*** a/src/bin/psql/tab-complete.c
--- b/src/bin/psql/tab-complete.c
***************
*** 813,820 **** static char *_complete_from_query(int is_schema_query,
  					 const char *text, int state);
  static char *complete_from_list(const char *text, int state);
  static char *complete_from_const(const char *text, int state);
  static char **complete_from_variables(const char *text,
! 						const char *prefix, const char *suffix);
  static char *complete_from_files(const char *text, int state);
  
  static char *pg_strdup_keyword_case(const char *s, const char *ref);
--- 813,823 ----
  					 const char *text, int state);
  static char *complete_from_list(const char *text, int state);
  static char *complete_from_const(const char *text, int state);
+ static void append_variable_names(char ***varnames, int *nvars,
+ 								  int *maxvars, const char *varname,
+ 								  const char *prefix, const char *suffix);
  static char **complete_from_variables(const char *text,
! 					  const char *prefix, const char *suffix, bool need_value);
  static char *complete_from_files(const char *text, int state);
  
  static char *pg_strdup_keyword_case(const char *s, const char *ref);
***************
*** 925,935 **** psql_completion(const char *text, int start, int end)
  	else if (text[0] == ':' && text[1] != ':')
  	{
  		if (text[1] == '\'')
! 			matches = complete_from_variables(text, ":'", "'");
  		else if (text[1] == '"')
! 			matches = complete_from_variables(text, ":\"", "\"");
  		else
! 			matches = complete_from_variables(text, ":", "");
  	}
  
  	/* If no previous word, suggest one of the basic sql commands */
--- 928,938 ----
  	else if (text[0] == ':' && text[1] != ':')
  	{
  		if (text[1] == '\'')
! 			matches = complete_from_variables(text, ":'", "'", true);
  		else if (text[1] == '"')
! 			matches = complete_from_variables(text, ":\"", "\"", true);
  		else
! 			matches = complete_from_variables(text, ":", "", true);
  	}
  
  	/* If no previous word, suggest one of the basic sql commands */
***************
*** 3604,3612 **** psql_completion(const char *text, int start, int end)
  			COMPLETE_WITH_LIST_CS(my_list);
  		}
  	}
  	else if (strcmp(prev_wd, "\\set") == 0)
  	{
! 		matches = complete_from_variables(text, "", "");
  	}
  	else if (strcmp(prev_wd, "\\sf") == 0 || strcmp(prev_wd, "\\sf+") == 0)
  		COMPLETE_WITH_SCHEMA_QUERY(Query_for_list_of_functions, NULL);
--- 3607,3677 ----
  			COMPLETE_WITH_LIST_CS(my_list);
  		}
  	}
+ 	else if (strcmp(prev_wd, "\\unset") == 0)
+ 	{
+ 		matches = complete_from_variables(text, "", "", true);
+ 	}
  	else if (strcmp(prev_wd, "\\set") == 0)
  	{
! 		matches = complete_from_variables(text, "", "", false);
! 	}
! 	else if (strcmp(prev2_wd, "\\set") == 0)
! 	{
! 		static const char *const boolean_value_list[] =
! 		{"on", "off", NULL};
! 
! 		if (strcmp(prev_wd, "AUTOCOMMIT") == 0)
! 			COMPLETE_WITH_LIST_CS(boolean_value_list);
! 		else if (strcmp(prev_wd, "COMP_KEYWORD_CASE") == 0)
! 		{
! 			static const char *const my_list[] =
! 			{"lower", "upper", "preserve-lower", "preserve-upper", NULL};
! 
! 			COMPLETE_WITH_LIST_CS(my_list);
! 		}
! 		else if (strcmp(prev_wd, "ECHO") == 0)
! 		{
! 			static const char *const my_list[] =
! 			{"errors", "queries", "all", NULL};
! 
! 			COMPLETE_WITH_LIST_CS(my_list);
! 		}
! 		else if (strcmp(prev_wd, "ECHO_HIDDEN") == 0)
! 		{
! 			static const char *const my_list[] =
! 			{"noexec", "off", "on", NULL};
! 
! 			COMPLETE_WITH_LIST_CS(my_list);
! 		}
! 		else if (strcmp(prev_wd, "HISTCONTROL") == 0)
! 		{
! 			static const char *const my_list[] =
! 			{"ignorespace", "ignoredups", "ignoreboth", NULL};
! 
! 			COMPLETE_WITH_LIST_CS(my_list);
! 		}
! 		else if (strcmp(prev_wd, "ON_ERROR_ROLLBACK") == 0)
! 		{
! 			static const char *const my_list[] =
! 			{"on", "off", "interactive", NULL};
! 
! 			COMPLETE_WITH_LIST_CS(my_list);
! 		}
! 		else if (strcmp(prev_wd, "ON_ERROR_STOP") == 0)
! 			COMPLETE_WITH_LIST_CS(boolean_value_list);
! 		else if (strcmp(prev_wd, "QUIET") == 0)
! 			COMPLETE_WITH_LIST_CS(boolean_value_list);
! 		else if (strcmp(prev_wd, "SINGLELINE") == 0)
! 			COMPLETE_WITH_LIST_CS(boolean_value_list);
! 		else if (strcmp(prev_wd, "SINGLESTEP") == 0)
! 			COMPLETE_WITH_LIST_CS(boolean_value_list);
! 		else if (strcmp(prev_wd, "VERBOSITY") == 0)
! 		{
! 			static const char *const my_list[] =
! 			{"default", "verbose", "terse", NULL};
! 
! 			COMPLETE_WITH_LIST_CS(my_list);
! 		}
  	}
  	else if (strcmp(prev_wd, "\\sf") == 0 || strcmp(prev_wd, "\\sf+") == 0)
  		COMPLETE_WITH_SCHEMA_QUERY(Query_for_list_of_functions, NULL);
***************
*** 4053,4064 **** complete_from_const(const char *text, int state)
  
