*** a/doc/src/sgml/plpgsql.sgml
--- b/doc/src/sgml/plpgsql.sgml
***************
*** 3528,3533 **** RAISE unique_violation USING MESSAGE = 'Duplicate user ID: ' || user_id;
--- 3528,3596 ----
      </para>
     </note>
  
+   <sect2 id="plpgsql-assert">
+    <title>Assertions</title>
+ 
+    <para>
+     <literal>Assertions</literal> provide a way to check that the
+     internal state of a function is as expected.  For example:
+ <programlisting>
+ CREATE FUNCTION delete_user(username text) RETURNS VOID AS $$
+ BEGIN
+ 	DELETE FROM users WHERE users.username = delete_user.username;
+ 	ASSERT FOUND;
+ END
+ $$ LANGUAGE plpgsql;
+ 
+ SELECT delete_user('mia');
+ ERROR:  Assertion on line 4 failed
+ CONTEXT:  PL/pgSQL function delete_user(text) line 4 at ASSERT
+ </programlisting>
+ 
+     One could implement the equivalent functionality with a conditional
+     RAISE EXCEPTION statement, but assertions have two major differences:
+         <itemizedlist>
+          <listitem>
+            <para>
+             They're a lot faster to write than the equivalent IF .. THEN RAISE EXCEPTION .. END IF constructs.
+            </para>
+          </listitem>
+          <listitem>
+            <para>
+             They can be (and are by default) disabled in production
+             environments.  A disabled assertion only incurs a negligible
+             compile-time overhead and no execution time overhead, so you
+             can write complex checks for development environments without
+             having to worry about performance.
+            </para>
+          </listitem>
+         </itemizedlist> 
+    </para>
+ 
+    <para>
+     The configuration parameter <varname>plpgsql.enable_assertions</>
+     controls whether assertions are enabled.  Note that the value of
+     this parameter only affects subsequent compilations of
+     <application>PL/pgSQL</> functions, but not statements already
+     compiled in the current session.
+    </para>
+ 
+    <para>
+     It is also possible to enable assertions in a single function
+     (possibly overriding the global setting) by using a compile
+     option:
+ <programlisting>
+ CREATE FUNCTION delete_user(username text) RETURNS VOID AS $$
+ #enable_assertions
+ BEGIN
+ 	DELETE FROM users WHERE users.username = delete_user.username;
+ 	ASSERT FOUND;
+ END
+ $$ LANGUAGE plpgsql;
+ </programlisting>
+    </para>
+   </sect2>
+ 
   </sect1>
  
   <sect1 id="plpgsql-trigger">
*** a/src/pl/plpgsql/src/pl_comp.c
--- b/src/pl/plpgsql/src/pl_comp.c
***************
*** 351,356 **** do_compile(FunctionCallInfo fcinfo,
--- 351,357 ----
  	function->fn_cxt = func_cxt;
  	function->out_param_varno = -1;		/* set up for no OUT param */
  	function->resolve_option = plpgsql_variable_conflict;
+ 	function->enable_assertions = plpgsql_enable_assertions;
  
  	if (is_dml_trigger)
  		function->fn_is_trigger = PLPGSQL_DML_TRIGGER;
***************
*** 847,852 **** plpgsql_compile_inline(char *proc_source)
--- 848,854 ----
  	function->fn_cxt = func_cxt;
  	function->out_param_varno = -1;		/* set up for no OUT param */
  	function->resolve_option = plpgsql_variable_conflict;
+ 	function->enable_assertions = plpgsql_enable_assertions;
  
  	plpgsql_ns_init();
  	plpgsql_ns_push(func_name);
*** a/src/pl/plpgsql/src/pl_exec.c
--- b/src/pl/plpgsql/src/pl_exec.c
***************
*** 133,138 **** static int exec_stmt_dynexecute(PLpgSQL_execstate *estate,
--- 133,140 ----
  					 PLpgSQL_stmt_dynexecute *stmt);
  static int exec_stmt_dynfors(PLpgSQL_execstate *estate,
  				  PLpgSQL_stmt_dynfors *stmt);
+ static int exec_stmt_assert(PLpgSQL_execstate *estate,
+ 				  PLpgSQL_stmt_assert *stmt);
  
  static void plpgsql_estate_setup(PLpgSQL_execstate *estate,
  					 PLpgSQL_function *func,
***************
*** 1466,1471 **** exec_stmt(PLpgSQL_execstate *estate, PLpgSQL_stmt *stmt)
--- 1468,1477 ----
  			rc = exec_stmt_close(estate, (PLpgSQL_stmt_close *) stmt);
  			break;
  
