Make superuser.c maintain a simple one-entry cache holding the superuser

status of the most recently queried userid.  Since the common pattern is
many successive queries about the same user (ie, the current user) this
can save a lot of syscache probes.

src/backend/utils/misc/superuser.c

@@ -2,7 +2,11 @@
  *
  * superuser.c
  *	  The superuser() function.  Determines if user has superuser privilege.
- *	  Also, a function to check for the owner (datdba) of a database.
+ *
+ * All code should use either of these two functions to find out
+ * whether a given user is a superuser, rather than examining
+ * pg_shadow.usesuper directly, so that the escape hatch built in for
+ * the single-user case works.
  *
  *
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
@@ -10,26 +14,33 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/misc/superuser.c,v 1.30 2004/12/31 22:02:45 pgsql Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/misc/superuser.c,v 1.31 2005/05/29 20:38:06 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
 #include "postgres.h"
 
-#include "access/heapam.h"
 #include "catalog/pg_shadow.h"
-#include "commands/dbcommands.h"
+#include "utils/inval.h"
 #include "utils/syscache.h"
 #include "miscadmin.h"
 
 
+/*
+ * In common cases the same userid (ie, the session or current ID) will
+ * be queried repeatedly.  So we maintain a simple one-entry cache for
+ * the status of the last requested userid.  The cache can be flushed
+ * at need by watching for cache update events on pg_shadow.
+ */
+static AclId	last_userid = 0;		/* 0 == cache not valid */
+static bool		last_userid_is_super = false;
+static bool		userid_callback_registered = false;
+
+static void UseridCallback(Datum arg, Oid relid);
+
+
 /*
  * The Postgres user running this command has Postgres superuser privileges
- *
- * All code should use either of these two functions to find out
- * whether a given user is a superuser, rather than evaluating
- * pg_shadow.usesuper directly, so that the escape hatch built in for
- * the single-user case works.
  */
 bool
 superuser(void)
@@ -38,16 +49,24 @@ superuser(void)
 }
 
 
+/*
+ * The specified userid has Postgres superuser privileges
+ */
 bool
 superuser_arg(AclId userid)
 {
-	bool		result = false;
+	bool		result;
 	HeapTuple	utup;
 
+	/* Quick out for cache hit */
+	if (AclIdIsValid(last_userid) && last_userid == userid)
+		return last_userid_is_super;
+
 	/* Special escape path in case you deleted all your users. */
 	if (!IsUnderPostmaster && userid == BOOTSTRAP_USESYSID)
 		return true;
 
+	/* OK, look up the information in pg_shadow */
 	utup = SearchSysCache(SHADOWSYSID,
 						  Int32GetDatum(userid),
 						  0, 0, 0);
@@ -56,5 +75,35 @@ superuser_arg(AclId userid)
 		result = ((Form_pg_shadow) GETSTRUCT(utup))->usesuper;
 		ReleaseSysCache(utup);
 	}
+	else
+	{
+		/* Report "not superuser" for invalid userids */
+		result = false;
+	}
+
+	/* If first time through, set up callback for cache flushes */
+	if (!userid_callback_registered)
+	{
+		CacheRegisterSyscacheCallback(SHADOWSYSID,
+									  UseridCallback,
+									  (Datum) 0);
+		userid_callback_registered = true;
+	}
+
+	/* Cache the result for next time */
+	last_userid = userid;
+	last_userid_is_super = result;
+
 	return result;
 }
+
+/*
+ * UseridCallback
+ *		Syscache inval callback function
+ */
+static void
+UseridCallback(Datum arg, Oid relid)
+{
+	/* Invalidate our local cache in case user's superuserness changed */
+	last_userid = 0;
+}