Improve the performance of relation deletes during recovery.

When multiple relations are deleted at the same transaction,
the files of those relations are deleted by one call to smgrdounlinkall(),
which leads to scan whole shared_buffers only one time. OTOH,
previously, during recovery, smgrdounlink() (not smgrdounlinkall()) was
called for each file to delete, which led to scan shared_buffers
multiple times. Obviously this could cause to increase the WAL replay
time very much especially when shared_buffers was huge.

To alleviate this situation, this commit changes the recovery so that
it also calls smgrdounlinkall() only one time to delete multiple
relation files.

This is just fix for oversight of commit 279628a, not new feature.
So, per discussion on pgsql-hackers, we concluded to backpatch this
to all supported versions.

Author: Fujii Masao
Reviewed-by: Michael Paquier, Andres Freund, Thomas Munro, Kyotaro Horiguchi, Takayuki Tsunakawa
Discussion: https://postgr.es/m/CAHGQGwHVQkdfDqtvGVkty+19cQakAydXn1etGND3X0PHbZ3+6w@mail.gmail.com

Author: MasaoFujii

src/backend/access/transam/twophase.c

@@ -1456,7 +1456,6 @@ FinishPreparedTransaction(const char *gid, bool isCommit)
 	RelFileNode *delrels;
 	int			ndelrels;
 	SharedInvalidationMessage *invalmsgs;
-	int			i;
 
 	/*
 	 * Validate the GID, and lock the GXACT to ensure that two backends do not
@@ -1549,13 +1548,9 @@ FinishPreparedTransaction(const char *gid, bool isCommit)
 		delrels = abortrels;
 		ndelrels = hdr->nabortrels;
 	}
-	for (i = 0; i < ndelrels; i++)
-	{
-		SMgrRelation srel = smgropen(delrels[i], InvalidBackendId);
 
-		smgrdounlink(srel, false);
-		smgrclose(srel);
-	}
+	/* Make sure files supposed to be dropped are dropped */
+	DropRelationFiles(delrels, ndelrels, false);
 
 	/*
 	 * Handle cache invalidation messages.

src/backend/access/transam/xact.c

@@ -5516,7 +5516,6 @@ xact_redo_commit(xl_xact_parsed_commit *parsed,
 				 RepOriginId origin_id)
 {
 	TransactionId max_xid;
-	int			i;
 	TimestampTz commit_time;
 
 	Assert(TransactionIdIsValid(xid));
@@ -5635,16 +5634,8 @@ xact_redo_commit(xl_xact_parsed_commit *parsed,
 		 */
 		XLogFlush(lsn);
 
-		for (i = 0; i < parsed->nrels; i++)
-		{
-			SMgrRelation srel = smgropen(parsed->xnodes[i], InvalidBackendId);
-			ForkNumber	fork;
-
-			for (fork = 0; fork <= MAX_FORKNUM; fork++)
-				XLogDropRelation(parsed->xnodes[i], fork);
-			smgrdounlink(srel, true);
-			smgrclose(srel);
-		}
+		/* Make sure files supposed to be dropped are dropped */
+		DropRelationFiles(parsed->xnodes, parsed->nrels, true);
 	}
 
 	/*
@@ -5683,7 +5674,6 @@ xact_redo_commit(xl_xact_parsed_commit *parsed,
 static void
 xact_redo_abort(xl_xact_parsed_abort *parsed, TransactionId xid)
 {
-	int			i;
 	TransactionId max_xid;
 
 	Assert(TransactionIdIsValid(xid));
@@ -5748,16 +5738,7 @@ xact_redo_abort(xl_xact_parsed_abort *parsed, TransactionId xid)
 	}
 
 	/* Make sure files supposed to be dropped are dropped */
-	for (i = 0; i < parsed->nrels; i++)
-	{
-		SMgrRelation srel = smgropen(parsed->xnodes[i], InvalidBackendId);
-		ForkNumber	fork;
-
-		for (fork = 0; fork <= MAX_FORKNUM; fork++)
-			XLogDropRelation(parsed->xnodes[i], fork);
-		smgrdounlink(srel, true);
-		smgrclose(srel);
-	}
+	DropRelationFiles(parsed->xnodes, parsed->nrels, true);
 }
 
 void

src/backend/storage/smgr/md.c

@@ -26,6 +26,7 @@
 #include <sys/file.h>
 
 #include "miscadmin.h"
+#include "access/xlogutils.h"
 #include "access/xlog.h"
 #include "pgstat.h"
 #include "portability/instr_time.h"
@@ -1703,6 +1704,43 @@ ForgetDatabaseFsyncRequests(Oid dbid)
 	}
 }
 
+/*
+ * DropRelationFiles -- drop files of all given relations
+ */
+void
+DropRelationFiles(RelFileNode *delrels, int ndelrels, bool isRedo)
+{
+	SMgrRelation *srels;
+	int			i;
+
+	srels = palloc(sizeof(SMgrRelation) * ndelrels);
+	for (i = 0; i < ndelrels; i++)
+	{
+		SMgrRelation srel = smgropen(delrels[i], InvalidBackendId);
+
+		if (isRedo)
+		{
+			ForkNumber	fork;
+
+			for (fork = 0; fork <= MAX_FORKNUM; fork++)
+				XLogDropRelation(delrels[i], fork);
+		}
+		srels[i] = srel;
+	}
+
+	smgrdounlinkall(srels, ndelrels, isRedo);
+
+	/*
+	 * Call smgrclose() in reverse order as when smgropen() is called.
+	 * This trick enables remove_from_unowned_list() in smgrclose()
+	 * to search the SMgrRelation from the unowned list,
+	 * with O(1) performance.
+	 */
+	for (i = ndelrels - 1; i >= 0; i--)
+		smgrclose(srels[i]);
+	pfree(srels);
+}
+
 
 /*
  *	_fdvec_resize() -- Resize the fork's open segments array

src/include/storage/smgr.h

@@ -143,5 +143,6 @@ extern void RememberFsyncRequest(RelFileNode rnode, ForkNumber forknum,
 					 BlockNumber segno);
 extern void ForgetRelationFsyncRequests(RelFileNode rnode, ForkNumber forknum);
 extern void ForgetDatabaseFsyncRequests(Oid dbid);
+extern void DropRelationFiles(RelFileNode *delrels, int ndelrels, bool isRedo);
 
 #endif							/* SMGR_H */