Date: Tue, 26 Mar 2019 21:04:53 +0100
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Rafia Sabih <rafia.pghackers@gmail.com>
Cc: Michael Paquier <michael@paquier.xyz>,
	John Naylor <john.naylor@2ndquadrant.com>,
	Sergei Agalakov <sergei.agalakov@gmail.com>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: explain plans with information about (modified) gucs
Message-ID: <20190326200453.GA16551@development>
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On Mon, Mar 18, 2019 at 11:31:48AM +0100, Rafia Sabih wrote:
>On Sun, 24 Feb 2019 at 00:06, Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote:
>>
>> Hi,
>>
>> attached is an updated patch, fixing and slightly tweaking the docs.
>>
>>
>> Barring objections, I'll get this committed later next week.
>>
>I was having a look at this patch, and this kept me wondering,
>
>+static void
>+ExplainShowSettings(ExplainState *es)
>+{
>Is there some reason for not providing any explanation above this
>function just like the rest of the functions in this file?
>
>Similarly, for
>
>struct config_generic **
>get_explain_guc_options(int *num)
>{
>
>/* also bail out of there are no options */
>+ if (!num)
>+ return;
>I think you meant 'if' instead if 'of' here.

Thanks for the review - attached is a patch adding the missing comments,
and doing two additional minor improvements:

1) Renaming ExplainShowSettings to ExplainPrintSettings, to make it more
consistent with naming of the other functions in explain.c.

2) Actually counting GUC_EXPLAIN options, and only allocating space for
those in get_explain_guc_options, instead of using num_guc_variables. The
diffrence is quite significant (~50 vs. ~300), and considering each entry
is 8B it makes a difference because such large chunks tend to have higher
palloc overhed (due to ALLOCSET_SEPARATE_THRESHOLD).


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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diff --git a/src/backend/commands/explain.c b/src/backend/commands/explain.c
index 8e48b94d4a..2a289b8b94 100644
--- a/src/backend/commands/explain.c
+++ b/src/backend/commands/explain.c
@@ -599,8 +599,12 @@ ExplainOnePlan(PlannedStmt *plannedstmt, IntoClause *into, ExplainState *es,
 	ExplainCloseGroup("Query", NULL, true, es);
 }
 
+/*
+ * ExplainPrintSettings -
+ *    Print summary of modified settings affecting query planning.
+ */
 static void
-ExplainShowSettings(ExplainState *es)
+ExplainPrintSettings(ExplainState *es)
 {
 	int		num;
 	struct config_generic **gucs;
@@ -700,10 +704,10 @@ ExplainPrintPlan(ExplainState *es, QueryDesc *queryDesc)
 	ExplainNode(ps, NIL, NULL, NULL, es);
 
 	/*
-	 * If requested, include information about GUC parameters that don't
-	 * match the built-in defaults.
+	 * If requested, include information about GUC parameters with values
+	 * that don't match the built-in defaults.
 	 */
-	ExplainShowSettings(es);
+	ExplainPrintSettings(es);
 }
 
 /*
diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index 7306d7c232..39f844ebc5 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -4532,6 +4532,7 @@ static struct config_generic **guc_variables;
 
 /* Current number of variables contained in the vector */
 static int	num_guc_variables;
+static int	num_guc_explain_variables;
 
 /* Vector capacity */
 static int	size_guc_variables;
@@ -4796,6 +4797,7 @@ build_guc_variables(void)
 {
 	int			size_vars;
 	int			num_vars = 0;
+	int			num_explain_vars = 0;
 	struct config_generic **guc_vars;
 	int			i;
 
@@ -4806,6 +4808,9 @@ build_guc_variables(void)
 		/* Rather than requiring vartype to be filled in by hand, do this: */
 		conf->gen.vartype = PGC_BOOL;
 		num_vars++;
+
+		if (conf->gen.flags & GUC_EXPLAIN)
+			num_explain_vars++;
 	}
 
 	for (i = 0; ConfigureNamesInt[i].gen.name; i++)
@@ -4814,6 +4819,9 @@ build_guc_variables(void)
 
 		conf->gen.vartype = PGC_INT;
 		num_vars++;
+
+		if (conf->gen.flags & GUC_EXPLAIN)
+			num_explain_vars++;
 	}
 
 	for (i = 0; ConfigureNamesReal[i].gen.name; i++)
@@ -4822,6 +4830,9 @@ build_guc_variables(void)
 
 		conf->gen.vartype = PGC_REAL;
 		num_vars++;
+
+		if (conf->gen.flags & GUC_EXPLAIN)
+			num_explain_vars++;
 	}
 
 	for (i = 0; ConfigureNamesString[i].gen.name; i++)
@@ -4830,6 +4841,9 @@ build_guc_variables(void)
 
 		conf->gen.vartype = PGC_STRING;
 		num_vars++;
+
+		if (conf->gen.flags & GUC_EXPLAIN)
+			num_explain_vars++;
 	}
 
 	for (i = 0; ConfigureNamesEnum[i].gen.name; i++)
@@ -4838,6 +4852,9 @@ build_guc_variables(void)
 
 		conf->gen.vartype = PGC_ENUM;
 		num_vars++;
+
+		if (conf->gen.flags & GUC_EXPLAIN)
+			num_explain_vars++;
 	}
 
 	/*
@@ -4869,6 +4886,7 @@ build_guc_variables(void)
 		free(guc_variables);
 	guc_variables = guc_vars;
 	num_guc_variables = num_vars;
+	num_guc_explain_variables = num_explain_vars;
 	size_guc_variables = size_vars;
 	qsort((void *) guc_variables, num_guc_variables,
 		  sizeof(struct config_generic *), guc_var_compare);
@@ -8819,6 +8837,11 @@ ShowAllGUCConfig(DestReceiver *dest)
 	end_tup_output(tstate);
 }
 
+/*
+ * Returns an array of modified GUC options to show in EXPLAIN. Only options
+ * related to query planning (marked with GUC_EXPLAIN), with values different
+ * from built-in defaults.
+ */
 struct config_generic **
 get_explain_guc_options(int *num)
 {
@@ -8826,7 +8849,13 @@ get_explain_guc_options(int *num)
 	struct config_generic **result;
 
 	*num = 0;
-	result = palloc(sizeof(struct config_generic *) * num_guc_variables);
+
+	/*
+	 * Allocate enough space to fit all GUC_EXPLAIN options. We may not
+	 * need all the space, but there are fairly few such options so we
+	 * don't waste a lot of memory.
+	 */
+	result = palloc(sizeof(struct config_generic *) * num_guc_explain_variables);
 
 	for (i = 0; i < num_guc_variables; i++)
 	{
@@ -8894,6 +8923,8 @@ get_explain_guc_options(int *num)
 		/* assign to the values array */
 		result[*num] = conf;
 		*num = *num + 1;
+
+		Assert(*num <= num_guc_explain_variables);
 	}
 
 	return result;

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