relation: Add .readyset_explain

This commit adds a `.readyset_explain` method to our relation extension.
This method invokes `EXPLAIN CREATE CACHE` upstream on ReadySet and
returns information about the query from ReadySet.

lib/readyset/relation_extension.rb

@@ -7,7 +7,7 @@ module RelationExtension
       # query already exists.
       #
       # NOTE: If the ActiveRecord query eager loads associations (e.g. via `#includes`), the
-      # the queries issues to do the eager loading will not have caches created. Those queries must
+      # the queries issued to do the eager loading will not have caches created. Those queries must
       # have their caches created separately.
       #
       # @return [void]
@@ -19,13 +19,21 @@ def create_readyset_cache!
       # for the query already doesn't exist.
       #
       # NOTE: If the ActiveRecord query eager loads associations (e.g. via `#includes`), the
-      # the queries issues to do the eager loading will not have caches dropped. Those queries must
+      # the queries issued to do the eager loading will not have caches dropped. Those queries must
       # have their caches dropped separately.
       #
       # @return [void]
       def drop_readyset_cache!
         Readyset.drop_cache!(sql: to_sql)
       end
+
+      # Gets information about this query from ReadySet, including the query's ID, the normalized
+      # query text, and whether the query is supported by ReadySet.
+      #
+      # @return [Readyset::Explain]
+      def readyset_explain
+        Readyset.explain(to_sql)
+      end
     end
   end
 end

spec/relation_extension_spec.rb

@@ -12,8 +12,7 @@
       subject
     end
 
-    it 'invokes `Readyset.create_cache!` with the parameterized query string that the ' \
-        'relation represents' do
+    it "invokes `Readyset.create_cache!` with the relation's query string" do
       expect(Readyset).to have_received(:create_cache!).with(sql: query_string)
     end
   end
@@ -29,9 +28,34 @@
       subject
     end
 
-    it 'invokes `Readyset.drop_cache!` with the parameterized query string that the relation ' \
-        'represents' do
+    it "invokes `Readyset.drop_cache!` with the relation's query string" do
       expect(Readyset).to have_received(:drop_cache!).with(sql: query_string)
     end
   end
+
+  describe '#readyset_explain' do
+    it "invokes `Readyset.readyset_explain` with the relation's query string" do
+      query = Cat.where(id: 1)
+      query_string = query.to_sql
+      allow(Readyset).to receive(:explain).with(query_string).
+        and_return(instance_double(Readyset::Explain))
+
+      query.readyset_explain
+
+      expect(Readyset).to have_received(:explain).with(query_string)
+    end
+
+    it 'returns the expected explain information' do
+      query = Cat.where(id: 1)
+      query_string = query.to_sql
+      explain = Readyset::Explain.new(id: 'q_0000000000000000',
+                                      text: 'SELECT * FROM "cats" WHERE ("id" = $1)',
+                                      supported: :yes)
+      allow(Readyset).to receive(:explain).with(query_string).and_return(explain)
+
+      output = query.readyset_explain
+
+      expect(output).to eq(explain)
+    end
+  end
 end