+ 		case PLPGSQL_STMT_ASSERT:
+ 			rc = exec_stmt_assert(estate, (PLpgSQL_stmt_assert *) stmt);
+ 			break;
+ 
  		default:
  			estate->err_stmt = save_estmt;
  			elog(ERROR, "unrecognized cmdtype: %d", stmt->cmd_type);
***************
*** 3629,3634 **** exec_stmt_dynfors(PLpgSQL_execstate *estate, PLpgSQL_stmt_dynfors *stmt)
--- 3635,3655 ----
  	return rc;
  }
  
+ static int
+ exec_stmt_assert(PLpgSQL_execstate *estate, PLpgSQL_stmt_assert *stmt)
+ {
+ 	bool value;
+ 	bool isnull;
+ 
+ 	value = exec_eval_boolean(estate, stmt->expr, &isnull);
+ 	exec_eval_cleanup(estate);
+ 	if (isnull || !value)
+ 		ereport(ERROR,
+ 				(errcode(ERRCODE_ASSERTION_FAILURE),
+ 				 errmsg("Assertion on line %d failed", stmt->lineno)));
+ 
+ 	return PLPGSQL_RC_OK;
+ }
  
  /* ----------
   * exec_stmt_open			Execute an OPEN cursor statement
*** a/src/pl/plpgsql/src/pl_funcs.c
--- b/src/pl/plpgsql/src/pl_funcs.c
***************
*** 260,265 **** plpgsql_stmt_typename(PLpgSQL_stmt *stmt)
--- 260,267 ----
  			return "CLOSE";
  		case PLPGSQL_STMT_PERFORM:
  			return "PERFORM";
+ 		case PLPGSQL_STMT_ASSERT:
+ 			return "ASSERT";
  	}
  
  	return "unknown";
***************
*** 338,343 **** static void free_open(PLpgSQL_stmt_open *stmt);
--- 340,346 ----
  static void free_fetch(PLpgSQL_stmt_fetch *stmt);
  static void free_close(PLpgSQL_stmt_close *stmt);
  static void free_perform(PLpgSQL_stmt_perform *stmt);
+ static void free_assert(PLpgSQL_stmt_assert *stmt);
  static void free_expr(PLpgSQL_expr *expr);
  
  
***************
*** 415,420 **** free_stmt(PLpgSQL_stmt *stmt)
--- 418,426 ----
  		case PLPGSQL_STMT_PERFORM:
  			free_perform((PLpgSQL_stmt_perform *) stmt);
  			break;
+ 		case PLPGSQL_STMT_ASSERT:
+ 			free_assert((PLpgSQL_stmt_assert *) stmt);
+ 			break;
  		default:
  			elog(ERROR, "unrecognized cmd_type: %d", stmt->cmd_type);
  			break;
***************
*** 563,568 **** free_perform(PLpgSQL_stmt_perform *stmt)
--- 569,580 ----
  }
  
  static void
+ free_assert(PLpgSQL_stmt_assert *stmt)
+ {
+ 	free_expr(stmt->expr);
+ }
+ 
+ static void
  free_exit(PLpgSQL_stmt_exit *stmt)
  {
  	free_expr(stmt->cond);
***************
*** 742,747 **** static void dump_cursor_direction(PLpgSQL_stmt_fetch *stmt);
--- 754,760 ----
  static void dump_close(PLpgSQL_stmt_close *stmt);
  static void dump_perform(PLpgSQL_stmt_perform *stmt);
  static void dump_expr(PLpgSQL_expr *expr);
+ static void dump_assert(PLpgSQL_stmt_assert *stmt);
  
