This patch reduces the size of the message header used by statistics

collector messages, per recent discussion on pgsql-patches. This
actually required quite a few changes -- for example,
"databaseid != InvalidOid" was used to check whether a slot in the
backend entry table was initialized, but that no longer works since
the slot might be initialized prior to receiving the BESTART message
which contains the database id. We now use procpid > 0 to indicate
that a slot is non-empty.

Other changes:

- various comment improvements and cleanups
- there's no need to zero-out the entire activity buffer in
  pgstat_add_backend(), we can just set activity[0] to '\0'.
- remove the counting of the # of connections to a database; this
  was not used anywhere

One change in behavior I wasn't sure about: previously, the code
would create a hash table entry for a database as soon as any message
was received whose header referenced that database. Now, we only
create hash table entries as needed (so for example BESTART won't
create a database hash table entry, since it doesn't need to
access anything in the per-db hash table). It would be easy enough
to retain the old behavior, but AFAICS it is not required.

src/backend/utils/adt/pgstatfuncs.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/adt/pgstatfuncs.c,v 1.21 2005/05/09 11:31:33 neilc Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/adt/pgstatfuncs.c,v 1.22 2005/05/11 01:41:41 neilc Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -283,6 +283,10 @@ pg_stat_get_backend_dbid(PG_FUNCTION_ARGS)
 	if ((beentry = pgstat_fetch_stat_beentry(beid)) == NULL)
 		PG_RETURN_NULL();
 
+	/* Not initialized yet? */
+	if (!OidIsValid(beentry->databaseid))
+		PG_RETURN_NULL();
+
 	PG_RETURN_OID(beentry->databaseid);
 }
 
@@ -298,6 +302,10 @@ pg_stat_get_backend_userid(PG_FUNCTION_ARGS)
 	if ((beentry = pgstat_fetch_stat_beentry(beid)) == NULL)
 		PG_RETURN_NULL();
 
+	/* Not initialized yet? */
+	if (!OidIsValid(beentry->userid))
+		PG_RETURN_NULL();
+
 	PG_RETURN_INT32(beentry->userid);
 }
 
@@ -405,6 +413,10 @@ pg_stat_get_backend_client_addr(PG_FUNCTION_ARGS)
 	if ((beentry = pgstat_fetch_stat_beentry(beid)) == NULL)
 		PG_RETURN_NULL();
 
+	/* Not initialized yet? */
+	if (!OidIsValid(beentry->userid))
+		PG_RETURN_NULL();
+
 	if (!superuser() && beentry->userid != GetUserId())
 		PG_RETURN_NULL();
 
@@ -420,7 +432,6 @@ pg_stat_get_backend_client_addr(PG_FUNCTION_ARGS)
 	}
 
 	remote_host[0] = '\0';
-
 	ret = getnameinfo_all(&beentry->clientaddr.addr, beentry->clientaddr.salen,
 						  remote_host, sizeof(remote_host),
 						  NULL, 0,
@@ -445,6 +456,10 @@ pg_stat_get_backend_client_port(PG_FUNCTION_ARGS)
 	if ((beentry = pgstat_fetch_stat_beentry(beid)) == NULL)
 		PG_RETURN_NULL();
 
+	/* Not initialized yet? */
+	if (!OidIsValid(beentry->userid))
+		PG_RETURN_NULL();
+
 	if (!superuser() && beentry->userid != GetUserId())
 		PG_RETURN_NULL();
 
@@ -462,7 +477,6 @@ pg_stat_get_backend_client_port(PG_FUNCTION_ARGS)
 	}
 
 	remote_port[0] = '\0';
-
 	ret = getnameinfo_all(&beentry->clientaddr.addr,
 						  beentry->clientaddr.salen,
 						  NULL, 0,

src/include/pgstat.h

@@ -5,7 +5,7 @@
  *
  *	Copyright (c) 2001-2005, PostgreSQL Global Development Group
  *
- *	$PostgreSQL: pgsql/src/include/pgstat.h,v 1.28 2005/05/09 11:31:33 neilc Exp $
+ *	$PostgreSQL: pgsql/src/include/pgstat.h,v 1.29 2005/05/11 01:41:41 neilc Exp $
  * ----------
  */
 #ifndef PGSTAT_H
@@ -52,9 +52,6 @@ typedef struct PgStat_MsgHdr
 	int			m_size;
 	int			m_backendid;
 	int			m_procpid;
-	Oid			m_databaseid;
-	AclId		m_userid;
-	SockAddr	m_clientaddr;
 } PgStat_MsgHdr;
 
 /* ----------
@@ -102,7 +99,10 @@ typedef struct PgStat_MsgDummy
  */
 typedef struct PgStat_MsgBestart
 {
-	PgStat_MsgHdr m_hdr;
+	PgStat_MsgHdr	m_hdr;
+	Oid				m_databaseid;
+	AclId			m_userid;
+	SockAddr		m_clientaddr;
 } PgStat_MsgBestart;
 
 /* ----------
@@ -138,6 +138,7 @@ typedef struct PgStat_MsgActivity
 typedef struct PgStat_MsgTabstat
 {
 	PgStat_MsgHdr m_hdr;
+	int			m_databaseid;
 	int			m_nentries;
 	int			m_xact_commit;
 	int			m_xact_rollback;
@@ -155,6 +156,7 @@ typedef struct PgStat_MsgTabstat
 typedef struct PgStat_MsgTabpurge
 {
 	PgStat_MsgHdr m_hdr;
+	Oid			m_databaseid;
 	int			m_nentries;
 	Oid			m_tableid[PGSTAT_NUM_TABPURGE];
 } PgStat_MsgTabpurge;
@@ -180,6 +182,7 @@ typedef struct PgStat_MsgDropdb
 typedef struct PgStat_MsgResetcounter
 {
 	PgStat_MsgHdr m_hdr;
+	Oid			m_databaseid;
 } PgStat_MsgResetcounter;
 
 
@@ -214,7 +217,6 @@ typedef struct PgStat_StatDBEntry
 	Oid			databaseid;
 	HTAB	   *tables;
 	int			n_backends;
-	PgStat_Counter n_connects;
 	PgStat_Counter n_xact_commit;
 	PgStat_Counter n_xact_rollback;
 	PgStat_Counter n_blocks_fetched;
@@ -229,15 +231,24 @@ typedef struct PgStat_StatDBEntry
  */
 typedef struct PgStat_StatBeEntry
 {
-	Oid			databaseid;
-	Oid			userid;
+	/* An entry is non-empty iff procpid > 0 */
 	int			procpid;
 	AbsoluteTime start_sec;
 	int         start_usec;
 	AbsoluteTime activity_start_sec;
 	int			activity_start_usec;
-	SockAddr    clientaddr;
 	char		activity[PGSTAT_ACTIVITY_SIZE];
+
+	/*
+	 * The following fields are initialized by the BESTART message. If
+	 * we have received messages from a backend before we have
+	 * received its BESTART, these fields will be uninitialized:
+	 * userid and databaseid will be InvalidOid, and clientaddr will
+	 * be undefined.
+	 */
+	Oid			userid;
+	Oid			databaseid;
+	SockAddr    clientaddr;
 } PgStat_StatBeEntry;