Make explain accept hash format syntax

activerecord/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   Allow either explain format syntax for EXPLAIN queries.
+
+    MySQL uses FORMAT=JSON whereas Postgres uses FORMAT JSON. We should be
+    able to accept both formats as options.
+
+    *Gannon McGibbon*
+
 *   On MySQL parallel test database table reset to use `DELETE` instead of `TRUNCATE`.
 
     Truncating on MySQL is very slow even on empty or nearly empty tables.

activerecord/lib/active_record/connection_adapters/mysql/database_statements.rb

@@ -36,6 +36,10 @@ def explain(arel, binds = [], options = [])
         def build_explain_clause(options = [])
           return "EXPLAIN" if options.empty?
 
+          options = options.flat_map do |option|
+            option.is_a?(Hash) ? option.to_a.map { |nested| nested.join("=") } : option
+          end
+
           explain_clause = "EXPLAIN #{options.join(" ").upcase}"
 
           if analyze_without_explain? && explain_clause.include?("ANALYZE")

activerecord/lib/active_record/connection_adapters/postgresql/database_statements.rb

@@ -96,6 +96,10 @@ def high_precision_current_timestamp
         def build_explain_clause(options = [])
           return "EXPLAIN" if options.empty?
 
+          options = options.flat_map do |option|
+            option.is_a?(Hash) ? option.to_a.map { |nested| nested.join(" ") } : option
+          end
+
           "EXPLAIN (#{options.join(", ").upcase})"
         end
 

activerecord/test/cases/adapters/abstract_mysql_adapter/mysql_explain_test.rb

@@ -37,6 +37,12 @@ def test_explain_options_with_eager_loading
     assert_match %(#{expected_analyze_clause} SELECT `posts`.* FROM `posts` WHERE `posts`.`author_id` = 1), explain
   end
 
+  def test_explain_format_option
+    explain = Author.all.explain(format: :json).inspect
+
+    assert_match(/\{.*\}/m, explain)
+  end
+
   private
     def explain_option
       supports_analyze? || supports_explain_analyze? ? :analyze : :extended

activerecord/test/cases/adapters/postgresql/explain_test.rb

@@ -38,4 +38,10 @@ def test_explain_options_with_eager_loading
     assert_match %r(EXPLAIN \(ANALYZE\) SELECT "authors"\.\* FROM "authors" WHERE "authors"\."id" = (?:\$1 \[\["id", 1\]\]|1)), explain
     assert_match %r(EXPLAIN \(ANALYZE\) SELECT "posts"\.\* FROM "posts" WHERE "posts"\."author_id" = (?:\$1 \[\["author_id", 1\]\]|1)), explain
   end
+
+  def test_explain_format_option
+    explain = Author.all.explain(format: :json).inspect
+
+    assert_match(/\{.*\}/m, explain)
+  end
 end