Add SETTINGS option to EXPLAIN, to print modified settings.

Query planning is affected by a number of configuration options, and it
may be crucial to know which of those options were set to non-default
values.  With this patch you can say EXPLAIN (SETTINGS ON) to include
that information in the query plan.  Only options affecting planning,
with values different from the built-in default are printed.

This patch also adds auto_explain.log_settings option, providing the
same capability in auto_explain module.

Author: Tomas Vondra
Reviewed-by: Rafia Sabih, John Naylor
Discussion: https://postgr.es/m/e1791b4c-df9c-be02-edc5-7c8874944be0@2ndquadrant.com

src/include/commands/explain.h

@@ -35,6 +35,7 @@ typedef struct ExplainState
 	bool		buffers;		/* print buffer usage */
 	bool		timing;			/* print detailed node timing */
 	bool		summary;		/* print total planning and execution timing */
+	bool		settings;		/* print modified settings */
 	ExplainFormat format;		/* output format */
 	/* state for output formatting --- not reset for each new plan tree */
 	int			indent;			/* current indentation level */

src/include/utils/guc.h

@@ -227,6 +227,8 @@ typedef enum
 #define GUC_UNIT_MIN		   0x30000	/* value is in minutes */
 #define GUC_UNIT_TIME		   0xF0000	/* mask for time-related units */
 
+#define GUC_EXPLAIN			  0x100000	/* include in explain */
+
 #define GUC_UNIT				(GUC_UNIT_MEMORY | GUC_UNIT_TIME)
 
 

src/include/utils/guc_tables.h

@@ -267,5 +267,6 @@ extern void build_guc_variables(void);
 extern const char *config_enum_lookup_by_value(struct config_enum *record, int val);
 extern bool config_enum_lookup_by_name(struct config_enum *record,
 						   const char *value, int *retval);
+extern struct config_generic **get_explain_guc_options(int *num);
 
 #endif							/* GUC_TABLES_H */