Subject: [PATCH] do only critical work during single-user vacuum?

From: John Naylor <[email-masked]>
Date: Fri, 21 Jan 2022 17:41:58 -0500

Jan 21 John Naylor
---
 src/backend/commands/vacuum.c | 76 +++++++++++++++++++++++++++++++++--
 src/include/commands/vacuum.h |  1 +
 2 files changed, 74 insertions(+), 3 deletions(-)

diff --git a/src/backend/commands/vacuum.c b/src/backend/commands/vacuum.c
index d1dadc54e47..c7bc97d3f76 100644
--- a/src/backend/commands/vacuum.c
+++ b/src/backend/commands/vacuum.c
@@ -52,6 +52,7 @@
 #include "storage/proc.h"
 #include "storage/procarray.h"
 #include "utils/acl.h"
+#include "utils/builtins.h"
 #include "utils/fmgroids.h"
 #include "utils/guc.h"
 #include "utils/memutils.h"
@@ -114,6 +115,7 @@ ExecVacuum(ParseState *pstate, VacuumStmt *vacstmt, bool isTopLevel)
 	bool		full = false;
 	bool		disable_page_skipping = false;
 	bool		process_toast = true;
+	bool		wraparound = false;
 	ListCell   *lc;
 
 	/* index_cleanup and truncate values unspecified for now */
@@ -200,6 +202,8 @@ ExecVacuum(ParseState *pstate, VacuumStmt *vacstmt, bool isTopLevel)
 					params.nworkers = nworkers;
 			}
 		}
+		else if (strcmp(opt->defname, "wraparound") == 0)
+			wraparound = defGetBoolean(opt);
 		else
 			ereport(ERROR,
 					(errcode(ERRCODE_SYNTAX_ERROR),
@@ -246,17 +250,51 @@ ExecVacuum(ParseState *pstate, VacuumStmt *vacstmt, bool isTopLevel)
 		}
 	}
 
+	if (wraparound)
+	{
+		/* exclude incompatible options */
+		foreach(lc, vacstmt->options)
+		{
+			DefElem    *opt = (DefElem *) lfirst(lc);
+
+			// WIP is there a better way?
+			if (strcmp(opt->defname, "wraparound") != 0 &&
+				strcmp(opt->defname, "verbose") != 0 &&
+				defGetBoolean(opt))
+
+				ereport(ERROR,
+						(errcode(ERRCODE_SYNTAX_ERROR),
+								errmsg("option \"%s\" is incompatible with WRAPAROUND", opt->defname),
+								parser_errposition(pstate, opt->location)));
+		}
+
+		/* skip unnecessary work, as in failsafe mode */
+		params.index_cleanup = VACOPTVALUE_DISABLED;
+		params.truncate = VACOPTVALUE_DISABLED;
+	}
+
 	/*
-	 * All freeze ages are zero if the FREEZE option is given; otherwise pass
-	 * them as -1 which means to use the default values.
+	 * Set freeze ages to zero where appropriate; otherwise pass
+	 * them as -1 which means to use the configured values.
 	 */
 	if (params.options & VACOPT_FREEZE)
 	{
+		/* All freeze ages are zero if the FREEZE option is given */
 		params.freeze_min_age = 0;
 		params.freeze_table_age = 0;
 		params.multixact_freeze_min_age = 0;
 		params.multixact_freeze_table_age = 0;
 	}
+	else if (params.options & VACOPT_MINIMAL)
+	{
+		/* it's unlikely any selected table will not be eligible for aggressive vacuum, but make sure */
+		params.freeze_table_age = 0;
+		params.multixact_freeze_table_age = 0;
+
+		// WIP: It might be worth trying to do less work here, or at least hard-coding the default values
+		params.freeze_min_age = -1;
+		params.multixact_freeze_min_age = -1;
+	}
 	else
 	{
 		params.freeze_min_age = -1;
@@ -894,6 +932,8 @@ get_all_vacuum_rels(int options)
 	Relation	pgclass;
 	TableScanDesc scan;
 	HeapTuple	tuple;
+	int32 		table_xid_age,
+				table_mxid_age;
 
 	pgclass = table_open(RelationRelationId, AccessShareLock);
 
@@ -909,12 +949,42 @@ get_all_vacuum_rels(int options)
 		if (!vacuum_is_relation_owner(relid, classForm, options))
 			continue;
 
+		if (options & VACOPT_MINIMAL)
+		{
+			/*
+			* Only consider relations able to hold unfrozen XIDs (anything else
+			* should have InvalidTransactionId in relfrozenxid anyway).
+			*/
+			if (classForm->relkind != RELKIND_RELATION &&
+				classForm->relkind != RELKIND_MATVIEW &&
+				classForm->relkind != RELKIND_TOASTVALUE)
+			{
+				Assert(!TransactionIdIsValid(classForm->relfrozenxid));
+				Assert(!MultiXactIdIsValid(classForm->relminmxid));
+				continue;
+			}
+
+			table_xid_age = DirectFunctionCall1(xid_age, classForm->relfrozenxid);
+			table_mxid_age = DirectFunctionCall1(mxid_age, classForm->relminmxid);
+
+			/* Hard-code 1 billion for the thresholds to avoid making assumptions
+			 * about the configuration. This leaves some headroom for when the user
+			 * returns to normal mode while also minimizing work.
+			 * WIP: consider passing these constants via the params struct
+			 */
+			// FIXME to speed up testing
+			// if ((table_xid_age < 1000 * 1000 * 1000) &&
+			// 	(table_mxid_age < 1000 * 1000 * 1000))
+			if ((table_xid_age < 1000 * 1000) &&
+				(table_mxid_age < 1000 * 1000))
+				continue;
+		}
 		/*
 		 * We include partitioned tables here; depending on which operation is
 		 * to be performed, caller will decide whether to process or ignore
 		 * them.
 		 */
-		if (classForm->relkind != RELKIND_RELATION &&
+		else if (classForm->relkind != RELKIND_RELATION &&
 			classForm->relkind != RELKIND_MATVIEW &&
 			classForm->relkind != RELKIND_PARTITIONED_TABLE)
 			continue;
diff --git a/src/include/commands/vacuum.h b/src/include/commands/vacuum.h
index 5d0bdfa4279..3d8b8fbbb1a 100644
--- a/src/include/commands/vacuum.h
+++ b/src/include/commands/vacuum.h
@@ -188,6 +188,7 @@ typedef struct VacAttrStats
 #define VACOPT_SKIP_LOCKED 0x20 /* skip if cannot get lock */
 #define VACOPT_PROCESS_TOAST 0x40	/* process the TOAST table, if any */
 #define VACOPT_DISABLE_PAGE_SKIPPING 0x80	/* don't skip any pages */
+#define VACOPT_MINIMAL 0x100	/* do minimal freezing work to prevent or get out of shutdown */
 
 /*
  * Values used by index_cleanup and truncate params.
-- 
2.17.1

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