Speed up plpgsql trigger startup by introducing "promises".

Over the years we've accreted quite a few special variables that are
predefined in plpgsql trigger functions.  The cost of initializing these
variables to their defined values turns out to be a significant part of
the runtime of simple triggers; but, undoubtedly, most real-world triggers
never examine the values of most of these variables.

To improve matters, invent the notion of a variable that has a "promise"
attached to it, specifying which of the predetermined values should be
assigned to the variable if anything ever reads it.  This eliminates all
the unneeded startup overhead, in return for a small penalty on accesses
to these variables.

Tom Lane, reviewed by Pavel Stehule

Discussion: https://postgr.es/m/11986.1514407114@sss.pgh.pa.us

src/pl/plpgsql/src/pl_comp.c

@@ -607,7 +607,9 @@ do_compile(FunctionCallInfo fcinfo,
 																-1,
 																InvalidOid),
 										 true);
-			function->tg_name_varno = var->dno;
+			Assert(var->dtype == PLPGSQL_DTYPE_VAR);
+			var->dtype = PLPGSQL_DTYPE_PROMISE;
+			((PLpgSQL_var *) var)->promise = PLPGSQL_PROMISE_TG_NAME;
 
 			/* Add the variable tg_when */
 			var = plpgsql_build_variable("tg_when", 0,
@@ -615,7 +617,9 @@ do_compile(FunctionCallInfo fcinfo,
 																-1,
 																function->fn_input_collation),
 										 true);
-			function->tg_when_varno = var->dno;
+			Assert(var->dtype == PLPGSQL_DTYPE_VAR);
+			var->dtype = PLPGSQL_DTYPE_PROMISE;
+			((PLpgSQL_var *) var)->promise = PLPGSQL_PROMISE_TG_WHEN;
 
 			/* Add the variable tg_level */
 			var = plpgsql_build_variable("tg_level", 0,
@@ -623,7 +627,9 @@ do_compile(FunctionCallInfo fcinfo,
 																-1,
 																function->fn_input_collation),
 										 true);
-			function->tg_level_varno = var->dno;
+			Assert(var->dtype == PLPGSQL_DTYPE_VAR);
+			var->dtype = PLPGSQL_DTYPE_PROMISE;
+			((PLpgSQL_var *) var)->promise = PLPGSQL_PROMISE_TG_LEVEL;
 
 			/* Add the variable tg_op */
 			var = plpgsql_build_variable("tg_op", 0,
@@ -631,7 +637,9 @@ do_compile(FunctionCallInfo fcinfo,
 																-1,
 																function->fn_input_collation),
 										 true);
-			function->tg_op_varno = var->dno;
+			Assert(var->dtype == PLPGSQL_DTYPE_VAR);
+			var->dtype = PLPGSQL_DTYPE_PROMISE;
+			((PLpgSQL_var *) var)->promise = PLPGSQL_PROMISE_TG_OP;
 
 			/* Add the variable tg_relid */
 			var = plpgsql_build_variable("tg_relid", 0,
@@ -639,7 +647,9 @@ do_compile(FunctionCallInfo fcinfo,
 																-1,
 																InvalidOid),
 										 true);
-			function->tg_relid_varno = var->dno;
+			Assert(var->dtype == PLPGSQL_DTYPE_VAR);
+			var->dtype = PLPGSQL_DTYPE_PROMISE;
+			((PLpgSQL_var *) var)->promise = PLPGSQL_PROMISE_TG_RELID;
 
 			/* Add the variable tg_relname */
 			var = plpgsql_build_variable("tg_relname", 0,
@@ -647,7 +657,9 @@ do_compile(FunctionCallInfo fcinfo,
 																-1,
 																InvalidOid),
 										 true);
-			function->tg_relname_varno = var->dno;
+			Assert(var->dtype == PLPGSQL_DTYPE_VAR);
+			var->dtype = PLPGSQL_DTYPE_PROMISE;
+			((PLpgSQL_var *) var)->promise = PLPGSQL_PROMISE_TG_TABLE_NAME;
 
 			/* tg_table_name is now preferred to tg_relname */
 			var = plpgsql_build_variable("tg_table_name", 0,
@@ -655,7 +667,9 @@ do_compile(FunctionCallInfo fcinfo,
 																-1,
 																InvalidOid),
 										 true);
-			function->tg_table_name_varno = var->dno;
+			Assert(var->dtype == PLPGSQL_DTYPE_VAR);
+			var->dtype = PLPGSQL_DTYPE_PROMISE;
+			((PLpgSQL_var *) var)->promise = PLPGSQL_PROMISE_TG_TABLE_NAME;
 
