-to-calculate-aggregate-values.
While this isn't perfect, and results in a bunch of subqueries running, it does get me all the data at once, and with a single trip to the DB.
It ended up like this:
"groups"."groupId",
coalesce(
"members"."member_count",
0
) as "numActiveMembers",
coalesce(
"members"."invitee_count",
0
) as "numInviteesInternal",
coalesce(
"discussions"."discussions_count",
0
) as "numDiscussions",
coalesce(
"discussions"."posts_count",
0
) as "numDiscussionPosts"
from (
select "public"."user_group"."fk_group_id" as "groupId"
from "public"."user_group"
where (
"public"."user_group"."fk_group_id" in (
1, 2, 3, 4, 5
)
and "public"."user_group"."role" = 'ADMINISTRATOR'
and "public"."user_group"."fk_user_id" = 123
)
group by "public"."user_group"."fk_group_id"
) as "groups"
left outer join (
select
"public"."user_group"."fk_group_id" as "members_group_id",
count(distinct case
when "public"."user_group"."role" in (
'ADMINISTRATOR', 'MODERATOR', 'MEMBER'
) then "public"."user_group"."pk_user_group_id"
end) as "member_count",
count(distinct case
when "public"."user_group"."role" = 'INVITEE' then "public"."user_group"."pk_user_group_id"
end) as "invitee_count"
from "public"."user_group"
group by "public"."user_group"."fk_group_id"
) as "members"
on "members_group_id" = "groupId"
left outer join (
select
"public"."message"."fk_group_id" as "discussions_group_id",
count(case
when (
"public"."message"."child_message_id" is null
and "public"."message"."pk_message_id" is not null
) then "public"."message"."pk_message_id"
end) as "discussions_count",
count("public"."message"."pk_message_id") as "posts_count"
from "public"."message"
group by "public"."message"."fk_group_id"
) as "discussions"
on "discussions_group_id" = "groupId"```