Allow Boolean reloptions to have ternary values

From the user's point of view these are just Boolean values; from the
implementation side we can now distinguish an option that hasn't been
set.  Reimplement the vacuum_truncate reloption using this type.

This could also be used for reloptions vacuum_index_cleanup and
buffering, but those additionally need a per-option "alias" for the
state where the variable is unset (currently the value "auto").

Author: Nikolay Shaplov <dhyan@nataraj.su>
Reviewed-by: Timur Magomedov <t.magomedov@postgrespro.ru>
Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://postgr.es/m/3474141.usfYGdeWWP@thinkpad-pgpro

3 files changed, 28 insertions, 15 deletions

diff --git a/src/include/access/reloptions.h b/src/include/access/reloptions.h
index 2f08e1b0cf0..a3f6f5a3990 100644
--- a/src/include/access/reloptions.h
+++ b/src/include/access/reloptions.h
@@ -29,6 +29,7 @@
 typedef enum relopt_type
 {
 	RELOPT_TYPE_BOOL,
+	RELOPT_TYPE_TERNARY,		/* on, off, unset */
 	RELOPT_TYPE_INT,
 	RELOPT_TYPE_REAL,
 	RELOPT_TYPE_ENUM,
@@ -80,6 +81,7 @@ typedef struct relopt_value
 	union
 	{
 		bool		bool_val;
+		pg_ternary	ternary_val;
 		int			int_val;
 		double		real_val;
 		int			enum_val;
@@ -94,6 +96,12 @@ typedef struct relopt_bool
 	bool		default_val;
 } relopt_bool;
 
+typedef struct relopt_ternary
+{
+	relopt_gen	gen;
+	/* ternaries have no default_val: otherwise they'd just be bools */
+} relopt_ternary;
+
 typedef struct relopt_int
 {
 	relopt_gen	gen;
@@ -152,19 +160,6 @@ typedef struct
 	const char *optname;		/* option's name */
 	relopt_type opttype;		/* option's datatype */
 	int			offset;			/* offset of field in result struct */
-
-	/*
-	 * isset_offset is an optional offset of a field in the result struct that
-	 * stores whether the option is explicitly set for the relation or if it
-	 * just picked up the default value.  In most cases, this can be
-	 * accomplished by giving the reloption a special out-of-range default
-	 * value (e.g., some integer reloptions use -2), but this isn't always
-	 * possible.  For example, a Boolean reloption cannot be given an
-	 * out-of-range default, so we need another way to discover the source of
-	 * its value.  This offset is only used if given a value greater than
-	 * zero.
-	 */
-	int			isset_offset;
 } relopt_parse_elt;
 
 /* Local reloption definition */
@@ -195,6 +190,8 @@ typedef struct local_relopts
 extern relopt_kind add_reloption_kind(void);
 extern void add_bool_reloption(bits32 kinds, const char *name, const char *desc,
 							   bool default_val, LOCKMODE lockmode);
+extern void add_ternary_reloption(bits32 kinds, const char *name,
+								  const char *desc, LOCKMODE lockmode);
 extern void add_int_reloption(bits32 kinds, const char *name, const char *desc,
 							  int default_val, int min_val, int max_val,
 							  LOCKMODE lockmode);
@@ -214,6 +211,9 @@ extern void register_reloptions_validator(local_relopts *relopts,
 extern void add_local_bool_reloption(local_relopts *relopts, const char *name,
 									 const char *desc, bool default_val,
 									 int offset);
+extern void add_local_ternary_reloption(local_relopts *relopts,
+										const char *name, const char *desc,
+										int offset);
 extern void add_local_int_reloption(local_relopts *relopts, const char *name,
 									const char *desc, int default_val,
 									int min_val, int max_val, int offset);
diff --git a/src/include/postgres.h b/src/include/postgres.h
index 7d93fbce709..8b92f453e7a 100644
--- a/src/include/postgres.h
+++ b/src/include/postgres.h
@@ -544,6 +544,20 @@ Float8GetDatum(float8 X)
  */
 
 /*
+ * pg_ternary
+ *		Boolean value with an extra "unset" value
+ *
+ * This enum can be used for values that want to distinguish between true,
+ * false, and unset.
+*/
+typedef enum pg_ternary
+{
+	PG_TERNARY_FALSE = 0,
+	PG_TERNARY_TRUE = 1,
+	PG_TERNARY_UNSET = -1
+} pg_ternary;
+
+/*
  * NON_EXEC_STATIC: It's sometimes useful to define a variable or function
  * that is normally static but extern when using EXEC_BACKEND (see
  * pg_config_manual.h).  There would then typically be some code in
diff --git a/src/include/utils/rel.h b/src/include/utils/rel.h
index d03ab247788..236830f6b93 100644
--- a/src/include/utils/rel.h
+++ b/src/include/utils/rel.h
@@ -347,8 +347,7 @@ typedef struct StdRdOptions
 	bool		user_catalog_table; /* use as an additional catalog relation */
 	int			parallel_workers;	/* max number of parallel workers */
 	StdRdOptIndexCleanup vacuum_index_cleanup;	/* controls index vacuuming */
-	bool		vacuum_truncate;	/* enables vacuum to truncate a relation */
-	bool		vacuum_truncate_set;	/* whether vacuum_truncate is set */
+	pg_ternary	vacuum_truncate;	/* enables vacuum to truncate a relation */
 
 	/*
 	 * Fraction of pages in a relation that vacuum can eagerly scan and fail