Make the table name too long error message more explicit for PostgreSQL

* Explain why the enforced limit is less than the Postgres limit

Extract the table name too long error message out into its own method

Fix tests for new table name too long error message

Update error message from Postgres to PostgreSQL

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -1767,16 +1767,20 @@ def check_constraint_for!(table_name, expression: nil, **options)
 
         def validate_index_length!(table_name, new_name, internal = false)
           if new_name.length > index_name_length
-            raise ArgumentError, "Index name '#{new_name}' on table '#{table_name}' is too long; the limit is #{index_name_length} characters"
+            raise ArgumentError, "Index name '#{new_name}' on table '#{table_name}' is too long; the limit is #{index_name_length} characters."
           end
         end
 
         def validate_table_length!(table_name)
           if table_name.length > table_name_length
-            raise ArgumentError, "Table name '#{table_name}' is too long; the limit is #{table_name_length} characters"
+            raise ArgumentError, table_name_too_long_error_message(table_name)
           end
         end
 
+        def table_name_too_long_error_message(table_name)
+          "Table name '#{table_name}' is too long; the limit is #{table_name_length} characters"
+        end
+
         def extract_new_default_value(default_or_changes)
           if default_or_changes.is_a?(Hash) && default_or_changes.has_key?(:from) && default_or_changes.has_key?(:to)
             default_or_changes[:to]

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -608,6 +608,10 @@ def table_name_length
         max_identifier_length - "_pkey".length
       end
 
+      def table_name_too_long_error_message(table_name)
+        "Table name '#{table_name}' is too long; PostgreSQL allows a maximum length of #{max_identifier_length} characters, but we also need to allow for the `_pkey` suffix that PostgreSQL adds to table names when creating default indexes, so our effective limit is #{table_name_length} characters."
+      end
+
       # Set the authorized user for this session
       def session_auth=(user)
         clear_cache!

activerecord/test/cases/migration/rename_table_test.rb

@@ -52,13 +52,18 @@ def test_rename_table
 
       def test_rename_table_raises_for_long_table_names
         name_limit = connection.table_name_length
+        max_identifier_length = connection.max_identifier_length
         long_name = "a" * (name_limit + 1)
         short_name = "a" * name_limit
 
         error = assert_raises(ArgumentError) do
           connection.rename_table :test_models, long_name
         end
-        assert_equal "Table name '#{long_name}' is too long; the limit is #{name_limit} characters", error.message
+        if current_adapter?(:PostgreSQLAdapter)
+          assert_equal "Table name '#{long_name}' is too long; PostgreSQL allows a maximum length of #{max_identifier_length} characters, but we also need to allow for the `_pkey` suffix that PostgreSQL adds to table names when creating default indexes, so our effective limit is #{name_limit} characters.", error.message
+        else
+          assert_equal "Table name '#{long_name}' is too long; the limit is #{name_limit} characters", error.message
+        end
 
         connection.rename_table :test_models, short_name
         assert connection.table_exists?(short_name)

activerecord/test/cases/migration_test.rb

@@ -200,13 +200,18 @@ def test_create_table_with_if_not_exists_true
   def test_create_table_raises_for_long_table_names
     connection = Person.connection
     name_limit = connection.table_name_length
+    max_identifier_length = connection.max_identifier_length
     long_name = "a" * (name_limit + 1)
     short_name = "a" * name_limit
 
     error = assert_raises(ArgumentError) do
       connection.create_table(long_name)
     end
-    assert_equal "Table name '#{long_name}' is too long; the limit is #{name_limit} characters", error.message
+    if current_adapter?(:PostgreSQLAdapter)
+      assert_equal "Table name '#{long_name}' is too long; PostgreSQL allows a maximum length of #{max_identifier_length} characters, but we also need to allow for the `_pkey` suffix that PostgreSQL adds to table names when creating default indexes, so our effective limit is #{name_limit} characters.", error.message
+    else
+      assert_equal "Table name '#{long_name}' is too long; the limit is #{name_limit} characters", error.message
+    end
 
     connection.create_table(short_name)
     assert connection.table_exists?(short_name)