  
  static void
***************
*** 828,833 **** dump_stmt(PLpgSQL_stmt *stmt)
--- 841,849 ----
  		case PLPGSQL_STMT_PERFORM:
  			dump_perform((PLpgSQL_stmt_perform *) stmt);
  			break;
+ 		case PLPGSQL_STMT_ASSERT:
+ 			dump_assert((PLpgSQL_stmt_assert *) stmt);
+ 			break;
  		default:
  			elog(ERROR, "unrecognized cmd_type: %d", stmt->cmd_type);
  			break;
***************
*** 1205,1210 **** dump_perform(PLpgSQL_stmt_perform *stmt)
--- 1221,1235 ----
  }
  
  static void
+ dump_assert(PLpgSQL_stmt_assert *stmt)
+ {
+ 	dump_ind();
+ 	printf("ASSERT expr = ");
+ 	dump_expr(stmt->expr);
+ 	printf("\n");
+ }
+ 
+ static void
  dump_exit(PLpgSQL_stmt_exit *stmt)
  {
  	dump_ind();
*** a/src/pl/plpgsql/src/pl_gram.y
--- b/src/pl/plpgsql/src/pl_gram.y
***************
*** 193,198 **** static	List			*read_raise_options(void);
--- 193,199 ----
  %type <stmt>	stmt_assign stmt_if stmt_loop stmt_while stmt_exit
  %type <stmt>	stmt_return stmt_raise stmt_execsql
  %type <stmt>	stmt_dynexecute stmt_for stmt_perform stmt_getdiag
+ %type <stmt>	stmt_assert
  %type <stmt>	stmt_open stmt_fetch stmt_move stmt_close stmt_null
  %type <stmt>	stmt_case stmt_foreach_a
  
***************
*** 245,250 **** static	List			*read_raise_options(void);
--- 246,252 ----
  %token <keyword>	K_ALIAS
  %token <keyword>	K_ALL
  %token <keyword>	K_ARRAY
+ %token <keyword>	K_ASSERT
  %token <keyword>	K_BACKWARD
  %token <keyword>	K_BEGIN
  %token <keyword>	K_BY
***************
*** 268,273 **** static	List			*read_raise_options(void);
--- 270,276 ----
  %token <keyword>	K_DUMP
  %token <keyword>	K_ELSE
  %token <keyword>	K_ELSIF
+ %token <keyword>	K_ENABLE_ASSERTIONS
  %token <keyword>	K_END
  %token <keyword>	K_ERRCODE
  %token <keyword>	K_ERROR
***************
*** 354,359 **** comp_option		: '#' K_OPTION K_DUMP
--- 357,366 ----
  					{
  						plpgsql_DumpExecTree = true;
  					}
+ 				| '#' K_ENABLE_ASSERTIONS
+ 					{
+ 						plpgsql_curr_compile->enable_assertions = true;
+ 					}
  				| '#' K_VARIABLE_CONFLICT K_ERROR
  					{
  						plpgsql_curr_compile->resolve_option = PLPGSQL_RESOLVE_ERROR;
***************
*** 833,838 **** proc_stmt		: pl_block ';'
--- 840,847 ----
  						{ $$ = $1; }
  				| stmt_getdiag
  						{ $$ = $1; }
+ 				| stmt_assert
+ 						{ $$ = $1; }
  				| stmt_open
  						{ $$ = $1; }
  				| stmt_fetch
***************
*** 934,939 **** stmt_getdiag	: K_GET getdiag_area_opt K_DIAGNOSTICS getdiag_list ';'
--- 943,969 ----
  					}
  				;
  
+ stmt_assert : K_ASSERT expr_until_semi
+ 					{
+ 						if (plpgsql_curr_compile->enable_assertions)
+ 						{
+ 							PLpgSQL_stmt_assert *new;
+ 
+ 							new = palloc0(sizeof(PLpgSQL_stmt_assert));
+ 							new->cmd_type = PLPGSQL_STMT_ASSERT;
+ 							new->lineno   = plpgsql_location_to_lineno(@1);
+ 							new->expr  = $2;
+ 
+ 							$$ = (PLpgSQL_stmt *)new;
+ 						}
+ 						else
+ 						{
+ 							/* skip if assertions are disabled */
+ 							$$ = NULL;
+ 						}
+ 					}
+ 				;
+ 
  getdiag_area_opt :
  					{
  						$$ = false;
*** a/src/pl/plpgsql/src/pl_handler.c
--- b/src/pl/plpgsql/src/pl_handler.c
***************
*** 36,41 **** static const struct config_enum_entry variable_conflict_options[] = {
--- 36,42 ----
  };
  
  int			plpgsql_variable_conflict = PLPGSQL_RESOLVE_ERROR;
+ bool		plpgsql_enable_assertions = false;
  