 			/* add the variable tg_table_schema */
 			var = plpgsql_build_variable("tg_table_schema", 0,
@@ -663,7 +677,9 @@ do_compile(FunctionCallInfo fcinfo,
 																-1,
 																InvalidOid),
 										 true);
-			function->tg_table_schema_varno = var->dno;
+			Assert(var->dtype == PLPGSQL_DTYPE_VAR);
+			var->dtype = PLPGSQL_DTYPE_PROMISE;
+			((PLpgSQL_var *) var)->promise = PLPGSQL_PROMISE_TG_TABLE_SCHEMA;
 
 			/* Add the variable tg_nargs */
 			var = plpgsql_build_variable("tg_nargs", 0,
@@ -671,7 +687,9 @@ do_compile(FunctionCallInfo fcinfo,
 																-1,
 																InvalidOid),
 										 true);
-			function->tg_nargs_varno = var->dno;
+			Assert(var->dtype == PLPGSQL_DTYPE_VAR);
+			var->dtype = PLPGSQL_DTYPE_PROMISE;
+			((PLpgSQL_var *) var)->promise = PLPGSQL_PROMISE_TG_NARGS;
 
 			/* Add the variable tg_argv */
 			var = plpgsql_build_variable("tg_argv", 0,
@@ -679,7 +697,9 @@ do_compile(FunctionCallInfo fcinfo,
 																-1,
 																function->fn_input_collation),
 										 true);
-			function->tg_argv_varno = var->dno;
+			Assert(var->dtype == PLPGSQL_DTYPE_VAR);
+			var->dtype = PLPGSQL_DTYPE_PROMISE;
+			((PLpgSQL_var *) var)->promise = PLPGSQL_PROMISE_TG_ARGV;
 
 			break;
 
@@ -701,7 +721,9 @@ do_compile(FunctionCallInfo fcinfo,
 																-1,
 																function->fn_input_collation),
 										 true);
-			function->tg_event_varno = var->dno;
+			Assert(var->dtype == PLPGSQL_DTYPE_VAR);
+			var->dtype = PLPGSQL_DTYPE_PROMISE;
+			((PLpgSQL_var *) var)->promise = PLPGSQL_PROMISE_TG_EVENT;
 
 			/* Add the variable tg_tag */
 			var = plpgsql_build_variable("tg_tag", 0,
@@ -709,7 +731,9 @@ do_compile(FunctionCallInfo fcinfo,
 																-1,
 																function->fn_input_collation),
 										 true);
-			function->tg_tag_varno = var->dno;
+			Assert(var->dtype == PLPGSQL_DTYPE_VAR);
+			var->dtype = PLPGSQL_DTYPE_PROMISE;
+			((PLpgSQL_var *) var)->promise = PLPGSQL_PROMISE_TG_TAG;
 
 			break;
 
@@ -1878,6 +1902,7 @@ build_row_from_vars(PLpgSQL_variable **vars, int numvars)
 		switch (var->dtype)
 		{
 			case PLPGSQL_DTYPE_VAR:
+			case PLPGSQL_DTYPE_PROMISE:
 				typoid = ((PLpgSQL_var *) var)->datatype->typoid;
 				typmod = ((PLpgSQL_var *) var)->datatype->atttypmod;
 				typcoll = ((PLpgSQL_var *) var)->datatype->collation;
@@ -2196,6 +2221,7 @@ plpgsql_finish_datums(PLpgSQL_function *function)
 		switch (function->datums[i]->dtype)
 		{
 			case PLPGSQL_DTYPE_VAR:
+			case PLPGSQL_DTYPE_PROMISE:
 				copiable_size += MAXALIGN(sizeof(PLpgSQL_var));
 				break;
 			case PLPGSQL_DTYPE_REC:

src/pl/plpgsql/src/pl_exec.c

@@ -237,6 +237,8 @@ static void coerce_function_result_tuple(PLpgSQL_execstate *estate,
 static void plpgsql_exec_error_callback(void *arg);
 static void copy_plpgsql_datums(PLpgSQL_execstate *estate,
 					PLpgSQL_function *func);
+static void plpgsql_fulfill_promise(PLpgSQL_execstate *estate,
+						PLpgSQL_var *var);
 static MemoryContext get_stmt_mcontext(PLpgSQL_execstate *estate);
 static void push_stmt_mcontext(PLpgSQL_execstate *estate);
 static void pop_stmt_mcontext(PLpgSQL_execstate *estate);
@@ -547,6 +549,7 @@ plpgsql_exec_function(PLpgSQL_function *func, FunctionCallInfo fcinfo,
 				break;
 
 			default:
+				/* Anything else should not be an argument variable */
 				elog(ERROR, "unrecognized dtype: %d", func->datums[i]->dtype);
 		}
 	}
@@ -834,10 +837,8 @@ plpgsql_exec_trigger(PLpgSQL_function *func,
 {
 	PLpgSQL_execstate estate;
 	ErrorContextCallback plerrcontext;
-	int			i;
 	int			rc;
 	TupleDesc	tupdesc;
-	PLpgSQL_var *var;
 	PLpgSQL_rec *rec_new,
 			   *rec_old;
 	HeapTuple	rettup;
@@ -846,6 +847,7 @@ plpgsql_exec_trigger(PLpgSQL_function *func,
 	 * Setup the execution state
 	 */
 	plpgsql_estate_setup(&estate, func, NULL, NULL);
+	estate.trigdata = trigdata;
 
