Arel和CTE包装查询

Question

I have to update a search builder which builds a relation with CTE. This is necessary because a complex relation (which includes DISTINCT, JOINs etc) is first built and then it's results have to be ordered – all in one query.

Here's a simplified look at things:

rel = User.select('DISTINCT ON (users.id) users.*').where(<lotsastuff>)
rel.to_sql

# SELECT DISTINCT ON (users.id) users.*
#   FROM "users"
#   WHERE <lotsastuff>

rel2 = User.from_cte('cte_table', rel).order(:created_at)
rel2.to_sql

# WITH "cte_table" AS (
#   SELECT DISTINCT ON (users.id) users.*
#     FROM "users"
#     WHERE <lotsastuff>
# ) SELECT "cte_table".* FROM "cte_table"
#   ORDER BY "cte_table"."created_at" ASC

The beauty of it is that rel2 responds as expected eg to count .

The from_cte method is provided by the "posgres_ext" gem which appears to have been abandoned. I'm therefore looking for another way to build the relation rel2 from rel .

The Arel docs mention a case which doesn't seem to help here.

Any hints on how to get there? Thanks a bunch!

---

PS: I know how to do this with to queries by selecting all user IDs in the first, then build a query with IN over the IDs and order there. However, I'm curious whether this is possible with one query (with or without CTE) as well.

Answer 1

Since your CTE is non-recursive, you can rewrite it as a subquery in the FROM clause. The only change is that Postgres's planner will optimize it as part of the main query instead of separately (because a CTE is an optimization fence ). In ActiveRecord this works for me (tested on 5.1.4):

2.4.1 :001 > rel = User.select("DISTINCT ON (users.id) users.*").where("1=1")
2.4.1 :002 > puts User.from(rel, 'users').order(:created_at).to_sql
SELECT "users".* FROM (SELECT DISTINCT ON (users.id) users.* FROM "users" WHERE (1=1)) users ORDER BY "users"."created_at" ASC

I don't see any way to squeeze a CTE into ActiveRecord without extending it though, like what postgres_ext does. Sorry!

Answer 2

From what you've mentioned, I did not understand why do you need to use CTE instead of just a nested query.

rel = User.select('DISTINCT ON (users.id) users.*').where(<lotsastuff>).arel
inner_query = Arel::Table.new(:inner_query)
composed_cte = Arel::Nodes::As.new(inner_query, rel)
select_manager = Arel::SelectManager.new(composed_cte)
rel2 = select_manager.project('COUNT(*)')
rel2.to_sql
rel3 = select_manager.order('created_at ASC')
rel3.to_sql

you can then execute that sql