I've got an app with over 2 million rows in a Postgresql database and I make complex queries with joins, unions and aggregate functions that take a lot of time. The results of some of these queries are used a lot, for example to render a cloropleth map on the app's homepage that changes based on filters on the map. This is a super expensive operation and can't scale!
In come Materialized Views to the rescue. A materialized view is a query that is persisted like a table and treated the same way. This means queries on this table are fast. No joins, or unions, just like querying any other table. Note that it needs to be refreshed to keep it up to date so it is not suited for realtime data.
Although we can map queries on our materialized view to a plain crystal class using DB.mapping, we would miss out on Lucky's awesome typesafe queries. So in this article we're going to look at treating materialized views just like normal tables and having Lucky model them for us!
ComplexPost
The sql for the query we'll be materializing is below. Later we'll make slight adjustments to it so it can play nicely with LuckyRecord::Model.
SELECT
posts.id,
posts.title,
posts.content,
users.name as author
FROM posts
JOIN users
ON users.id = posts.user_id
This isn't a complex query but the way we implement this feature can be expanded to include any query regardless of complexity. The only thing that matters is that the columns returned by our SELECT statement match our LuckyRecord model definition.
Setup
If you want to test this out yourself you can use my demo app, just clone the repo and checkout the matviews-0 branch to follow along, or matviews-3-complete to see the finished code.
git clone git@github.com:mikeeus/lucky_api_demo.git
cd lucky_api_demo
bin/setup
git checkout matviews-0
Create Spec
git checkout matviews-0
Although we don't have a materialized view or a model to access it, we're going to write a spec for how we want to be able to use it.
# spec/matview_spec.cr
require "./spec_helper"
describe App do
describe "ComplexPost matview" do
it "should refresh and query" do
user = UserBox.new.create
first_post = PostBox.new.title("First").user_id(user.id).create
second_post = PostBox.new.title("Second").user_id(user.id).create
ComplexPost.refresh
complex = ComplexPostQuery.new.title(first_post.title).first
complex.title.should eq first_post.title
end
end
end
We want to be able to refresh the materialized view using the model, and we want to query the view with Lucky's own queries which will generate helper methods for each column, eg: ComplexPostQuery.new.title(...), ComplexPostQuery.new.content.ilike(...), etc.
Create Materialized View
git checkout matviews-1-create-matview
First lets generate a migration with lucky gen.migration CreateComplexPostMatview.
class CreateComplexPostsMatview::V20180101010 < LuckyMigrator::Migration::V1
def migrate
execute <<-SQL
CREATE MATERIALIZED VIEW complex_posts AS
SELECT
posts.id,
posts.title,
posts.content,
users.name as author,
NOW() as created_at,
NOW() as updated_at
FROM posts
JOIN users
ON users.id = posts.user_id
SQL
execute "CREATE UNIQUE INDEX complex_posts_id_index ON complex_posts (id)"
end
def rollback
execute "DROP MATERIALIZED VIEW complex_posts"
end
end
We need to have id, created_at and updated_at columns because LuckyRecord::Model will expect these when it generates helper methods for us. In this case we actually use id and even add a unique index on it to speed up our queries, but I should note that neither an id column or a primary key is needed for a materialized view.
Now if we migrate with lucky db.migrate it should pass without issue.
Generate ComplexPost Model and Query
git checkout matviews-2-create-model
Now that we have the materialized view in the database we could define a ComplexPost class that calls DB.mapping (like in my previous post) and just query it with LuckyRecord::Repo like this.
LuckyRecord::Repo.run do |db|
db.query_all "SELECT * FROM complex_posts", as: ComplexPost
end
But that wouldn't give us the benefits of LuckyRecord::Model or ComplexPost::Query.
So instead we'll generate our ComplexPost model by running lucky gen.model ComplexPost. We can go ahead and delete the generated ComplexPost::Form because we'll never be inserting rows into our materialized view.
Let's fill it in as if it was a normal database model and add a class method to refresh the materialized view.
# src/models/complex_post.cr
class ComplexPost < BaseModel
table :complex_posts do
column title : String
column content : String
column author : String
end
def self.refresh
LuckyRecord::Repo.db.exec "REFRESH MATERIALIZED VIEW complex_posts"
end
end
Run The Specs
git clone matviews-3-complete
Run crystal spec spec/matviews_spec.cr and... BAM! It's working.
Although this was a simple example we can expand on it to account for complex queries. An example of a query that I use in my app is this.
class CreateCountryAnnualTradeMatview::V0000001 < LuckyMigrator::Migration::V1
def migrate
execute <<-SQL
CREATE MATERIALIZED VIEW country_annual_trades AS
SELECT
merged.id, -- FOR LUCKY
merged.name,
merged.short,
merged.year,
COALESCE(sum(merged.total_imports_cents))::bigint as total_imports_cents,
COALESCE(sum(merged.total_exports_cents))::bigint as total_exports_cents,
NOW() as created_at, -- FOR LUCKY
NOW() as updated_at -- FOR LUCKY
FROM (
( SELECT ... ) -- some complex query
UNION
( SELECT ... ) -- another complex query
) merged
GROUP BY id, name, short, year
ORDER BY name
SQL
execute <<-SQL
CREATE UNIQUE INDEX country_annual_trades_id_year -- we can even add indexes!
ON country_annual_trades(id, year)
SQL
end
def rollback
execute <<-SQL
DROP MATERIALIZED VIEW country_annual_trades
SQL
end
end
And the model.
class CountryAnnualTrade < BaseModel
table :country_annual_trades do
column name : String
column short : String
column year : Int32
column total_imports_cents : Int64
column total_exports_cents : Int64
end
def self.refresh
LuckyRecord::Repo.db.exec "REFRESH MATERIALIZED VIEW country_annual_trades"
end
end
Note that I cast my aggregated columns total_imports_cents and total_exports_cents to ::bigint then define them in the model as Int64 with column total_imports_cents : Int64.
Join Us
I hope you enjoyed this tutorial and found it useful. Join us on the Lucky gitter channel to stay up to date on the framework or checkout the docs for more information on how to bring your app idea to life with Lucky.
Top comments (4)
You're allowed to have union in material views in Postgres?!
Hi Evaldas, I hope you enjoyed the article. You can actually create whatever kind of query you want. A materialized view is just a query whose resulting rows are persisted as if it was a normal table.
I did. I'm just used to SQL Server materialized views implementation. It has way more restrictions. That's why I was surprised 😊
Interesting, I've never used SQL server. tbh I've used Postgresql ever since I learned to code and I've never had a reason to try anything eles