 	/*
 	 * Setup error traceback support for ereport()
@@ -906,106 +908,6 @@ plpgsql_exec_trigger(PLpgSQL_function *func,
 	rc = SPI_register_trigger_data(trigdata);
 	Assert(rc >= 0);
 
-	/*
-	 * Assign the special tg_ variables
-	 */
-
-	var = (PLpgSQL_var *) (estate.datums[func->tg_op_varno]);
-	if (TRIGGER_FIRED_BY_INSERT(trigdata->tg_event))
-		assign_text_var(&estate, var, "INSERT");
-	else if (TRIGGER_FIRED_BY_UPDATE(trigdata->tg_event))
-		assign_text_var(&estate, var, "UPDATE");
-	else if (TRIGGER_FIRED_BY_DELETE(trigdata->tg_event))
-		assign_text_var(&estate, var, "DELETE");
-	else if (TRIGGER_FIRED_BY_TRUNCATE(trigdata->tg_event))
-		assign_text_var(&estate, var, "TRUNCATE");
-	else
-		elog(ERROR, "unrecognized trigger action: not INSERT, DELETE, UPDATE, or TRUNCATE");
-
-	var = (PLpgSQL_var *) (estate.datums[func->tg_name_varno]);
-	assign_simple_var(&estate, var,
-					  DirectFunctionCall1(namein,
-										  CStringGetDatum(trigdata->tg_trigger->tgname)),
-					  false, true);
-
-	var = (PLpgSQL_var *) (estate.datums[func->tg_when_varno]);
-	if (TRIGGER_FIRED_BEFORE(trigdata->tg_event))
-		assign_text_var(&estate, var, "BEFORE");
-	else if (TRIGGER_FIRED_AFTER(trigdata->tg_event))
-		assign_text_var(&estate, var, "AFTER");
-	else if (TRIGGER_FIRED_INSTEAD(trigdata->tg_event))
-		assign_text_var(&estate, var, "INSTEAD OF");
-	else
-		elog(ERROR, "unrecognized trigger execution time: not BEFORE, AFTER, or INSTEAD OF");
-
-	var = (PLpgSQL_var *) (estate.datums[func->tg_level_varno]);
-	if (TRIGGER_FIRED_FOR_ROW(trigdata->tg_event))
-		assign_text_var(&estate, var, "ROW");
-	else if (TRIGGER_FIRED_FOR_STATEMENT(trigdata->tg_event))
-		assign_text_var(&estate, var, "STATEMENT");
-	else
-		elog(ERROR, "unrecognized trigger event type: not ROW or STATEMENT");
-
-	var = (PLpgSQL_var *) (estate.datums[func->tg_relid_varno]);
-	assign_simple_var(&estate, var,
-					  ObjectIdGetDatum(trigdata->tg_relation->rd_id),
-					  false, false);
-
-	var = (PLpgSQL_var *) (estate.datums[func->tg_relname_varno]);
-	assign_simple_var(&estate, var,
-					  DirectFunctionCall1(namein,
-										  CStringGetDatum(RelationGetRelationName(trigdata->tg_relation))),
-					  false, true);
-
-	var = (PLpgSQL_var *) (estate.datums[func->tg_table_name_varno]);
-	assign_simple_var(&estate, var,
-					  DirectFunctionCall1(namein,
-										  CStringGetDatum(RelationGetRelationName(trigdata->tg_relation))),
-					  false, true);
-
-	var = (PLpgSQL_var *) (estate.datums[func->tg_table_schema_varno]);
-	assign_simple_var(&estate, var,
-					  DirectFunctionCall1(namein,
-										  CStringGetDatum(get_namespace_name(
-																			 RelationGetNamespace(
-																								  trigdata->tg_relation)))),
-					  false, true);
-
-	var = (PLpgSQL_var *) (estate.datums[func->tg_nargs_varno]);
-	assign_simple_var(&estate, var,
-					  Int16GetDatum(trigdata->tg_trigger->tgnargs),
-					  false, false);
-
-	var = (PLpgSQL_var *) (estate.datums[func->tg_argv_varno]);
-	if (trigdata->tg_trigger->tgnargs > 0)
-	{
-		/*
-		 * For historical reasons, tg_argv[] subscripts start at zero not one.
-		 * So we can't use construct_array().
-		 */
-		int			nelems = trigdata->tg_trigger->tgnargs;
-		Datum	   *elems;
-		int			dims[1];
-		int			lbs[1];
-
-		elems = palloc(sizeof(Datum) * nelems);
-		for (i = 0; i < nelems; i++)
-			elems[i] = CStringGetTextDatum(trigdata->tg_trigger->tgargs[i]);
-		dims[0] = nelems;
-		lbs[0] = 0;
-
-		assign_simple_var(&estate, var,
-						  PointerGetDatum(construct_md_array(elems, NULL,
-															 1, dims, lbs,
-															 TEXTOID,
-															 -1, false, 'i')),
-						  false, true);
-	}
-	else
-	{
-		assign_simple_var(&estate, var, (Datum) 0, true, false);
-	}
-
 	estate.err_text = gettext_noop("during function entry");
 