  /* Hook for plugins */
  PLpgSQL_plugin **plugin_ptr = NULL;
***************
*** 66,71 **** _PG_init(void)
--- 67,81 ----
  							 PGC_SUSET, 0,
  							 NULL, NULL, NULL);
  
+ 	DefineCustomBoolVariable("plpgsql.enable_assertions",
+ 							 gettext_noop("Enables assertions in PL/PgSQL functions."),
+ 							 NULL,
+ 							 &plpgsql_enable_assertions,
+ 							 false,
+ 							 PGC_USERSET, 0,
+ 							 NULL, NULL, NULL);
+ 
+ 
  	EmitWarningsOnPlaceholders("plpgsql");
  
  	plpgsql_HashTableInit();
*** a/src/pl/plpgsql/src/pl_scanner.c
--- b/src/pl/plpgsql/src/pl_scanner.c
***************
*** 60,65 **** IdentifierLookup plpgsql_IdentifierLookup = IDENTIFIER_LOOKUP_NORMAL;
--- 60,66 ----
  
  static const ScanKeyword reserved_keywords[] = {
  	PG_KEYWORD("all", K_ALL, RESERVED_KEYWORD)
+ 	PG_KEYWORD("assert", K_ASSERT, RESERVED_KEYWORD)
  	PG_KEYWORD("begin", K_BEGIN, RESERVED_KEYWORD)
  	PG_KEYWORD("by", K_BY, RESERVED_KEYWORD)
  	PG_KEYWORD("case", K_CASE, RESERVED_KEYWORD)
***************
*** 120,125 **** static const ScanKeyword unreserved_keywords[] = {
--- 121,127 ----
  	PG_KEYWORD("debug", K_DEBUG, UNRESERVED_KEYWORD)
  	PG_KEYWORD("detail", K_DETAIL, UNRESERVED_KEYWORD)
  	PG_KEYWORD("dump", K_DUMP, UNRESERVED_KEYWORD)
+ 	PG_KEYWORD("enable_assertions", K_ENABLE_ASSERTIONS, UNRESERVED_KEYWORD)
  	PG_KEYWORD("errcode", K_ERRCODE, UNRESERVED_KEYWORD)
  	PG_KEYWORD("error", K_ERROR, UNRESERVED_KEYWORD)
  	PG_KEYWORD("first", K_FIRST, UNRESERVED_KEYWORD)
*** a/src/pl/plpgsql/src/plpgsql.h
--- b/src/pl/plpgsql/src/plpgsql.h
***************
*** 100,106 **** enum PLpgSQL_stmt_types
  	PLPGSQL_STMT_OPEN,
  	PLPGSQL_STMT_FETCH,
  	PLPGSQL_STMT_CLOSE,
! 	PLPGSQL_STMT_PERFORM
  };
  
  
--- 100,107 ----
  	PLPGSQL_STMT_OPEN,
  	PLPGSQL_STMT_FETCH,
  	PLPGSQL_STMT_CLOSE,
! 	PLPGSQL_STMT_PERFORM,
! 	PLPGSQL_STMT_ASSERT
  };
  
  
***************
*** 403,408 **** typedef struct
--- 404,416 ----
  	List	   *diag_items;		/* List of PLpgSQL_diag_item */
  } PLpgSQL_stmt_getdiag;
  
+ typedef struct
+ {								/* ASSERT statement */
+ 	int			cmd_type;
+ 	int			lineno;
+ 	PLpgSQL_expr *expr;
+ } PLpgSQL_stmt_assert;
+ 
  
  typedef struct
  {								/* IF statement				*/
***************
*** 736,741 **** typedef struct PLpgSQL_function
--- 744,750 ----
  	int			tg_tag_varno;
  
