Preliminary refactoring of plpgsql expression construction.

This short and boring patch simply moves the responsibility for
initializing PLpgSQL_expr.target_param into plpgsql parsing,
rather than doing it at first execution of the expr as before.
This doesn't save anything in terms of runtime, since the work was
trivial and done only once per expr anyway.  But it makes the info
available during parsing, which will be useful for the next step.

Likewise set PLpgSQL_expr.func during parsing.  According to the
comments, this was once impossible; but it's certainly possible
since we invented the plpgsql_curr_compile variable.  Again, this
saves little runtime, but it seems far cleaner conceptually.

While at it, I reordered stuff in struct PLpgSQL_expr to make it
clearer which fields are filled when, and merged some duplicative
code in pl_gram.y.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru>
Reviewed-by: Pavel Borisov <pashkin.elfe@gmail.com>
Discussion: https://postgr.es/m/CACxu=vJaKFNsYxooSnW1wEgsAO5u_v1XYBacfVJ14wgJV_PYeg@mail.gmail.com

Author: tglsfdc

src/pl/plpgsql/src/pl_exec.c

@@ -4174,12 +4174,6 @@ exec_prepare_plan(PLpgSQL_execstate *estate,
 	SPIPlanPtr	plan;
 	SPIPrepareOptions options;
 
-	/*
-	 * The grammar can't conveniently set expr->func while building the parse
-	 * tree, so make sure it's set before parser hooks need it.
-	 */
-	expr->func = estate->func;
-
 	/*
 	 * Generate and save the plan
 	 */
@@ -5016,21 +5010,7 @@ exec_assign_expr(PLpgSQL_execstate *estate, PLpgSQL_datum *target,
 	 * If first time through, create a plan for this expression.
 	 */
 	if (expr->plan == NULL)
-	{
-		/*
-		 * Mark the expression as being an assignment source, if target is a
-		 * simple variable.  (This is a bit messy, but it seems cleaner than
-		 * modifying the API of exec_prepare_plan for the purpose.  We need to
-		 * stash the target dno into the expr anyway, so that it will be
-		 * available if we have to replan.)
-		 */
-		if (target->dtype == PLPGSQL_DTYPE_VAR)
-			expr->target_param = target->dno;
-		else
-			expr->target_param = -1;	/* should be that already */
-
 		exec_prepare_plan(estate, expr, 0);
-	}
 
 	value = exec_eval_expr(estate, expr, &isnull, &valtype, &valtypmod);
 	exec_assign_value(estate, target, value, isnull, valtype, valtypmod);
@@ -6282,13 +6262,6 @@ setup_param_list(PLpgSQL_execstate *estate, PLpgSQL_expr *expr)
 		 * that they are interrupting an active use of parameters.
 		 */
 		paramLI->parserSetupArg = expr;
-
-		/*
-		 * Also make sure this is set before parser hooks need it.  There is
-		 * no need to save and restore, since the value is always correct once
-		 * set.  (Should be set already, but let's be sure.)
-		 */
-		expr->func = estate->func;
 	}
 	else
 	{

src/pl/plpgsql/src/pl_gram.y

@@ -61,6 +61,10 @@ static	bool			tok_is_keyword(int token, union YYSTYPE *lval,
 static	void			word_is_not_variable(PLword *word, int location, yyscan_t yyscanner);
 static	void			cword_is_not_variable(PLcword *cword, int location, yyscan_t yyscanner);
 static	void			current_token_is_not_variable(int tok, YYSTYPE *yylvalp, YYLTYPE *yyllocp, yyscan_t yyscanner);
+static	PLpgSQL_expr	*make_plpgsql_expr(const char *query,
+										   RawParseMode parsemode);
+static	void			mark_expr_as_assignment_source(PLpgSQL_expr *expr,
+													   PLpgSQL_datum *target);
 static	PLpgSQL_expr	*read_sql_construct(int until,
 											int until2,
 											int until3,
@@ -536,6 +540,10 @@ decl_statement	: decl_varname decl_const decl_datatype decl_collate decl_notnull
 									 errmsg("variable \"%s\" must have a default value, since it's declared NOT NULL",
 											var->refname),
 									 parser_errposition(@5)));
+
+						if (var->default_val != NULL)
+							mark_expr_as_assignment_source(var->default_val,
+														   (PLpgSQL_datum *) var);
 					}
 				| decl_varname K_ALIAS K_FOR decl_aliasitem ';'
 					{
@@ -996,6 +1004,7 @@ stmt_assign		: T_DATUM
 													   false, true,
 													   NULL, NULL,
 													   &yylval, &yylloc, yyscanner);
+						mark_expr_as_assignment_source(new->expr, $1.datum);
 
 						$$ = (PLpgSQL_stmt *) new;
 					}
@@ -2651,6 +2660,38 @@ current_token_is_not_variable(int tok, YYSTYPE *yylvalp, YYLTYPE *yyllocp, yysca
 		yyerror(yyllocp, NULL, yyscanner, "syntax error");
 }
 