 	/*
@@ -1153,12 +1055,12 @@ plpgsql_exec_event_trigger(PLpgSQL_function *func, EventTriggerData *trigdata)
 	PLpgSQL_execstate estate;
 	ErrorContextCallback plerrcontext;
 	int			rc;
-	PLpgSQL_var *var;
 
 	/*
 	 * Setup the execution state
 	 */
 	plpgsql_estate_setup(&estate, func, NULL, NULL);
+	estate.evtrigdata = trigdata;
 
 	/*
 	 * Setup error traceback support for ereport()
@@ -1174,15 +1076,6 @@ plpgsql_exec_event_trigger(PLpgSQL_function *func, EventTriggerData *trigdata)
 	estate.err_text = gettext_noop("during initialization of execution state");
 	copy_plpgsql_datums(&estate, func);
 
-	/*
-	 * Assign the special tg_ variables
-	 */
-	var = (PLpgSQL_var *) (estate.datums[func->tg_event_varno]);
-	assign_text_var(&estate, var, trigdata->event);
-
-	var = (PLpgSQL_var *) (estate.datums[func->tg_tag_varno]);
-	assign_text_var(&estate, var, trigdata->tag);
-
 	/*
 	 * Let the instrumentation plugin peek at this function
 	 */
@@ -1321,6 +1214,7 @@ copy_plpgsql_datums(PLpgSQL_execstate *estate,
 		switch (indatum->dtype)
 		{
 			case PLPGSQL_DTYPE_VAR:
+			case PLPGSQL_DTYPE_PROMISE:
 				outdatum = (PLpgSQL_datum *) ws_next;
 				memcpy(outdatum, indatum, sizeof(PLpgSQL_var));
 				ws_next += MAXALIGN(sizeof(PLpgSQL_var));
@@ -1356,6 +1250,166 @@ copy_plpgsql_datums(PLpgSQL_execstate *estate,
 	Assert(ws_next == workspace + func->copiable_size);
 }
 
