Add VACUUM (DISABLE_PAGE_SKIPPING) for emergencies.

If you really want to vacuum every single page in the relation,
regardless of apparent visibility status or anything else, you can use
this option.  In previous releases, this behavior could be achieved
using VACUUM (FREEZE), but because we can now recognize all-frozen
pages as not needing to be frozen again, that no longer works.  There
should be no need for routine use of this option, but maybe bugs or
disaster recovery will necessitate its use.

Patch by me, reviewed by Andres Freund.

doc/src/sgml/ref/vacuum.sgml

@@ -21,7 +21,7 @@ PostgreSQL documentation
 
  <refsynopsisdiv>
 <synopsis>
-VACUUM [ ( { FULL | FREEZE | VERBOSE | ANALYZE } [, ...] ) ] [ <replaceable class="PARAMETER">table_name</replaceable> [ (<replaceable class="PARAMETER">column_name</replaceable> [, ...] ) ] ]
+VACUUM [ ( { FULL | FREEZE | VERBOSE | ANALYZE | DISABLE_PAGE_SKIPPING } [, ...] ) ] [ <replaceable class="PARAMETER">table_name</replaceable> [ (<replaceable class="PARAMETER">column_name</replaceable> [, ...] ) ] ]
 VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] [ <replaceable class="PARAMETER">table_name</replaceable> ]
 VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] ANALYZE [ <replaceable class="PARAMETER">table_name</replaceable> [ (<replaceable class="PARAMETER">column_name</replaceable> [, ...] ) ] ]
 </synopsis>
@@ -129,6 +129,25 @@ VACUUM [ FULL ] [ FREEZE ] [ VERBOSE ] ANALYZE [ <replaceable class="PARAMETER">
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><literal>DISABLE_PAGE_SKIPPING</literal></term>
+    <listitem>
+     <para>
+      Normally, <command>VACUUM</> will skip pages based on the <link
+      linkend="vacuum-for-visibility-map">visibility map</>.  Pages where
+      all tuples are known to be frozen can always be skipped, and those
+      where all tuples are known to be visible to all transactions may be
+      skipped except when performing an aggressive vacuum.  Furthermore,
+      except when performing an aggressive vacuum, some pages may be skipped
+      in order to avoid waiting for other sessions to finish using them.
+      This option disables all page-skipping behavior, and is intended to
+      be used only the contents of the visibility map are thought to
+      be suspect, which should happen only if there is a hardware or software
+      issue causing database corruption.
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><replaceable class="PARAMETER">table_name</replaceable></term>
     <listitem>

src/backend/commands/vacuum.c

@@ -185,6 +185,15 @@ vacuum(int options, RangeVar *relation, Oid relid, VacuumParams *params,
 				 errmsg("%s cannot be executed from VACUUM or ANALYZE",
 						stmttype)));
 
+	/*
+	 * Sanity check DISABLE_PAGE_SKIPPING option.
+	 */
+	if ((options & VACOPT_FULL) != 0 &&
+		(options & VACOPT_DISABLE_PAGE_SKIPPING) != 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("VACUUM option DISABLE_PAGE_SKIPPING cannot be used with FULL")));
+
 	/*
 	 * Send info about dead objects to the statistics collector, unless we are
 	 * in autovacuum --- autovacuum.c does this for itself.

src/backend/commands/vacuumlazy.c

@@ -137,8 +137,9 @@ static BufferAccessStrategy vac_strategy;
 
 
 /* non-export function prototypes */
-static void lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
-			   Relation *Irel, int nindexes, bool aggressive);
+static void lazy_scan_heap(Relation onerel, int options,
+			   LVRelStats *vacrelstats, Relation *Irel, int nindexes,
+			   bool aggressive);
 static void lazy_vacuum_heap(Relation onerel, LVRelStats *vacrelstats);
 static bool lazy_check_needs_freeze(Buffer buf, bool *hastup);
 static void lazy_vacuum_index(Relation indrel,
@@ -223,15 +224,17 @@ lazy_vacuum_rel(Relation onerel, int options, VacuumParams *params,
 						  &MultiXactCutoff, &mxactFullScanLimit);
 
