Allow compute_query_id to be set to 'auto' and make it default

Allowing only on/off meant that all either all existing configuration
guides would become obsolete if we disabled it by default, or that we
would have to accept a performance loss in the default config if we
enabled it by default.  By allowing 'auto' as a middle ground, the
performance cost is only paid by those who enable pg_stat_statements and
similar modules.

I only edited the release notes to comment-out a paragraph that is now
factually wrong; further edits are probably needed to describe the
related change in more detail.

Author: Julien Rouhaud <rjuju123@gmail.com>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/20210513002623.eugftm4nk2lvvks3@nol

src/backend/utils/misc/queryjumble.c

@@ -39,6 +39,12 @@
 
 #define JUMBLE_SIZE				1024	/* query serialization buffer size */
 
+/* GUC parameters */
+int			compute_query_id = COMPUTE_QUERY_ID_AUTO;
+
+/* True when compute_query_id is ON, or AUTO and a module requests them */
+bool		query_id_enabled = false;
+
 static uint64 compute_utility_query_id(const char *str, int query_location, int query_len);
 static void AppendJumble(JumbleState *jstate,
 						 const unsigned char *item, Size size);
@@ -96,6 +102,8 @@ JumbleQuery(Query *query, const char *querytext)
 {
 	JumbleState *jstate = NULL;
 
+	Assert(IsQueryIdEnabled());
+
 	if (query->utilityStmt)
 	{
 		query->queryId = compute_utility_query_id(querytext,
@@ -132,6 +140,19 @@ JumbleQuery(Query *query, const char *querytext)
 	return jstate;
 }
 
+/*
+ * Enables query identifier computation.
+ *
+ * Third-party plugins can use this function to inform core that they require
+ * a query identifier to be computed.
+ */
+void
+EnableQueryId(void)
+{
+	if (compute_query_id != COMPUTE_QUERY_ID_OFF)
+		query_id_enabled = true;
+}
+
 /*
  * Compute a query identifier for the given utility query string.
  */