+/*
+ * If the variable has an armed "promise", compute the promised value
+ * and assign it to the variable.
+ * The assignment automatically disarms the promise.
+ */
+static void
+plpgsql_fulfill_promise(PLpgSQL_execstate *estate,
+						PLpgSQL_var *var)
+{
+	MemoryContext oldcontext;
+
+	if (var->promise == PLPGSQL_PROMISE_NONE)
+		return;					/* nothing to do */
+
+	/*
+	 * This will typically be invoked in a short-lived context such as the
+	 * mcontext.  We must create variable values in the estate's datum
+	 * context.  This quick-and-dirty solution risks leaking some additional
+	 * cruft there, but since any one promise is honored at most once per
+	 * function call, it's probably not worth being more careful.
+	 */
+	oldcontext = MemoryContextSwitchTo(estate->datum_context);
+
+	switch (var->promise)
+	{
+		case PLPGSQL_PROMISE_TG_NAME:
+			if (estate->trigdata == NULL)
+				elog(ERROR, "trigger promise is not in a trigger function");
+			assign_simple_var(estate, var,
+							  DirectFunctionCall1(namein,
+												  CStringGetDatum(estate->trigdata->tg_trigger->tgname)),
+							  false, true);
+			break;
+
+		case PLPGSQL_PROMISE_TG_WHEN:
+			if (estate->trigdata == NULL)
+				elog(ERROR, "trigger promise is not in a trigger function");
+			if (TRIGGER_FIRED_BEFORE(estate->trigdata->tg_event))
+				assign_text_var(estate, var, "BEFORE");
+			else if (TRIGGER_FIRED_AFTER(estate->trigdata->tg_event))
+				assign_text_var(estate, var, "AFTER");
+			else if (TRIGGER_FIRED_INSTEAD(estate->trigdata->tg_event))
+				assign_text_var(estate, var, "INSTEAD OF");
+			else
+				elog(ERROR, "unrecognized trigger execution time: not BEFORE, AFTER, or INSTEAD OF");
+			break;
+
+		case PLPGSQL_PROMISE_TG_LEVEL:
+			if (estate->trigdata == NULL)
+				elog(ERROR, "trigger promise is not in a trigger function");
+			if (TRIGGER_FIRED_FOR_ROW(estate->trigdata->tg_event))
+				assign_text_var(estate, var, "ROW");
+			else if (TRIGGER_FIRED_FOR_STATEMENT(estate->trigdata->tg_event))
+				assign_text_var(estate, var, "STATEMENT");
+			else
+				elog(ERROR, "unrecognized trigger event type: not ROW or STATEMENT");
+			break;
+
+		case PLPGSQL_PROMISE_TG_OP:
+			if (estate->trigdata == NULL)
+				elog(ERROR, "trigger promise is not in a trigger function");
+			if (TRIGGER_FIRED_BY_INSERT(estate->trigdata->tg_event))
+				assign_text_var(estate, var, "INSERT");
+			else if (TRIGGER_FIRED_BY_UPDATE(estate->trigdata->tg_event))
+				assign_text_var(estate, var, "UPDATE");
+			else if (TRIGGER_FIRED_BY_DELETE(estate->trigdata->tg_event))
+				assign_text_var(estate, var, "DELETE");
+			else if (TRIGGER_FIRED_BY_TRUNCATE(estate->trigdata->tg_event))
+				assign_text_var(estate, var, "TRUNCATE");
+			else
+				elog(ERROR, "unrecognized trigger action: not INSERT, DELETE, UPDATE, or TRUNCATE");
+			break;
+
+		case PLPGSQL_PROMISE_TG_RELID:
+			if (estate->trigdata == NULL)
+				elog(ERROR, "trigger promise is not in a trigger function");
+			assign_simple_var(estate, var,
+							  ObjectIdGetDatum(estate->trigdata->tg_relation->rd_id),
+							  false, false);
+			break;
+
+		case PLPGSQL_PROMISE_TG_TABLE_NAME:
+			if (estate->trigdata == NULL)
+				elog(ERROR, "trigger promise is not in a trigger function");
+			assign_simple_var(estate, var,
+							  DirectFunctionCall1(namein,
+												  CStringGetDatum(RelationGetRelationName(estate->trigdata->tg_relation))),
+							  false, true);
+			break;
+
+		case PLPGSQL_PROMISE_TG_TABLE_SCHEMA:
+			if (estate->trigdata == NULL)
+				elog(ERROR, "trigger promise is not in a trigger function");
+			assign_simple_var(estate, var,
+							  DirectFunctionCall1(namein,
+												  CStringGetDatum(get_namespace_name(RelationGetNamespace(estate->trigdata->tg_relation)))),
+							  false, true);
+			break;
+
+		case PLPGSQL_PROMISE_TG_NARGS:
+			if (estate->trigdata == NULL)
+				elog(ERROR, "trigger promise is not in a trigger function");
+			assign_simple_var(estate, var,
+							  Int16GetDatum(estate->trigdata->tg_trigger->tgnargs),
+							  false, false);
+			break;
+
+		case PLPGSQL_PROMISE_TG_ARGV:
+			if (estate->trigdata == NULL)
+				elog(ERROR, "trigger promise is not in a trigger function");
+			if (estate->trigdata->tg_trigger->tgnargs > 0)
+			{
+				/*
+				 * For historical reasons, tg_argv[] subscripts start at zero
+				 * not one.  So we can't use construct_array().
+				 */
+				int			nelems = estate->trigdata->tg_trigger->tgnargs;
+				Datum	   *elems;
+				int			dims[1];
+				int			lbs[1];
+				int			i;
+
+				elems = palloc(sizeof(Datum) * nelems);
+				for (i = 0; i < nelems; i++)
+					elems[i] = CStringGetTextDatum(estate->trigdata->tg_trigger->tgargs[i]);
+				dims[0] = nelems;
+				lbs[0] = 0;
+
+				assign_simple_var(estate, var,
+								  PointerGetDatum(construct_md_array(elems, NULL,
+																	 1, dims, lbs,
+																	 TEXTOID,
+																	 -1, false, 'i')),
+								  false, true);
+			}
+			else
+			{
+				assign_simple_var(estate, var, (Datum) 0, true, false);
+			}
+			break;
+
+		case PLPGSQL_PROMISE_TG_EVENT:
+			if (estate->evtrigdata == NULL)
+				elog(ERROR, "event trigger promise is not in an event trigger function");
+			assign_text_var(estate, var, estate->evtrigdata->event);
+			break;
+
+		case PLPGSQL_PROMISE_TG_TAG:
+			if (estate->evtrigdata == NULL)
+				elog(ERROR, "event trigger promise is not in an event trigger function");
+			assign_text_var(estate, var, estate->evtrigdata->tag);
+			break;
+
+		default:
+			elog(ERROR, "unrecognized promise type: %d", var->promise);
+	}
+
+	MemoryContextSwitchTo(oldcontext);
+}
+
 /*
  * Create a memory context for statement-lifespan variables, if we don't
  * have one already.  It will be a child of stmt_mcontext_parent, which is
@@ -1464,6 +1518,10 @@ exec_stmt_block(PLpgSQL_execstate *estate, PLpgSQL_stmt_block *block)
 
