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

Author: tglsfdc

src/pl/plpgsql/src/pl_comp.c

@@ -607,79 +607,99 @@ 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,
 										 plpgsql_build_datatype(TEXTOID,
 																-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,
 										 plpgsql_build_datatype(TEXTOID,
 																-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,
 										 plpgsql_build_datatype(TEXTOID,
 																-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,
 										 plpgsql_build_datatype(OIDOID,
 																-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,
 										 plpgsql_build_datatype(NAMEOID,
 																-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,
 										 plpgsql_build_datatype(NAMEOID,
 																-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,
 										 plpgsql_build_datatype(NAMEOID,
 																-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,
 										 plpgsql_build_datatype(INT4OID,
 																-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,
 										 plpgsql_build_datatype(TEXTARRAYOID,
 																-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;
 
@@ -701,15 +721,19 @@ 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,
 										 plpgsql_build_datatype(TEXTOID,
 																-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: