fix: limit the number of fields fetched from MongoDB (#9021)

It does not solve issue
#8792 as
we're still over Memcached limit for main page, but it should still
be faster.
Furthermore, knowing which fields to fetched to compute attributes
will be necessary once search-a-licious project is deployed, as data
will be fetched from Elasticsearch (and not MongoDB).

lib/ProductOpener/Attributes.pm

@@ -31,6 +31,10 @@ the same structured format for all attributes.
 
 See https://wiki.openfoodfacts.org/Product_Attributes
 
+If new attributes are added, make sure *to update the list of fields* fetched from MongoDB
+in Display.pm (in search_and_display_products subroutine).
+
+
 =cut
 
 package ProductOpener::Attributes;

lib/ProductOpener/Display.pm

@@ -5111,8 +5111,42 @@ sub search_and_display_products ($request_ref, $query_ref, $sort_by, $limit, $pa
 	}
 	# - if we use user preferences, we need a lot of fields to compute product attributes: load them all
 	elsif ($request_ref->{user_preferences}) {
-		# when product attributes become more stable, we could try to restrict the fields
-		$fields_ref = {};
+		# we restrict the fields that are queried to MongoDB, and use the basic ones and those necessary
+		# by Attributes.pm to compute attributes.
+		# This list should be updated if new attributes are added.
+		$fields_ref = {
+			# generic fields
+			"lc" => 1,
+			"code" => 1,
+			"product_name" => 1,
+			"product_name_$lc" => 1,
+			"generic_name" => 1,
+			"generic_name_$lc" => 1,
+			"abbreviated_product_name" => 1,
+			"abbreviated_product_name_$lc" => 1,
+			"brands" => 1,
+			"images" => 1,
+			"quantity" => 1,
+			# fields necessary for personal search
+			"additives_n" => 1,
+			"allergens_tags" => 1,
+			"categories_tags" => 1,
+			"ecoscore_data" => 1,
+			"ecoscore_grade" => 1,
+			"ecoscore_score" => 1,
+			"forest_footprint_data" => 1,
+			"ingredients_analysis_tags" => 1,
+			"ingredients_n" => 1,
+			"labels_tags" => 1,
+			"nova_group" => 1,
+			"nutrient_levels" => 1,
+			"nutriments" => 1,
+			"nutriscore_data" => 1,
+			"nutriscore_grade" => 1,
+			"nutrition_grades" => 1,
+			"traces_tags" => 1,
+			"unknown_ingredients_n" => 1
+		};
 	}
 	else {
 		#for HTML, limit the fields we retrieve from MongoDB