 		/*
 		 * The set of dtypes handled here must match plpgsql_add_initdatums().
+		 *
+		 * Note that we currently don't support promise datums within blocks,
+		 * only at a function's outermost scope, so we needn't handle those
+		 * here.
 		 */
 		switch (datum->dtype)
 		{
@@ -2778,6 +2836,12 @@ exec_stmt_return(PLpgSQL_execstate *estate, PLpgSQL_stmt_return *stmt)
 
 		switch (retvar->dtype)
 		{
+			case PLPGSQL_DTYPE_PROMISE:
+				/* fulfill promise if needed, then handle like regular var */
+				plpgsql_fulfill_promise(estate, (PLpgSQL_var *) retvar);
+
+				/* FALL THRU */
+
 			case PLPGSQL_DTYPE_VAR:
 				{
 					PLpgSQL_var *var = (PLpgSQL_var *) retvar;
@@ -2917,6 +2981,12 @@ exec_stmt_return_next(PLpgSQL_execstate *estate,
 
 		switch (retvar->dtype)
 		{
+			case PLPGSQL_DTYPE_PROMISE:
+				/* fulfill promise if needed, then handle like regular var */
+				plpgsql_fulfill_promise(estate, (PLpgSQL_var *) retvar);
+
+				/* FALL THRU */
+
 			case PLPGSQL_DTYPE_VAR:
 				{
 					PLpgSQL_var *var = (PLpgSQL_var *) retvar;
@@ -3487,6 +3557,8 @@ plpgsql_estate_setup(PLpgSQL_execstate *estate,
 	func->cur_estate = estate;
 
 	estate->func = func;
+	estate->trigdata = NULL;
+	estate->evtrigdata = NULL;
 
 	estate->retval = (Datum) 0;
 	estate->retisnull = true;
@@ -4542,6 +4614,7 @@ exec_assign_value(PLpgSQL_execstate *estate,
 	switch (target->dtype)
 	{
 		case PLPGSQL_DTYPE_VAR:
+		case PLPGSQL_DTYPE_PROMISE:
 			{
 				/*
 				 * Target is a variable
@@ -4604,10 +4677,16 @@ exec_assign_value(PLpgSQL_execstate *estate,
 				 * cannot reliably be made any earlier; we have to be looking
 				 * at the object's standard R/W pointer to be sure pointer
 				 * equality is meaningful.
+				 *
+				 * Also, if it's a promise variable, we should disarm the
+				 * promise in any case --- otherwise, assigning null to an
+				 * armed promise variable would fail to disarm the promise.
 				 */
 				if (var->value != newvalue || var->isnull || isNull)
 					assign_simple_var(estate, var, newvalue, isNull,
 									  (!var->datatype->typbyval && !isNull));
+				else
+					var->promise = PLPGSQL_PROMISE_NONE;
 				break;
 			}
 
@@ -4951,6 +5030,12 @@ exec_eval_datum(PLpgSQL_execstate *estate,
 
 	switch (datum->dtype)
 	{
+		case PLPGSQL_DTYPE_PROMISE:
+			/* fulfill promise if needed, then handle like regular var */
+			plpgsql_fulfill_promise(estate, (PLpgSQL_var *) datum);
+
+			/* FALL THRU */
+
 		case PLPGSQL_DTYPE_VAR:
 			{
 				PLpgSQL_var *var = (PLpgSQL_var *) datum;
@@ -5093,6 +5178,7 @@ plpgsql_exec_get_datum_type(PLpgSQL_execstate *estate,
 	switch (datum->dtype)
 	{
 		case PLPGSQL_DTYPE_VAR:
+		case PLPGSQL_DTYPE_PROMISE:
 			{
 				PLpgSQL_var *var = (PLpgSQL_var *) datum;
 
@@ -5176,6 +5262,7 @@ plpgsql_exec_get_datum_type_info(PLpgSQL_execstate *estate,
 	switch (datum->dtype)
 	{
 		case PLPGSQL_DTYPE_VAR:
+		case PLPGSQL_DTYPE_PROMISE:
 			{
 				PLpgSQL_var *var = (PLpgSQL_var *) datum;
 
