feat: add command_tag_omit_oid GUC to drop legacy OID from INSERT tag

Add a new boolean GUC command_tag_omit_oid (default off) that controls
whether the legacy OID field is included in INSERT command completion
tags. When enabled, INSERT returns 'INSERT N' instead of 'INSERT 0 N'.

The '0' is a vestige from PostgreSQL 11 and earlier when tables could
have OIDs. The OID field has been hardcoded to 0 (InvalidOid) since
PostgreSQL 12, but remains in the wire protocol for backward
compatibility.

This GUC is per-session (PGC_USERSET) and supports SET LOCAL for
per-transaction control.

Also update libpq's PQcmdTuples() to handle both formats gracefully,
so client applications work correctly regardless of the server setting.

Files changed:
- src/backend/tcop/cmdtag.c: GUC variable + conditional OID emission
- src/backend/utils/misc/guc_parameters.dat: GUC registration
- src/include/tcop/cmdtag.h: extern declaration
- src/interfaces/libpq/fe-exec.c: handle INSERT with/without OID field

Author: NikolayS

src/backend/tcop/cmdtag.c

@@ -14,6 +14,7 @@
 #include "postgres.h"
 
 #include "tcop/cmdtag.h"
+#include "utils/guc.h"
 #include "utils/builtins.h"
 
 
@@ -36,6 +37,9 @@ static const CommandTagBehavior tag_behavior[] = {
 
 #undef PG_CMDTAG
 
+/* GUC variable: omit legacy OID field from INSERT command tag */
+bool		command_tag_omit_oid = false;
+
 void
 InitializeQueryCompletion(QueryCompletion *qc)
 {
@@ -145,7 +149,7 @@ BuildQueryCompletionString(char *buff, const QueryCompletion *qc,
 	 */
 	if (command_tag_display_rowcount(tag) && !nameonly)
 	{
-		if (tag == CMDTAG_INSERT)
+		if (tag == CMDTAG_INSERT && !command_tag_omit_oid)
 		{
 			*bufp++ = ' ';
 			*bufp++ = '0';

src/backend/utils/misc/guc_parameters.dat

@@ -426,6 +426,7 @@
   options => 'client_message_level_options',
 },
 
+
 { name => 'cluster_name', type => 'string', context => 'PGC_POSTMASTER', group => 'PROCESS_TITLE',
   short_desc => 'Sets the name of the cluster, which is included in the process title.',
   flags => 'GUC_IS_NAME',
@@ -435,6 +436,14 @@
 },
 
 # we have no microseconds designation, so can't supply units here
+
+{ name => 'command_tag_omit_oid', type => 'bool', context => 'PGC_USERSET', group => 'CLIENT_CONN_STATEMENT',
+  short_desc => 'Omits the legacy OID field from INSERT command completion tags.',
+  long_desc => 'When enabled, INSERT returns "INSERT N" instead of "INSERT 0 N". The "0" is a legacy placeholder from when tables could have OIDs (removed in PostgreSQL 12).',
+  variable => 'command_tag_omit_oid',
+  boot_val => 'false',
+  includes => 'tcop/cmdtag.h',
+},
 { name => 'commit_delay', type => 'int', context => 'PGC_SUSET', group => 'WAL_SETTINGS',
   short_desc => 'Sets the delay in microseconds between transaction commit and flushing WAL to disk.',
   variable => 'CommitDelay',

src/include/tcop/cmdtag.h

@@ -56,6 +56,9 @@ extern bool command_tag_display_rowcount(CommandTag commandTag);
 extern bool command_tag_event_trigger_ok(CommandTag commandTag);
 extern bool command_tag_table_rewrite_ok(CommandTag commandTag);
 extern CommandTag GetCommandTagEnum(const char *commandname);
+
+/* GUC variable: controls legacy OID in INSERT command tag */
+extern bool command_tag_omit_oid;
 extern Size BuildQueryCompletionString(char *buff, const QueryCompletion *qc,
 									   bool nameonly);
 

src/interfaces/libpq/fe-exec.c

@@ -3847,12 +3847,18 @@ PQcmdTuples(PGresult *res)
 	if (strncmp(res->cmdStatus, "INSERT ", 7) == 0)
 	{
 		p = res->cmdStatus + 7;
-		/* INSERT: skip oid and space */
-		while (*p && *p != ' ')
-			p++;
-		if (*p == 0)
-			goto interpret_error;	/* no space? */
-		p++;
+		/* INSERT: handle both "INSERT oid count" and "INSERT count" */
+		{
+			char *q = p;
+			while (*q && *q != ' ')
+				q++;
+			if (*q == ' ')
+			{
+				/* Old format: "INSERT oid count" - skip oid and space */
+				p = q + 1;
+			}
+			/* else: new format "INSERT count" - p already points to count */
+		}
 	}
 	else if (strncmp(res->cmdStatus, "SELECT ", 7) == 0 ||
 			 strncmp(res->cmdStatus, "DELETE ", 7) == 0 ||