  
  /*
   * This function supports completion with the name of a psql variable.
   * The variable names can be prefixed and suffixed with additional text
!  * to support quoting usages.
   */
  static char **
! complete_from_variables(const char *text, const char *prefix, const char *suffix)
  {
  	char	  **matches;
  	char	  **varnames;
--- 4118,4156 ----
  
  
  /*
+  * This function appends the variable name with prefix and suffix to
+  * the variable names array.
+  */
+ static void
+ append_variable_names(char ***varnames, int *nvars,
+ 					  int *maxvars, const char *varname,
+ 					  const char *prefix, const char *suffix)
+ {
+ 	if (*nvars >= *maxvars)
+ 	{
+ 		*maxvars *= 2;
+ 		*varnames = (char **) realloc(*varnames,
+ 									  ((*maxvars) + 1) * sizeof(char *));
+ 		if (!(*varnames))
+ 		{
+ 			psql_error("out of memory\n");
+ 			exit(EXIT_FAILURE);
+ 		}
+ 	}
+ 
+ 	(*varnames)[(*nvars)++] = psprintf("%s%s%s", prefix, varname, suffix);
+ }
+ 
+ 
+ /*
   * This function supports completion with the name of a psql variable.
   * The variable names can be prefixed and suffixed with additional text
!  * to support quoting usages. If need_value is true, only the variables
!  * that have the set values are picked up.
   */
  static char **
! complete_from_variables(const char *text, const char *prefix, const char *suffix,
! 						bool need_value)
  {
  	char	  **matches;
  	char	  **varnames;
***************
*** 4067,4089 **** complete_from_variables(const char *text, const char *prefix, const char *suffix
  	int			i;
  	struct _variable *ptr;
  
  	varnames = (char **) pg_malloc((maxvars + 1) * sizeof(char *));
  
! 	for (ptr = pset.vars->next; ptr; ptr = ptr->next)
  	{
! 		if (nvars >= maxvars)
! 		{
! 			maxvars *= 2;
! 			varnames = (char **) realloc(varnames,
! 										 (maxvars + 1) * sizeof(char *));
! 			if (!varnames)
! 			{
! 				psql_error("out of memory\n");
! 				exit(EXIT_FAILURE);
! 			}
! 		}
  
! 		varnames[nvars++] = psprintf("%s%s%s", prefix, ptr->name, suffix);
  	}
  
  	varnames[nvars] = NULL;
--- 4159,4187 ----
  	int			i;
  	struct _variable *ptr;
  
+ 	static const char *const known_varnames[] = {
+ 		"AUTOCOMMIT", "COMP_KEYWORD_CASE", "DBNAME", "ECHO", "ECHO_HIDDEN",
+ 		"ENCODING", "FETCH_COUNT", "HISTCONTROL", "HISTFILE", "HISTSIZE",
+ 		"HOST", "IGNOREEOF", "LASTOID", "ON_ERROR_ROLLBACK", "ON_ERROR_STOP",
+ 		"PORT", "PROMPT1", "PROMPT2", "PROMPT3", "QUIET", "SINGLELINE",
+ 		"SINGLESTEP", "USER", "VERBOSITY",	NULL
+ 	};
+ 
  	varnames = (char **) pg_malloc((maxvars + 1) * sizeof(char *));
  
! 	if (!need_value)
  	{
! 		for (i = 0; known_varnames[i] && nvars < maxvars; i++)
! 			append_variable_names(&varnames, &nvars, &maxvars,
! 								  known_varnames[i], prefix, suffix);
! 	}
  
! 	for (ptr = pset.vars->next; ptr; ptr = ptr->next)
! 	{
! 		if (need_value && !(ptr->value))
! 			continue;
! 		append_variable_names(&varnames, &nvars, &maxvars, ptr->name,
! 							  prefix, suffix);
  	}
  
  	varnames[nvars] = NULL;