Simplify and speed up Postgres query for primary_keys

primary_keys(table) needs to query various metadata tables in Postgres to
determine the primary key for the table. Previously, it did so using a
complex common table expression against pg_constraint and pg_attribute.

This patch simplifies the query by joining pg_index against pg_attribute
instead of going through pg_constraint. This avoids an expensive unnest,
window function query, and common table expression.

EXPLAINing these queries in Postgres against a database with a single
table with a composite primary key shows a 66% reduction in the plan and
execute latencies. This is significant during application startup time,
especially against very large schemas, where these queries would be even
slower and more numerous.

Closes #27949

1 files changed, 9 insertions, 0 deletions

diff --git a/activerecord/test/cases/primary_keys_test.rb b/activerecord/test/cases/primary_keys_test.rb
index 1baf7c4902..0c6799c015 100644
--- a/activerecord/test/cases/primary_keys_test.rb
+++ b/activerecord/test/cases/primary_keys_test.rb
@@ -298,6 +298,10 @@ class CompositePrimaryKeyTest < ActiveRecord::TestCase
       t.string :region
       t.integer :code
     end
+    @connection.create_table(:barcodes_reverse, primary_key: ["code", "region"], force: true) do |t|
+      t.string :region
+      t.integer :code
+    end
   end
 
   def teardown
@@ -308,6 +312,11 @@ class CompositePrimaryKeyTest < ActiveRecord::TestCase
     assert_equal ["region", "code"], @connection.primary_keys("barcodes")
   end
 
+  def test_composite_primary_key_out_of_order
+    skip if current_adapter?(:SQLite3Adapter)
+    assert_equal ["region", "code"], @connection.primary_keys("barcodes")
+  end
+
   def test_primary_key_issues_warning
     model = Class.new(ActiveRecord::Base) do
       def self.table_name