+/* Convenience routine to construct a PLpgSQL_expr struct */
+static PLpgSQL_expr *
+make_plpgsql_expr(const char *query,
+				  RawParseMode parsemode)
+{
+	PLpgSQL_expr *expr = palloc0(sizeof(PLpgSQL_expr));
+
+	expr->query = pstrdup(query);
+	expr->parseMode = parsemode;
+	expr->func = plpgsql_curr_compile;
+	expr->ns = plpgsql_ns_top();
+	/* might get changed later during parsing: */
+	expr->target_param = -1;
+	/* other fields are left as zeroes until first execution */
+	return expr;
+}
+
+/* Mark a PLpgSQL_expr as being the source of an assignment to target */
+static void
+mark_expr_as_assignment_source(PLpgSQL_expr *expr, PLpgSQL_datum *target)
+{
+	/*
+	 * Mark the expression as being an assignment source, if target is a
+	 * simple variable.  We don't currently support optimized assignments to
+	 * other DTYPEs, so no need to mark in other cases.
+	 */
+	if (target->dtype == PLPGSQL_DTYPE_VAR)
+		expr->target_param = target->dno;
+	else
+		expr->target_param = -1;	/* should be that already */
+}
+
 /* Convenience routine to read an expression with one possible terminator */
 static PLpgSQL_expr *
 read_sql_expression(int until, const char *expected, YYSTYPE *yylvalp, YYLTYPE *yyllocp, yyscan_t yyscanner)
@@ -2794,13 +2835,7 @@ read_sql_construct(int until,
 	 */
 	plpgsql_append_source_text(&ds, startlocation, endlocation, yyscanner);
 
-	expr = palloc0(sizeof(PLpgSQL_expr));
-	expr->query = pstrdup(ds.data);
-	expr->parseMode = parsemode;
-	expr->plan = NULL;
-	expr->paramnos = NULL;
-	expr->target_param = -1;
-	expr->ns = plpgsql_ns_top();
+	expr = make_plpgsql_expr(ds.data, parsemode);
 	pfree(ds.data);
 
 	if (valid_sql)
@@ -3122,13 +3157,7 @@ make_execsql_stmt(int firsttoken, int location, PLword *word, YYSTYPE *yylvalp,
 	while (ds.len > 0 && scanner_isspace(ds.data[ds.len - 1]))
 		ds.data[--ds.len] = '\0';
 
-	expr = palloc0(sizeof(PLpgSQL_expr));
-	expr->query = pstrdup(ds.data);
-	expr->parseMode = RAW_PARSE_DEFAULT;
-	expr->plan = NULL;
-	expr->paramnos = NULL;
-	expr->target_param = -1;
-	expr->ns = plpgsql_ns_top();
+	expr = make_plpgsql_expr(ds.data, RAW_PARSE_DEFAULT);
 	pfree(ds.data);
 
 	check_sql_expr(expr->query, expr->parseMode, location, yyscanner);
@@ -4006,13 +4035,7 @@ read_cursor_args(PLpgSQL_var *cursor, int until, YYSTYPE *yylvalp, YYLTYPE *yyll
 			appendStringInfoString(&ds, ", ");
 	}
 
-	expr = palloc0(sizeof(PLpgSQL_expr));
-	expr->query = pstrdup(ds.data);
-	expr->parseMode = RAW_PARSE_PLPGSQL_EXPR;
-	expr->plan = NULL;
-	expr->paramnos = NULL;
-	expr->target_param = -1;
-	expr->ns = plpgsql_ns_top();
+	expr = make_plpgsql_expr(ds.data, RAW_PARSE_PLPGSQL_EXPR);
 	pfree(ds.data);
 
 	/* Next we'd better find the until token */

src/pl/plpgsql/src/plpgsql.h

@@ -219,14 +219,22 @@ typedef struct PLpgSQL_expr
 {
 	char	   *query;			/* query string, verbatim from function body */
 	RawParseMode parseMode;		/* raw_parser() mode to use */
-	SPIPlanPtr	plan;			/* plan, or NULL if not made yet */
-	Bitmapset  *paramnos;		/* all dnos referenced by this query */
+	struct PLpgSQL_function *func;	/* function containing this expr */
+	struct PLpgSQL_nsitem *ns;	/* namespace chain visible to this expr */
 
-	/* function containing this expr (not set until we first parse query) */
-	struct PLpgSQL_function *func;
+	/*
+	 * These fields are used to help optimize assignments to expanded-datum
+	 * variables.  If this expression is the source of an assignment to a
+	 * simple variable, target_param holds that variable's dno (else it's -1).
+	 */
+	int			target_param;	/* dno of assign target, or -1 if none */
 
-	/* namespace chain visible to this expr */
-	struct PLpgSQL_nsitem *ns;
+	/*
+	 * Fields above are set during plpgsql parsing.  Remaining fields are left
+	 * as zeroes/NULLs until we first parse/plan the query.
+	 */
+	SPIPlanPtr	plan;			/* plan, or NULL if not made yet */
+	Bitmapset  *paramnos;		/* all dnos referenced by this query */
 
 	/* fields for "simple expression" fast-path execution: */
 	Expr	   *expr_simple_expr;	/* NULL means not a simple expr */
@@ -235,14 +243,11 @@ typedef struct PLpgSQL_expr
 	bool		expr_simple_mutable;	/* true if simple expr is mutable */
 
 	/*
-	 * These fields are used to optimize assignments to expanded-datum
-	 * variables.  If this expression is the source of an assignment to a
-	 * simple variable, target_param holds that variable's dno; else it's -1.
-	 * If we match a Param within expr_simple_expr to such a variable, that
-	 * Param's address is stored in expr_rw_param; then expression code
-	 * generation will allow the value for that Param to be passed read/write.
+	 * If we match a Param within expr_simple_expr to the variable identified
+	 * by target_param, that Param's address is stored in expr_rw_param; then
+	 * expression code generation will allow the value for that Param to be
+	 * passed as a read/write expanded-object pointer.
 	 */
-	int			target_param;	/* dno of assign target, or -1 if none */
 	Param	   *expr_rw_param;	/* read/write Param within expr, if any */
 
 	/*