@@ -5874,6 +5961,7 @@ plpgsql_param_fetch(ParamListInfo params,
 		switch (datum->dtype)
 		{
 			case PLPGSQL_DTYPE_VAR:
+			case PLPGSQL_DTYPE_PROMISE:
 				/* always safe */
 				break;
 
@@ -5989,8 +6077,8 @@ plpgsql_param_compile(ParamListInfo params, Param *param,
 	 * Select appropriate eval function.  It seems worth special-casing
 	 * DTYPE_VAR and DTYPE_RECFIELD for performance.  Also, we can determine
 	 * in advance whether MakeExpandedObjectReadOnly() will be required.
-	 * Currently, only VAR and REC datums could contain read/write expanded
-	 * objects.
+	 * Currently, only VAR/PROMISE and REC datums could contain read/write
+	 * expanded objects.
 	 */
 	if (datum->dtype == PLPGSQL_DTYPE_VAR)
 	{
@@ -6002,6 +6090,14 @@ plpgsql_param_compile(ParamListInfo params, Param *param,
 	}
 	else if (datum->dtype == PLPGSQL_DTYPE_RECFIELD)
 		scratch.d.cparam.paramfunc = plpgsql_param_eval_recfield;
+	else if (datum->dtype == PLPGSQL_DTYPE_PROMISE)
+	{
+		if (dno != expr->rwparam &&
+			((PLpgSQL_var *) datum)->datatype->typlen == -1)
+			scratch.d.cparam.paramfunc = plpgsql_param_eval_generic_ro;
+		else
+			scratch.d.cparam.paramfunc = plpgsql_param_eval_generic;
+	}
 	else if (datum->dtype == PLPGSQL_DTYPE_REC &&
 			 dno != expr->rwparam)
 		scratch.d.cparam.paramfunc = plpgsql_param_eval_generic_ro;
@@ -7680,7 +7776,8 @@ static void
 assign_simple_var(PLpgSQL_execstate *estate, PLpgSQL_var *var,
 				  Datum newvalue, bool isnull, bool freeable)
 {
-	Assert(var->dtype == PLPGSQL_DTYPE_VAR);
+	Assert(var->dtype == PLPGSQL_DTYPE_VAR ||
+		   var->dtype == PLPGSQL_DTYPE_PROMISE);
 	/* Free the old value if needed */
 	if (var->freeval)
 	{
@@ -7695,6 +7792,13 @@ assign_simple_var(PLpgSQL_execstate *estate, PLpgSQL_var *var,
 	var->value = newvalue;
 	var->isnull = isnull;
 	var->freeval = freeable;
+
+	/*
+	 * If it's a promise variable, then either we just assigned the promised
+	 * value, or the user explicitly assigned an overriding value.  Either
+	 * way, cancel the promise.
+	 */
+	var->promise = PLPGSQL_PROMISE_NONE;
 }
 
 /*

src/pl/plpgsql/src/pl_funcs.c

@@ -729,6 +729,7 @@ plpgsql_free_function_memory(PLpgSQL_function *func)
 		switch (d->dtype)
 		{
 			case PLPGSQL_DTYPE_VAR:
+			case PLPGSQL_DTYPE_PROMISE:
 				{
 					PLpgSQL_var *var = (PLpgSQL_var *) d;
 
@@ -1582,6 +1583,7 @@ plpgsql_dumptree(PLpgSQL_function *func)
 		switch (d->dtype)
 		{
 			case PLPGSQL_DTYPE_VAR:
+			case PLPGSQL_DTYPE_PROMISE:
 				{
 					PLpgSQL_var *var = (PLpgSQL_var *) d;
 
@@ -1608,6 +1610,9 @@ plpgsql_dumptree(PLpgSQL_function *func)
 						dump_expr(var->cursor_explicit_expr);
 						printf("\n");
 					}
+					if (var->promise != PLPGSQL_PROMISE_NONE)
+						printf("                                  PROMISE %d\n",
+							   (int) var->promise);
 				}
 				break;
 			case PLPGSQL_DTYPE_ROW:

src/pl/plpgsql/src/pl_gram.y

@@ -3170,6 +3170,7 @@ make_return_stmt(int location)
 
 		if (tok == T_DATUM && plpgsql_peek() == ';' &&
 			(yylval.wdatum.datum->dtype == PLPGSQL_DTYPE_VAR ||
+			 yylval.wdatum.datum->dtype == PLPGSQL_DTYPE_PROMISE ||
 			 yylval.wdatum.datum->dtype == PLPGSQL_DTYPE_ROW ||
 			 yylval.wdatum.datum->dtype == PLPGSQL_DTYPE_REC))
 		{
@@ -3231,6 +3232,7 @@ make_return_next_stmt(int location)
 
