diff --git a/contrib/auth_delay/auth_delay.c b/contrib/auth_delay/auth_delay.c
index c3d78e5020..4ca9b4afb1 100644
--- a/contrib/auth_delay/auth_delay.c
+++ b/contrib/auth_delay/auth_delay.c
@@ -21,7 +21,7 @@
 PG_MODULE_MAGIC;
 
 /* GUC Variables */
-static int	auth_delay_milliseconds;
+static int	auth_delay_milliseconds = 0;
 
 /* Original Hook */
 static ClientAuthentication_hook_type original_client_auth_hook = NULL;
diff --git a/contrib/pg_prewarm/autoprewarm.c b/contrib/pg_prewarm/autoprewarm.c
index 1843b1862e..d02a6a1ba0 100644
--- a/contrib/pg_prewarm/autoprewarm.c
+++ b/contrib/pg_prewarm/autoprewarm.c
@@ -103,7 +103,7 @@ static AutoPrewarmSharedState *apw_state = NULL;
 
 /* GUC variables. */
 static bool autoprewarm = true; /* start worker? */
-static int	autoprewarm_interval;	/* dump interval */
+static int	autoprewarm_interval = 300; /* dump interval */
 
 /*
  * Module load callback.
diff --git a/contrib/pg_stat_statements/pg_stat_statements.c b/contrib/pg_stat_statements/pg_stat_statements.c
index e5aa429995..0beb56a804 100644
--- a/contrib/pg_stat_statements/pg_stat_statements.c
+++ b/contrib/pg_stat_statements/pg_stat_statements.c
@@ -283,11 +283,12 @@ static const struct config_enum_entry track_options[] =
 	{NULL, 0, false}
 };
 
-static int	pgss_max;			/* max # statements to track */
-static int	pgss_track;			/* tracking level */
-static bool pgss_track_utility; /* whether to track utility commands */
-static bool pgss_track_planning;	/* whether to track planning duration */
-static bool pgss_save;			/* whether to save stats across shutdown */
+static int	pgss_max = 5000;	/* max # statements to track */
+static int	pgss_track = PGSS_TRACK_TOP;	/* tracking level */
+static bool pgss_track_utility = true;	/* whether to track utility commands */
+static bool pgss_track_planning = false;	/* whether to track planning
+											 * duration */
+static bool pgss_save = true;	/* whether to save stats across shutdown */
 
 
 #define pgss_enabled(level) \
diff --git a/contrib/pg_trgm/trgm_op.c b/contrib/pg_trgm/trgm_op.c
index 154346398a..2c644bc148 100644
--- a/contrib/pg_trgm/trgm_op.c
+++ b/contrib/pg_trgm/trgm_op.c
@@ -68,7 +68,7 @@ _PG_init(void)
 							 "Sets the threshold used by the % operator.",
 							 "Valid range is 0.0 .. 1.0.",
 							 &similarity_threshold,
-							 0.3,
+							 0.3f,
 							 0.0,
 							 1.0,
 							 PGC_USERSET,
@@ -80,7 +80,7 @@ _PG_init(void)
 							 "Sets the threshold used by the <% operator.",
 							 "Valid range is 0.0 .. 1.0.",
 							 &word_similarity_threshold,
-							 0.6,
+							 0.6f,
 							 0.0,
 							 1.0,
 							 PGC_USERSET,
@@ -92,7 +92,7 @@ _PG_init(void)
 							 "Sets the threshold used by the <<% operator.",
 							 "Valid range is 0.0 .. 1.0.",
 							 &strict_word_similarity_threshold,
-							 0.5,
+							 0.5f,
 							 0.0,
 							 1.0,
 							 PGC_USERSET,
diff --git a/contrib/sepgsql/hooks.c b/contrib/sepgsql/hooks.c
index 87fdd972c2..363ac06700 100644
--- a/contrib/sepgsql/hooks.c
+++ b/contrib/sepgsql/hooks.c
@@ -57,7 +57,7 @@ static sepgsql_context_info_t sepgsql_context_info;
 /*
  * GUC: sepgsql.permissive = (on|off)
  */
-static bool sepgsql_permissive;
+static bool sepgsql_permissive = false;
 
 bool
 sepgsql_get_permissive(void)
@@ -68,7 +68,7 @@ sepgsql_get_permissive(void)
 /*
  * GUC: sepgsql.debug_audit = (on|off)
  */
-static bool sepgsql_debug_audit;
+static bool sepgsql_debug_audit = false;
 
