Clarify that COPY FREEZE is not a hard rule.

Remove message when FREEZE not honoured,
clarify reasons in comments and docs.

doc/src/sgml/ref/copy.sgml

@@ -186,17 +186,19 @@ COPY { <replaceable class="parameter">table_name</replaceable> [ ( <replaceable
     <term><literal>FREEZE</literal></term>
     <listitem>
      <para>
-      Specifies copying the data with rows already frozen, just as they
+      Requests copying the data with rows already frozen, just as they
       would be after running the <command>VACUUM FREEZE</> command.
       This is intended as a performance option for initial data loading.
       Rows will be frozen only if the table being loaded has been created
       in the current subtransaction, there are no cursors open and there
       are no older snapshots held by this transaction. If those conditions
       are not met the command will continue without error though will not
-      freeze rows.
+      freeze rows. It is also possible in rare cases that the request
+      cannot be honoured for internal reasons, hence <literal>FREEZE</literal>
+      is more of a guideline than a hard rule.
      </para>
      <para>
-      Note that all sessions will immediately be able to see the data
+      Note that all other sessions will immediately be able to see the data
       once it has been successfully loaded. This violates the normal rules
       of MVCC visibility and by specifying this option the user acknowledges
       explicitly that this is understood.

src/backend/commands/copy.c

@@ -1993,6 +1993,11 @@ CopyFrom(CopyState cstate)
 		 * after xact cleanup. Note that the stronger test of exactly
 		 * which subtransaction created it is crucial for correctness
 		 * of this optimisation.
+		 *
+		 * Note that because the test is unreliable in case of relcache reset
+		 * we cannot guarantee that we can honour the request to FREEZE.
+		 * If we cannot honour the request we do so silently, firstly to
+		 * avoid noise for the user and also to avoid obscure test failures.
 		 */
 		if (cstate->freeze &&
 			ThereAreNoPriorRegisteredSnapshots() &&
@@ -2001,11 +2006,6 @@ CopyFrom(CopyState cstate)
 			hi_options |= HEAP_INSERT_FROZEN;
 	}
 
-	if (cstate->freeze && (hi_options & HEAP_INSERT_FROZEN) == 0)
-		ereport(NOTICE,
-				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-				 errmsg("FREEZE option specified but pre-conditions not met")));
-
 	/*
 	 * We need a ResultRelInfo so we can use the regular executor's
 	 * index-entry-making machinery.  (There used to be a huge amount of code