Revise host.inet_addresses query to use gin index (#1550)

* Revise host.inet_addresses query to use gin index

core/src/main/java/google/registry/model/host/HostResource.java

@@ -36,6 +36,17 @@ import javax.persistence.AccessType;
 @javax.persistence.Entity(name = "Host")
 @javax.persistence.Table(
     name = "Host",
+    /**
+     * A gin index defined on the inet_addresses field ({@link HostBase#inetAddresses} cannot be
+     * declared here because JPA/Hibernate does not support index type specification. As a result,
+     * the hibernate-generated schema (which is for reference only) does not have this index.
+     *
+     * <p>There are Hibernate-specific solutions for adding this index to Hibernate's domain model.
+     * We could either declare the index in hibernate.cfg.xml or add it to the {@link
+     * org.hibernate.cfg.Configuration} instance for {@link SessionFactory} instantiation (which
+     * would prevent us from using JPA standard bootstrapping). For now, there is no obvious benefit
+     * doing either.
+     */
     indexes = {@javax.persistence.Index(columnList = "hostName")})
 @ExternalMessagingName("host")
 @WithStringVKey

core/src/main/java/google/registry/whois/NameserverLookupByIpCommand.java

@@ -64,11 +64,17 @@ final class NameserverLookupByIpCommand implements WhoisCommand {
           jpaTm()
               .transact(
                   () ->
-                      // We cannot query @Convert-ed fields in HQL so we must use native Postgres
+                      // We cannot query @Convert-ed fields in HQL so we must use native Postgres.
                       jpaTm()
                           .getEntityManager()
+                          /**
+                           * Using array_operator <@ (contained-by) with gin index on inet_address.
+                           * Without gin index, this is slightly slower than the alternative form of
+                           * ':address = ANY(inet_address)'.
+                           */
                           .createNativeQuery(
-                              "SELECT * From \"Host\" WHERE :address = ANY(inet_addresses) AND "
+                              "SELECT * From \"Host\" WHERE "
+                                  + "ARRAY[ CAST(:address AS TEXT) ] <@ inet_addresses AND "
                                   + "deletion_time > CAST(:now AS timestamptz)",
                               HostResource.class)
                           .setParameter("address", InetAddresses.toAddrString(ipAddress))