 		if (tok == T_DATUM && plpgsql_peek() == ';' &&
 			(yylval.wdatum.datum->dtype == PLPGSQL_DTYPE_VAR ||
+			 yylval.wdatum.datum->dtype == PLPGSQL_DTYPE_PROMISE ||
 			 yylval.wdatum.datum->dtype == PLPGSQL_DTYPE_ROW ||
 			 yylval.wdatum.datum->dtype == PLPGSQL_DTYPE_REC))
 		{
@@ -3318,6 +3320,7 @@ check_assignable(PLpgSQL_datum *datum, int location)
 	switch (datum->dtype)
 	{
 		case PLPGSQL_DTYPE_VAR:
+		case PLPGSQL_DTYPE_PROMISE:
 			if (((PLpgSQL_var *) datum)->isconst)
 				ereport(ERROR,
 						(errcode(ERRCODE_ERROR_IN_ASSIGNMENT),

src/pl/plpgsql/src/plpgsql.h

@@ -63,9 +63,29 @@ typedef enum PLpgSQL_datum_type
 	PLPGSQL_DTYPE_ROW,
 	PLPGSQL_DTYPE_REC,
 	PLPGSQL_DTYPE_RECFIELD,
-	PLPGSQL_DTYPE_ARRAYELEM
+	PLPGSQL_DTYPE_ARRAYELEM,
+	PLPGSQL_DTYPE_PROMISE
 } PLpgSQL_datum_type;
 
+/*
+ * DTYPE_PROMISE datums have these possible ways of computing the promise
+ */
+typedef enum PLpgSQL_promise_type
+{
+	PLPGSQL_PROMISE_NONE = 0,	/* not a promise, or promise satisfied */
+	PLPGSQL_PROMISE_TG_NAME,
+	PLPGSQL_PROMISE_TG_WHEN,
+	PLPGSQL_PROMISE_TG_LEVEL,
+	PLPGSQL_PROMISE_TG_OP,
+	PLPGSQL_PROMISE_TG_RELID,
+	PLPGSQL_PROMISE_TG_TABLE_NAME,
+	PLPGSQL_PROMISE_TG_TABLE_SCHEMA,
+	PLPGSQL_PROMISE_TG_NARGS,
+	PLPGSQL_PROMISE_TG_ARGV,
+	PLPGSQL_PROMISE_TG_EVENT,
+	PLPGSQL_PROMISE_TG_TAG
+} PLpgSQL_promise_type;
+
 /*
  * Variants distinguished in PLpgSQL_type structs
  */
@@ -248,6 +268,14 @@ typedef struct PLpgSQL_variable
 
 /*
  * Scalar variable
+ *
+ * DTYPE_VAR and DTYPE_PROMISE datums both use this struct type.
+ * A PROMISE datum works exactly like a VAR datum for most purposes,
+ * but if it is read without having previously been assigned to, then
+ * a special "promised" value is computed and assigned to the datum
+ * before the read is performed.  This technique avoids the overhead of
+ * computing the variable's value in cases where we expect that many
+ * functions will never read it.
  */
 typedef struct PLpgSQL_var
 {
@@ -271,9 +299,18 @@ typedef struct PLpgSQL_var
 	int			cursor_explicit_argrow;
 	int			cursor_options;
 
+	/* Fields below here can change at runtime */
+
 	Datum		value;
 	bool		isnull;
 	bool		freeval;
+
+	/*
+	 * The promise field records which "promised" value to assign if the
+	 * promise must be honored.  If it's a normal variable, or the promise has
+	 * been fulfilled, this is PLPGSQL_PROMISE_NONE.
+	 */
+	PLpgSQL_promise_type promise;
 } PLpgSQL_var;
 
 /*
@@ -869,20 +906,6 @@ typedef struct PLpgSQL_function
 	int			found_varno;
 	int			new_varno;
 	int			old_varno;
-	int			tg_name_varno;
-	int			tg_when_varno;
-	int			tg_level_varno;
-	int			tg_op_varno;
-	int			tg_relid_varno;
-	int			tg_relname_varno;
-	int			tg_table_name_varno;
-	int			tg_table_schema_varno;
-	int			tg_nargs_varno;
-	int			tg_argv_varno;
-
-	/* for event triggers */
-	int			tg_event_varno;
-	int			tg_tag_varno;
 
 	PLpgSQL_resolve_option resolve_option;
 
@@ -912,6 +935,9 @@ typedef struct PLpgSQL_execstate
 {
 	PLpgSQL_function *func;		/* function being executed */
 
+	TriggerData *trigdata;		/* if regular trigger, data about firing */
+	EventTriggerData *evtrigdata;	/* if event trigger, data about firing */
+
 	Datum		retval;
 	bool		retisnull;
 	Oid			rettype;		/* type of current retval */