 	/*
-	 * We request an aggressive scan if either the table's frozen Xid is now
-	 * older than or equal to the requested Xid full-table scan limit; or if
-	 * the table's minimum MultiXactId is older than or equal to the requested
-	 * mxid full-table scan limit.
+	 * We request an aggressive scan if the table's frozen Xid is now older
+	 * than or equal to the requested Xid full-table scan limit; or if the
+	 * table's minimum MultiXactId is older than or equal to the requested
+	 * mxid full-table scan limit; or if DISABLE_PAGE_SKIPPING was specified.
 	 */
 	aggressive = TransactionIdPrecedesOrEquals(onerel->rd_rel->relfrozenxid,
 											   xidFullScanLimit);
 	aggressive |= MultiXactIdPrecedesOrEquals(onerel->rd_rel->relminmxid,
 											  mxactFullScanLimit);
+	if (options & VACOPT_DISABLE_PAGE_SKIPPING)
+		aggressive = true;
 
 	vacrelstats = (LVRelStats *) palloc0(sizeof(LVRelStats));
 
@@ -246,7 +249,7 @@ lazy_vacuum_rel(Relation onerel, int options, VacuumParams *params,
 	vacrelstats->hasindex = (nindexes > 0);
 
 	/* Do the vacuuming */
-	lazy_scan_heap(onerel, vacrelstats, Irel, nindexes, aggressive);
+	lazy_scan_heap(onerel, options, vacrelstats, Irel, nindexes, aggressive);
 
 	/* Done with indexes */
 	vac_close_indexes(nindexes, Irel, NoLock);
@@ -441,7 +444,7 @@ vacuum_log_cleanup_info(Relation rel, LVRelStats *vacrelstats)
  *		reference them have been killed.
  */
 static void
-lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
+lazy_scan_heap(Relation onerel, int options, LVRelStats *vacrelstats,
 			   Relation *Irel, int nindexes, bool aggressive)
 {
 	BlockNumber nblocks,
@@ -542,25 +545,28 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
 	 * the last page.  This is worth avoiding mainly because such a lock must
 	 * be replayed on any hot standby, where it can be disruptive.
 	 */
-	for (next_unskippable_block = 0;
-		 next_unskippable_block < nblocks;
-		 next_unskippable_block++)
+	next_unskippable_block = 0;
+	if ((options & VACOPT_DISABLE_PAGE_SKIPPING) == 0)
 	{
-		uint8		vmstatus;
-
-		vmstatus = visibilitymap_get_status(onerel, next_unskippable_block,
-											&vmbuffer);
-		if (aggressive)
+		while (next_unskippable_block < nblocks)
 		{
-			if ((vmstatus & VISIBILITYMAP_ALL_FROZEN) == 0)
-				break;
-		}
-		else
-		{
-			if ((vmstatus & VISIBILITYMAP_ALL_VISIBLE) == 0)
-				break;
+			uint8		vmstatus;
+
+			vmstatus = visibilitymap_get_status(onerel, next_unskippable_block,
+												&vmbuffer);
+			if (aggressive)
+			{
+				if ((vmstatus & VISIBILITYMAP_ALL_FROZEN) == 0)
+					break;
+			}
+			else
+			{
+				if ((vmstatus & VISIBILITYMAP_ALL_VISIBLE) == 0)
+					break;
+			}
+			vacuum_delay_point();
+			next_unskippable_block++;
 		}
-		vacuum_delay_point();
 	}
 