 bool
 sepgsql_get_debug_audit(void)
diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index 6cb7dab124..883d6c0f70 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -75,7 +75,7 @@
  *	User-tweakable parameters
  */
 int			DefaultXactIsoLevel = XACT_READ_COMMITTED;
-int			XactIsoLevel;
+int			XactIsoLevel = XACT_READ_COMMITTED;
 
 bool		DefaultXactReadOnly = false;
 bool		XactReadOnly;
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 8f10effe3a..be54c23187 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -131,7 +131,7 @@ bool		wal_init_zero = true;
 bool		wal_recycle = true;
 bool		log_checkpoints = true;
 int			sync_method = DEFAULT_SYNC_METHOD;
-int			wal_level = WAL_LEVEL_MINIMAL;
+int			wal_level = WAL_LEVEL_REPLICA;
 int			CommitDelay = 0;	/* precommit delay in microseconds */
 int			CommitSiblings = 5; /* # concurrent xacts needed to sleep */
 int			wal_retrieve_retry_interval = 5000;
diff --git a/src/backend/libpq/be-secure.c b/src/backend/libpq/be-secure.c
index e3e54713e8..c8963572a7 100644
--- a/src/backend/libpq/be-secure.c
+++ b/src/backend/libpq/be-secure.c
@@ -58,8 +58,8 @@ char	   *SSLECDHCurve;
 /* GUC variable: if false, prefer client ciphers */
 bool		SSLPreferServerCiphers;
 
-int			ssl_min_protocol_version;
-int			ssl_max_protocol_version;
+int			ssl_min_protocol_version = PG_TLS1_2_VERSION;
+int			ssl_max_protocol_version = PG_TLS_ANY;
 
 /* ------------------------------------------------------------ */
 /*			 Procedures common to all secure sessions			*/
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 30fb576ac3..0b637ba6a2 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -196,7 +196,7 @@ BackgroundWorker *MyBgworkerEntry = NULL;
 
 
 /* The socket number we are listening for connections on */
-int			PostPortNumber;
+int			PostPortNumber = DEF_PGPORT;
 
 /* The directory names for Unix socket(s) */
 char	   *Unix_socket_directories;
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 6b95381481..73d30bf619 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -142,22 +142,22 @@ bool		track_io_timing = false;
  * for buffers not belonging to tablespaces that have their
  * effective_io_concurrency parameter set.
  */
-int			effective_io_concurrency = 0;
+int			effective_io_concurrency = DEFAULT_EFFECTIVE_IO_CONCURRENCY;
 
 /*
  * Like effective_io_concurrency, but used by maintenance code paths that might
  * benefit from a higher setting because they work on behalf of many sessions.
  * Overridden by the tablespace setting of the same name.
  */
-int			maintenance_io_concurrency = 0;
+int			maintenance_io_concurrency = DEFAULT_MAINTENANCE_IO_CONCURRENCY;
 
 /*
  * GUC variables about triggering kernel writeback for buffers written; OS
  * dependent defaults are set via the GUC mechanism.
  */
-int			checkpoint_flush_after = 0;
-int			bgwriter_flush_after = 0;
-int			backend_flush_after = 0;
+int			checkpoint_flush_after = DEFAULT_CHECKPOINT_FLUSH_AFTER;
+int			bgwriter_flush_after = DEFAULT_BGWRITER_FLUSH_AFTER;
+int			backend_flush_after = DEFAULT_BACKEND_FLUSH_AFTER;
 
 /* local state for StartBufferIO and related functions */
 static BufferDesc *InProgressBuf = NULL;
diff --git a/src/backend/storage/ipc/dsm_impl.c b/src/backend/storage/ipc/dsm_impl.c
index e1b90c5de4..6ddd46a4e7 100644
--- a/src/backend/storage/ipc/dsm_impl.c
+++ b/src/backend/storage/ipc/dsm_impl.c
@@ -109,7 +109,7 @@ const struct config_enum_entry dynamic_shared_memory_options[] = {
 };
 
 /* Implementation selector. */
-int			dynamic_shared_memory_type;
+int			dynamic_shared_memory_type = DEFAULT_DYNAMIC_SHARED_MEMORY_TYPE;
 
