Theoretically, it makes sense that it would be prudent to cache access to a database resource. Although caching the SELECT query plan is pre-Ecto v3.0, this is a good opportunity to go over why and how this strategy improves performance for Phoenix applications. It was this commit to the PostgreSQL driver for Elixir, effectively caching the planning phase of INSERT, UPDATE and DELETE operations. However, there was one improvement that caught my eye. Some projects may not use Ecto to talk to a database but rather as a data mapping and validation layer. In addition to the official release, Ecto v3.0 was published! The latest major Ecto release moved SQL adapters to a separate repository ecto_sql. Chris McCord recently outlined what’s new in Phoenix 1.4. With subqueries, that's straight-forward: last_lendings = from l in MyApp.Lending, group_by : l. To do so, we need to find the last lending ID of every book, and then join on the book and visitor tables. Now consider we want to retrieve the name of every book alongside the name of the last person the library has lent it to. The "lendings" table uses an auto-incrementing primary key and can be backed by the following schema: defmodule Library.Lending do use Ecto.Schema schema "lendings" do belongs_to :book, MyApp.Book # defines book_id belongs_to :visitor, MyApp.Visitor # defines visitor_id end end Imagine you manage a library (as in an actual library in the real world) and there is a table that logs every time the library lends a book. For example, if you write q.field_that_does_not_exist, your Ecto query won't compile.Įcto also allows an Elixir map to be returned from a subquery, making the map keys directly available to the parent query. In fact, Ecto will keep the schema properties across queries. Since the query will return all fields in MyApp.Post, when we convert it to a subquery, all of the fields from MyApp.Post will be available on the parent query, such as q.visits. one ( query )īecause the query does not specify a :select clause, it will return select: p where p is controlled by MyApp.Post schema. Let's revisit the aggregate query we saw in the previous section: inner_query = from MyApp.Post, order_by :, limit : 10 query = from q in subquery ( inner_query ), select : avg ( q. All fields selected in a subquery can be accessed from the parent query. In Ecto, it is allowed for a subquery to select a whole table ( p) or a field ( p.field). This function receives any data structure that can be converted to a query, via the Ecto.Queryable protocol, and returns a subquery construct (which is also queryable). Subqueries in Ecto are created by calling /1. That's one of many examples that caused subqueries to be added to Ecto. In the previous section we have already learned some queries that would be hard to express without support for subqueries. Therefore the query executed by aggregate/4 above is rather equivalent to: inner_query = from MyApp.Post, order_by :, limit : 10 query = from q in subquery ( inner_query ), select : avg ( q. When limit, offset or distinct is specified in the query, aggregate/4 automatically wraps the given query in a subquery. aggregate ( query, :avg, :visits ) #=> #Decimal That's exactly what aggregate/4 does: query = from MyApp.Post, order_by :, limit : 10 MyApp.Repo. In order to retrieve the correct result, we would need to first find the top 10 posts and only then aggregate. The option limit: 10 has no effect here since it is limiting the aggregated result and queries with aggregates return only a single row anyway. The query above returned the same value as the queries before. one ( from p in MyApp.Post, order_by :, limit : 10, select : avg ( p. ![]() Imagine that instead of calculating the average of all posts, you want the average of only the top 10. However, complexities start to arise on queries that rely on limit, offset or distinct clauses. You could even start to wonder why it was added to Ecto in the first place. The /4 function supports any of the aggregate operations listed in the module.Īt first, it looks like the implementation of aggregate/4 is quite straight-forward. one ( from p in MyApp.Post, select : avg ( p. aggregate ( MyApp.Post, :avg, :visits ) #=> #Decimalīehind the scenes, the query above translates to: MyApp.Repo. ![]() As we will learn, one builds directly on the other.Įcto includes a convenience function in repositories to calculate aggregates.įor example, if we assume every post has an integer column named visits, we can find the average number of visits across all posts with: MyApp.Repo. Now it's time to discuss aggregates and subqueries. ![]() Settings View Source Aggregates and subqueries
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