From fabe03a1e3cca701e35e6394c82e218369cd63a1 Mon Sep 17 00:00:00 2001
From: Melanie Plageman <melanieplageman@gmail.com>
Date: Mon, 3 Apr 2023 12:36:35 -0400
Subject: [PATCH v13 3/4] VACUUM reloads config file more often

Previously, VACUUM would not reload the configuration file, so changes
to cost-based delay parameters could only take effect on the next
invocation of VACUUM.

Now, check if a reload is pending roughly once per block, when checking
if we need to delay.

Note that autovacuum is unaffected by this change. Autovacuum workers
overwrite the value of VacuumCostLimit and VacuumCostDelay with their
own WorkerInfo->wi_cost_limit and wi_cost_delay instead of using
potentially refreshed values of autovacuum_vacuum_cost_limit and
autovacuum_vacuum_cost_delay. Locking considerations and needed updates
to the worker balancing logic make enabling this feature for autovacuum
worthy of an independent commit.

Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://www.postgresql.org/message-id/flat/CAAKRu_ZngzqnEODc7LmS1NH04Kt6Y9huSjz5pp7%2BDXhrjDA0gw%40mail.gmail.com
---
 src/backend/commands/vacuum.c         | 41 +++++++++++++++++++++++++--
 src/backend/commands/vacuumparallel.c |  5 ++--
 src/backend/postmaster/autovacuum.c   | 18 ++++++++++++
 3 files changed, 58 insertions(+), 6 deletions(-)

diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index 96df5e2920..888eb022cc 100644
--- a/src/backend/commands/vacuum.c
+++ b/src/backend/commands/vacuum.c
@@ -48,6 +48,7 @@
 #include "pgstat.h"
 #include "postmaster/autovacuum.h"
 #include "postmaster/bgworker_internals.h"
+#include "postmaster/interrupt.h"
 #include "storage/bufmgr.h"
 #include "storage/lmgr.h"
 #include "storage/pmsignal.h"
@@ -83,6 +84,7 @@ int			vacuum_cost_limit;
  */
 int			VacuumCostLimit = 0;
 double		VacuumCostDelay = -1;
+static bool vacuum_can_reload_config = false;
 
 /*
  * VacuumFailsafeActive is a defined as a global so that we can determine
@@ -354,6 +356,8 @@ vacuum(List *relations, VacuumParams *params,
 	else
 		in_outer_xact = IsInTransactionBlock(isTopLevel);
 
+	vacuum_can_reload_config = !in_outer_xact;
+
 	/*
 	 * Check for and disallow recursive calls.  This could happen when VACUUM
 	 * FULL or ANALYZE calls a hostile index expression that itself calls
@@ -510,9 +514,9 @@ vacuum(List *relations, VacuumParams *params,
 	{
 		ListCell   *cur;
 
-		VacuumUpdateCosts();
 		in_vacuum = true;
-		VacuumCostActive = (VacuumCostDelay > 0);
+		VacuumFailsafeActive = false;
+		VacuumUpdateCosts();
 		VacuumCostBalance = 0;
 		VacuumPageHit = 0;
 		VacuumPageMiss = 0;
@@ -566,12 +570,21 @@ vacuum(List *relations, VacuumParams *params,
 					CommandCounterIncrement();
 				}
 			}
+
+			/*
+			 * Ensure VacuumFailsafeActive has been reset before vacuuming the
+			 * next relation relation.
+			 */
+			VacuumFailsafeActive = false;
 		}
 	}
 	PG_FINALLY();
 	{
 		in_vacuum = false;
 		VacuumCostActive = false;
+		VacuumFailsafeActive = false;
+		VacuumCostBalance = 0;
+		vacuum_can_reload_config = false;
 	}
 	PG_END_TRY();
 
@@ -2238,7 +2251,29 @@ vacuum_delay_point(void)
 	/* Always check for interrupts */
 	CHECK_FOR_INTERRUPTS();
 
-	if (!VacuumCostActive || InterruptPending)
+	if (InterruptPending ||
+		(!VacuumCostActive && !ConfigReloadPending))
+		return;
+
+	/*
+	 * Reload the configuration file if requested. This allows changes to
+	 * vacuum_cost_limit and vacuum_cost_delay to take effect while a table is
+	 * being vacuumed or analyzed. Analyze should not reload configuration
+	 * file if it is in an outer transaction, as we currently only allow
+	 * configuration reload when in top-level statements.
+	 */
+	if (ConfigReloadPending && vacuum_can_reload_config)
+	{
+		ConfigReloadPending = false;
+		ProcessConfigFile(PGC_SIGHUP);
+		VacuumUpdateCosts();
+	}
+
+	/*
+	 * If we disabled cost-based delays after reloading the config file,
+	 * return.
+	 */
+	if (!VacuumCostActive)
 		return;
 
 	/*
diff --git a/src/backend/commands/vacuumparallel.c b/src/backend/commands/vacuumparallel.c
index cf8cf89927..4bc1c14dff 100644
--- a/src/backend/commands/vacuumparallel.c
+++ b/src/backend/commands/vacuumparallel.c
@@ -995,10 +995,9 @@ parallel_vacuum_main(dsm_segment *seg, shm_toc *toc)
 												 false);
 
 	/* Set cost-based vacuum delay */
-	VacuumFailsafeActive = false;
-	VacuumUpdateCosts();
-	VacuumCostActive = (VacuumCostDelay > 0);
+	Assert(!VacuumFailsafeActive);
 	VacuumCostBalance = 0;
+	VacuumUpdateCosts();
 	VacuumPageHit = 0;
 	VacuumPageMiss = 0;
 	VacuumPageDirty = 0;
diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index 27d0d5f9e2..7a9738202f 100644
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -1796,6 +1796,24 @@ VacuumUpdateCosts(void)
 		VacuumCostLimit = vacuum_cost_limit;
 		VacuumCostDelay = vacuum_cost_delay;
 	}
+
+	/*
+	 * If configuration changes are allowed to impact VacuumCostActive, make
+	 * sure it is updated.
+	 */
+	if (VacuumFailsafeActive)
+	{
+		Assert(!VacuumCostActive);
+		return;
+	}
+
+	if (VacuumCostDelay > 0)
+		VacuumCostActive = true;
+	else
+	{
+		VacuumCostActive = false;
+		VacuumCostBalance = 0;
+	}
 }
 
 /*
-- 
2.37.2