 /* Amount of space reserved for DSM segments in the main area. */
 int			min_dynamic_shared_memory;
diff --git a/src/backend/utils/adt/xml.c b/src/backend/utils/adt/xml.c
index e4f2adb1f9..3884babc1b 100644
--- a/src/backend/utils/adt/xml.c
+++ b/src/backend/utils/adt/xml.c
@@ -94,8 +94,8 @@
 
 
 /* GUC variables */
-int			xmlbinary;
-int			xmloption;
+int			xmlbinary = XMLBINARY_BASE64;
+int			xmloption = XMLOPTION_CONTENT;
 
 #ifdef USE_LIBXML
 
diff --git a/src/backend/utils/cache/plancache.c b/src/backend/utils/cache/plancache.c
index 0d6a295674..cc943205d3 100644
--- a/src/backend/utils/cache/plancache.c
+++ b/src/backend/utils/cache/plancache.c
@@ -116,7 +116,7 @@ static void PlanCacheObjectCallback(Datum arg, int cacheid, uint32 hashvalue);
 static void PlanCacheSysCallback(Datum arg, int cacheid, uint32 hashvalue);
 
 /* GUC parameter */
-int			plan_cache_mode;
+int			plan_cache_mode = PLAN_CACHE_MODE_AUTO;
 
 /*
  * InitPlanCache: initialize module during InitPostgres.
diff --git a/src/backend/utils/error/elog.c b/src/backend/utils/error/elog.c
index 6e0a66c29e..2585e24845 100644
--- a/src/backend/utils/error/elog.c
+++ b/src/backend/utils/error/elog.c
@@ -107,7 +107,7 @@ extern bool redirection_done;
 emit_log_hook_type emit_log_hook = NULL;
 
 /* GUC parameters */
-int			Log_error_verbosity = PGERROR_VERBOSE;
+int			Log_error_verbosity = PGERROR_DEFAULT;
 char	   *Log_line_prefix = NULL; /* format for extra log line info */
 int			Log_destination = LOG_DESTINATION_STDERR;
 char	   *Log_destination_string = NULL;
diff --git a/src/backend/utils/init/globals.c b/src/backend/utils/init/globals.c
index 1a5d29ac9b..00bceec8fa 100644
--- a/src/backend/utils/init/globals.c
+++ b/src/backend/utils/init/globals.c
@@ -133,8 +133,8 @@ int			max_parallel_maintenance_workers = 2;
  * MaxBackends is computed by PostmasterMain after modules have had a chance to
  * register background workers.
  */
-int			NBuffers = 1000;
-int			MaxConnections = 90;
+int			NBuffers = 16384;
+int			MaxConnections = 100;
 int			max_worker_processes = 8;
 int			max_parallel_workers = 8;
 int			MaxBackends = 0;
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 05ab087934..836b49484a 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -526,7 +526,7 @@ int			ssl_renegotiation_limit;
  * This really belongs in pg_shmem.c, but is defined here so that it doesn't
  * need to be duplicated in all the different implementations of pg_shmem.c.
  */
-int			huge_pages;
+int			huge_pages = HUGE_PAGES_TRY;
 int			huge_page_size;
 
 /*
@@ -543,7 +543,14 @@ static char *locale_ctype;
 static char *server_encoding_string;
 static char *server_version_string;
 static int	server_version_num;
-static int	syslog_facility;
+
+#ifdef HAVE_SYSLOG
+#define	DEFAULT_SYSLOG_FACILITY LOG_LOCAL0
+#else
+#define	DEFAULT_SYSLOG_FACILITY 0
+#endif
+static int	syslog_facility = DEFAULT_SYSLOG_FACILITY;
+
 static char *timezone_string;
 static char *log_timezone_string;
 static char *timezone_abbreviations_string;
@@ -559,7 +566,14 @@ static int	shared_memory_size_in_huge_pages;
 static int	wal_block_size;
 static bool data_checksums;
 static bool integer_datetimes;
-static bool assert_enabled;
+
+#ifdef USE_ASSERT_CHECKING
+#define DEFAULT_ASSERT_ENABLED true
+#else
+#define DEFAULT_ASSERT_ENABLED false
+#endif
+static bool assert_enabled = DEFAULT_ASSERT_ENABLED;
+
 static char *recovery_target_timeline_string;
 static char *recovery_target_string;
 static char *recovery_target_xid_string;
@@ -1181,11 +1195,7 @@ struct config_bool ConfigureNamesBool[] =
 			GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
 		},
 		&assert_enabled,
-#ifdef USE_ASSERT_CHECKING
-		true,
-#else
-		false,
-#endif
+		DEFAULT_ASSERT_ENABLED,
 		NULL, NULL, NULL
 	},
 