  	PLpgSQL_resolve_option resolve_option;
+ 	bool		enable_assertions;
  
  	int			ndatums;
  	PLpgSQL_datum **datums;
***************
*** 873,878 **** extern IdentifierLookup plpgsql_IdentifierLookup;
--- 882,889 ----
  
  extern int	plpgsql_variable_conflict;
  
+ extern bool plpgsql_enable_assertions;
+ 
  extern bool plpgsql_check_syntax;
  extern bool plpgsql_DumpExecTree;
  
*** a/src/test/regress/expected/plpgsql.out
--- b/src/test/regress/expected/plpgsql.out
***************
*** 5085,5087 **** NOTICE:  outer_func() done
--- 5085,5161 ----
  drop function outer_outer_func(int);
  drop function outer_func(int);
  drop function inner_func(int);
+ -- check assertions
+ set plpgsql.enable_assertions to true;
+ create function foof()
+ returns void
+ as $$
+ begin
+ -- failure
+ assert 1 > 2;
+ end
+ $$ language plpgsql;
+ select foof();
+ ERROR:  Assertion on line 4 failed
+ CONTEXT:  PL/pgSQL function foof() line 4 at ASSERT
+ drop function foof();
+ create function foof()
+ returns void
+ as $$
+ begin
+ -- no failure
+ assert 1 < 2;
+ end
+ $$ language plpgsql;
+ select foof();
+  foof 
+ ------
+  
+ (1 row)
+ 
+ drop function foof();
+ create function foof()
+ returns void
+ as $$
+ begin
+ -- NULL is failure
+ assert 1 < NULL;
+ end
+ $$ language plpgsql;
+ select foof();
+ ERROR:  Assertion on line 4 failed
+ CONTEXT:  PL/pgSQL function foof() line 4 at ASSERT
+ reset plpgsql.enable_assertions;
+ -- still enabled for foof
+ select foof();
+ ERROR:  Assertion on line 4 failed
+ CONTEXT:  PL/pgSQL function foof() line 4 at ASSERT
+ drop function foof();
+ create function foof()
+ returns void
+ as $$
+ #enable_assertions
+ begin
+ -- failure
+ assert 1 > 2;
+ end
+ $$ language plpgsql;
+ select foof();
+ ERROR:  Assertion on line 5 failed
+ CONTEXT:  PL/pgSQL function foof() line 5 at ASSERT
+ drop function foof();
+ create function foof()
+ returns void
+ as $$
+ begin
+ -- failure, but assertions are disabled
+ assert 1 > 2;
+ end
+ $$ language plpgsql;
+ select foof();
+  foof 
+ ------
+  
+ (1 row)
+ 
+ drop function foof();
*** a/src/test/regress/sql/plpgsql.sql
--- b/src/test/regress/sql/plpgsql.sql
***************
*** 3983,3985 **** drop function outer_outer_func(int);
--- 3983,4055 ----
  drop function outer_func(int);
  drop function inner_func(int);
  
+ -- check assertions
+ 
+ set plpgsql.enable_assertions to true;
+ 
+ create function foof()
+ returns void
+ as $$
+ begin
+ -- failure
+ assert 1 > 2;
+ end
+ $$ language plpgsql;
+ 
+ select foof();
+ drop function foof();
+ 
+ create function foof()
+ returns void
+ as $$
+ begin
+ -- no failure
+ assert 1 < 2;
+ end
+ $$ language plpgsql;
+ 
+ select foof();
+ drop function foof();
+ 
+ create function foof()
+ returns void
+ as $$
+ begin
+ -- NULL is failure
+ assert 1 < NULL;
+ end
+ $$ language plpgsql;
+ 
+ select foof();
+ 
+ reset plpgsql.enable_assertions;
+ -- still enabled for foof
+ select foof();
+ 
+ drop function foof();
+ 
+ create function foof()
+ returns void
+ as $$
+ #enable_assertions
+ begin
+ -- failure
+ assert 1 > 2;
+ end
+ $$ language plpgsql;
+ 
+ select foof();
+ drop function foof();
+ 
+ create function foof()
+ returns void
+ as $$
+ begin
+ -- failure, but assertions are disabled
+ assert 1 > 2;
+ end
+ $$ language plpgsql;
+ 
+ select foof();
+ drop function foof();
+