Reduce memory usage when loading types in PG

We were never clearing the `PG::Result` object used to query the types
when the connection is first established. This would lead to a
potentially large amount of memory being retained for the life of the
connection.

Investigating this issue also revealed several low hanging fruit on the
performance of these methods, and the number of allocations has been
reduced by ~90%.

Fixes #19578

4 files changed, 40 insertions, 10 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index be3a461eda..8793d6de70 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   Reduce memory usage from loading types on pg.
+
+    Fixes #19578.
+
+    *Sean Griffin*
+
 *   Add `config.active_record.warn_on_records_fetched_greater_than` option
 
     When set to an integer, a warning will be logged whenever a result set
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/oid/type_map_initializer.rb b/activerecord/lib/active_record/connection_adapters/postgresql/oid/type_map_initializer.rb
index 9d421d6975..191c828e60 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/oid/type_map_initializer.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/oid/type_map_initializer.rb
@@ -15,10 +15,10 @@ module ActiveRecord
           def run(records)
             nodes = records.reject { |row| @store.key? row['oid'].to_i }
             mapped, nodes = nodes.partition { |row| @store.key? row['typname'] }
-            ranges, nodes = nodes.partition { |row| row['typtype'] == 'r' }
-            enums, nodes = nodes.partition { |row| row['typtype'] == 'e' }
-            domains, nodes = nodes.partition { |row| row['typtype'] == 'd' }
-            arrays, nodes = nodes.partition { |row| row['typinput'] == 'array_in' }
+            ranges, nodes = nodes.partition { |row| row['typtype'] == 'r'.freeze }
+            enums, nodes = nodes.partition { |row| row['typtype'] == 'e'.freeze }
+            domains, nodes = nodes.partition { |row| row['typtype'] == 'd'.freeze }
+            arrays, nodes = nodes.partition { |row| row['typinput'] == 'array_in'.freeze }
             composites, nodes = nodes.partition { |row| row['typelem'].to_i != 0 }
 
             mapped.each     { |row| register_mapped_type(row)    }
@@ -29,6 +29,18 @@ module ActiveRecord
             composites.each { |row| register_composite_type(row) }
           end
 
+          def query_conditions_for_initial_load(type_map)
+            known_type_names = type_map.keys.map { |n| "'#{n}'" }
+            known_type_types = %w('r' 'e' 'd')
+            <<-SQL % [known_type_names.join(", "), known_type_types.join(", ")]
+              WHERE
+                t.typname IN (%s)
+                OR t.typtype IN (%s)
+                OR t.typinput::varchar = 'array_in'
+                OR t.typelem != 0
+            SQL
+          end
+
           private
           def register_mapped_type(row)
             alias_type row['oid'], row['typname']
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
index e616cee5eb..00fc69c878 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
@@ -594,6 +594,8 @@ module ActiveRecord
         end
 
         def load_additional_types(type_map, oids = nil) # :nodoc:
+          initializer = OID::TypeMapInitializer.new(type_map)
+
           if supports_ranges?
             query = <<-SQL
               SELECT t.oid, t.typname, t.typelem, t.typdelim, t.typinput, r.rngsubtype, t.typtype, t.typbasetype
@@ -609,11 +611,13 @@ module ActiveRecord
 
           if oids
             query += "WHERE t.oid::integer IN (%s)" % oids.join(", ")
+          else
+            query += initializer.query_conditions_for_initial_load(type_map)
           end
 
-          initializer = OID::TypeMapInitializer.new(type_map)
-          records = execute(query, 'SCHEMA')
-          initializer.run(records)
+          execute_and_clear(query, 'SCHEMA', []) do |records|
+            initializer.run(records)
+          end
         end
 
         FEATURE_NOT_SUPPORTED = "0A000" #:nodoc:
@@ -823,9 +827,11 @@ module ActiveRecord
             'float8' => PG::TextDecoder::Float,
             'bool' => PG::TextDecoder::Boolean,
           }
-          query = <<-SQL
+          known_coder_types = coders_by_name.keys.map { |n| quote(n) }
+          query = <<-SQL % known_coder_types.join(", ")
             SELECT t.oid, t.typname
             FROM pg_type as t
+            WHERE t.typname IN (%s)
           SQL
           coders = execute_and_clear(query, "SCHEMA", []) do |result|
             result
diff --git a/activerecord/lib/active_record/type/hash_lookup_type_map.rb b/activerecord/lib/active_record/type/hash_lookup_type_map.rb
index 82d9327fc0..3b01e3f8ca 100644
--- a/activerecord/lib/active_record/type/hash_lookup_type_map.rb
+++ b/activerecord/lib/active_record/type/hash_lookup_type_map.rb
@@ -1,12 +1,18 @@
 module ActiveRecord
   module Type
     class HashLookupTypeMap < TypeMap # :nodoc:
-      delegate :key?, to: :@mapping
-
       def alias_type(type, alias_type)
         register_type(type) { |_, *args| lookup(alias_type, *args) }
       end
 
+      def key?(key)
+        @mapping.key?(key)
+      end
+
+      def keys
+        @mapping.keys
+      end
+
       private
 
       def perform_fetch(type, *args, &block)