@@ -1357,11 +1367,7 @@ struct config_bool ConfigureNamesBool[] =
 			gettext_noop("Enables updating of the process title every time a new SQL command is received by the server.")
 		},
 		&update_process_title,
-#ifdef WIN32
-		false,
-#else
-		true,
-#endif
+		DEFAULT_UPDATE_PROCESS_TITLE,
 		NULL, NULL, NULL
 	},
 
@@ -2888,11 +2894,7 @@ struct config_int ConfigureNamesInt[] =
 			GUC_EXPLAIN
 		},
 		&effective_io_concurrency,
-#ifdef USE_PREFETCH
-		1,
-#else
-		0,
-#endif
+		DEFAULT_EFFECTIVE_IO_CONCURRENCY,
 		0, MAX_IO_CONCURRENCY,
 		check_effective_io_concurrency, NULL, NULL
 	},
@@ -2906,11 +2908,7 @@ struct config_int ConfigureNamesInt[] =
 			GUC_EXPLAIN
 		},
 		&maintenance_io_concurrency,
-#ifdef USE_PREFETCH
-		10,
-#else
-		0,
-#endif
+		DEFAULT_MAINTENANCE_IO_CONCURRENCY,
 		0, MAX_IO_CONCURRENCY,
 		check_maintenance_io_concurrency, assign_maintenance_io_concurrency,
 		NULL
@@ -4613,11 +4611,7 @@ struct config_enum ConfigureNamesEnum[] =
 			NULL
 		},
 		&syslog_facility,
-#ifdef HAVE_SYSLOG
-		LOG_LOCAL0,
-#else
-		0,
-#endif
+		DEFAULT_SYSLOG_FACILITY,
 		syslog_facility_options,
 		NULL, assign_syslog_facility, NULL
 	},
diff --git a/src/backend/utils/misc/ps_status.c b/src/backend/utils/misc/ps_status.c
index 8520ce76bb..d81a67be79 100644
--- a/src/backend/utils/misc/ps_status.c
+++ b/src/backend/utils/misc/ps_status.c
@@ -30,8 +30,9 @@
 #include "utils/ps_status.h"
 
 extern char **environ;
-bool		update_process_title = true;
 
+/* GUC variable */
+bool		update_process_title = DEFAULT_UPDATE_PROCESS_TITLE;
 
 /*
  * Alternative ways of updating ps display:
diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h
index 6f4dfa0960..e1bd22441b 100644
--- a/src/include/storage/bufmgr.h
+++ b/src/include/storage/bufmgr.h
@@ -69,6 +69,15 @@ extern PGDLLIMPORT bool zero_damaged_pages;
 extern PGDLLIMPORT int bgwriter_lru_maxpages;
 extern PGDLLIMPORT double bgwriter_lru_multiplier;
 extern PGDLLIMPORT bool track_io_timing;
+
+/* only applicable when prefetching is available */
+#ifdef USE_PREFETCH
+#define DEFAULT_EFFECTIVE_IO_CONCURRENCY 1
+#define DEFAULT_MAINTENANCE_IO_CONCURRENCY 10
+#else
+#define DEFAULT_EFFECTIVE_IO_CONCURRENCY 0
+#define DEFAULT_MAINTENANCE_IO_CONCURRENCY 0
+#endif
 extern PGDLLIMPORT int effective_io_concurrency;
 extern PGDLLIMPORT int maintenance_io_concurrency;
 
diff --git a/src/include/utils/ps_status.h b/src/include/utils/ps_status.h
index bba463591f..6953a326f1 100644
--- a/src/include/utils/ps_status.h
+++ b/src/include/utils/ps_status.h
@@ -12,6 +12,13 @@
 #ifndef PS_STATUS_H
 #define PS_STATUS_H
 
+/* disabled on Windows as the performance overhead can be significant */
+#ifdef WIN32
+#define DEFAULT_UPDATE_PROCESS_TITLE false
+#else
+#define DEFAULT_UPDATE_PROCESS_TITLE true
+#endif
+
 extern PGDLLIMPORT bool update_process_title;
 
 extern char **save_ps_display_args(int argc, char **argv);