 	if (next_unskippable_block >= SKIP_PAGES_THRESHOLD)
@@ -594,26 +600,29 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
 		if (blkno == next_unskippable_block)
 		{
 			/* Time to advance next_unskippable_block */
-			for (next_unskippable_block++;
-				 next_unskippable_block < nblocks;
-				 next_unskippable_block++)
+			next_unskippable_block++;
+			if ((options & VACOPT_DISABLE_PAGE_SKIPPING) == 0)
 			{
-				uint8		vmskipflags;
-
-				vmskipflags = visibilitymap_get_status(onerel,
-													   next_unskippable_block,
-													   &vmbuffer);
-				if (aggressive)
+				while (next_unskippable_block < nblocks)
 				{
-					if ((vmskipflags & VISIBILITYMAP_ALL_FROZEN) == 0)
-						break;
-				}
-				else
-				{
-					if ((vmskipflags & VISIBILITYMAP_ALL_VISIBLE) == 0)
-						break;
+					uint8		vmskipflags;
+
+					vmskipflags = visibilitymap_get_status(onerel,
+													  next_unskippable_block,
+														   &vmbuffer);
+					if (aggressive)
+					{
+						if ((vmskipflags & VISIBILITYMAP_ALL_FROZEN) == 0)
+							break;
+					}
+					else
+					{
+						if ((vmskipflags & VISIBILITYMAP_ALL_VISIBLE) == 0)
+							break;
+					}
+					vacuum_delay_point();
+					next_unskippable_block++;
 				}
-				vacuum_delay_point();
 			}
 
 			/*
@@ -1054,7 +1063,7 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
 			}
 			else
 			{
-				bool	tuple_totally_frozen;
+				bool		tuple_totally_frozen;
 
 				num_tuples += 1;
 				hastup = true;
@@ -1064,8 +1073,8 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,
 				 * freezing.  Note we already have exclusive buffer lock.
 				 */
 				if (heap_prepare_freeze_tuple(tuple.t_data, FreezeLimit,
-										  MultiXactCutoff, &frozen[nfrozen],
-											&tuple_totally_frozen))
+										   MultiXactCutoff, &frozen[nfrozen],
+											  &tuple_totally_frozen))
 					frozen[nfrozen++].offset = offnum;
 
 				if (!tuple_totally_frozen)

src/backend/parser/gram.y

@@ -9370,6 +9370,16 @@ vacuum_option_elem:
 			| VERBOSE			{ $$ = VACOPT_VERBOSE; }
 			| FREEZE			{ $$ = VACOPT_FREEZE; }
 			| FULL				{ $$ = VACOPT_FULL; }
+			| IDENT
+				{
+					if (strcmp($1, "disable_page_skipping") == 0)
+						$$ = VACOPT_DISABLE_PAGE_SKIPPING;
+					else
+						ereport(ERROR,
+								(errcode(ERRCODE_SYNTAX_ERROR),
+							 errmsg("unrecognized VACUUM option \"%s\"", $1),
+									 parser_errposition(@1)));
+				}
 		;
 
 AnalyzeStmt:

src/include/nodes/parsenodes.h

@@ -2822,7 +2822,8 @@ typedef enum VacuumOption
 	VACOPT_FREEZE = 1 << 3,		/* FREEZE option */
 	VACOPT_FULL = 1 << 4,		/* FULL (non-concurrent) vacuum */
 	VACOPT_NOWAIT = 1 << 5,		/* don't wait to get lock (autovacuum only) */
-	VACOPT_SKIPTOAST = 1 << 6	/* don't process the TOAST table, if any */
+	VACOPT_SKIPTOAST = 1 << 6,	/* don't process the TOAST table, if any */
+	VACOPT_DISABLE_PAGE_SKIPPING = 1 << 7	/* don't skip any pages */
 } VacuumOption;
 
 typedef struct VacuumStmt

src/test/regress/expected/vacuum.out

@@ -79,5 +79,6 @@ ERROR:  ANALYZE cannot be executed from VACUUM or ANALYZE
 CONTEXT:  SQL function "do_analyze" statement 1
 SQL function "wrap_do_analyze" statement 1
 VACUUM FULL vactst;
+VACUUM (DISABLE_PAGE_SKIPPING) vaccluster;
 DROP TABLE vaccluster;
 DROP TABLE vactst;

src/test/regress/sql/vacuum.sql

@@ -60,5 +60,7 @@ VACUUM FULL pg_database;
 VACUUM FULL vaccluster;
 VACUUM FULL vactst;
 
+VACUUM (DISABLE_PAGE_SKIPPING) vaccluster;
+
 DROP TABLE vaccluster;
 DROP TABLE vactst;