Make DROP DATABASE command generate less WAL records.

Previously DROP DATABASE generated as many XLOG_DBASE_DROP WAL records
as the number of tablespaces that the database to drop uses. This caused
the scans of shared_buffers as many times as the number of the tablespaces
during recovery because WAL replay of one XLOG_DBASE_DROP record needs
that full scan. This could make the recovery time longer especially
when shared_buffers is large.

This commit changes DROP DATABASE so that it generates only one
XLOG_DBASE_DROP record, and registers the information of all the tablespaces
into it. Then, WAL replay of XLOG_DBASE_DROP record needs full scan of
shared_buffers only once, and which may improve the recovery performance.

Author: Fujii Masao
Reviewed-by: Kirk Jamison, Simon Riggs
Discussion: https://postgr.es/m/CAHGQGwF8YwNH0ZaL+2wjZPkj+ji9UhC+Z4ScnG97WKtVY5L9iw@mail.gmail.com

diff --git a/src/backend/access/rmgrdesc/dbasedesc.c b/src/backend/access/rmgrdesc/dbasedesc.c
index c7d60ce10d6845e71cbce6ba5275df5ae55659cc..d08c57587296b5ccc98eb76dae750932c3338d6d 100644 (file)
--- a/src/backend/access/rmgrdesc/dbasedesc.c
+++ b/src/backend/access/rmgrdesc/dbasedesc.c
@@ -35,9 +35,12 @@ dbase_desc(StringInfo buf, XLogReaderState *record)
        else if (info == XLOG_DBASE_DROP)
        {
                xl_dbase_drop_rec *xlrec = (xl_dbase_drop_rec *) rec;
+               int             i;
 
-               appendStringInfo(buf, "dir %u/%u",
-                                                xlrec->tablespace_id, xlrec->db_id);
+               appendStringInfo(buf, "dir");
+               for (i = 0; i < xlrec->ntablespaces; i++)
+                       appendStringInfo(buf, " %u/%u",
+                                                        xlrec->tablespace_ids[i], xlrec->db_id);
        }
 }
 

diff --git a/src/backend/commands/dbcommands.c b/src/backend/commands/dbcommands.c
index 446813f0f0b2a0530c0bdc9b0d36585da9019ab1..a70e75a219e3a0c7d18a18845129a6a89886841b 100644 (file)
--- a/src/backend/commands/dbcommands.c
+++ b/src/backend/commands/dbcommands.c
@@ -1411,10 +1411,11 @@ movedb(const char *dbname, const char *tblspcname)
                xl_dbase_drop_rec xlrec;
 
                xlrec.db_id = db_id;
-               xlrec.tablespace_id = src_tblspcoid;
+               xlrec.ntablespaces = 1;
 
                XLogBeginInsert();
                XLogRegisterData((char *) &xlrec, sizeof(xl_dbase_drop_rec));
+               XLogRegisterData((char *) &src_tblspcoid, sizeof(Oid));
 
                (void) XLogInsert(RM_DBASE_ID,
                                                  XLOG_DBASE_DROP | XLR_SPECIAL_REL_UPDATE);
@@ -1946,6 +1947,11 @@ remove_dbtablespaces(Oid db_id)
        Relation        rel;
        TableScanDesc scan;
        HeapTuple       tuple;
+       List            *ltblspc = NIL;
+       ListCell        *cell;
+       int             ntblspc;
+       int             i;
+       Oid             *tablespace_ids;
 
        rel = table_open(TableSpaceRelationId, AccessShareLock);
        scan = table_beginscan_catalog(rel, 0, NULL);
@@ -1974,23 +1980,41 @@ remove_dbtablespaces(Oid db_id)
                                        (errmsg("some useless files may be left behind in old database directory \"%s\"",
                                                        dstpath)));
 
-               /* Record the filesystem change in XLOG */
-               {
-                       xl_dbase_drop_rec xlrec;
+               ltblspc = lappend_oid(ltblspc, dsttablespace);
+               pfree(dstpath);
+       }
 
-                       xlrec.db_id = db_id;
-                       xlrec.tablespace_id = dsttablespace;
+       ntblspc = list_length(ltblspc);
+       if (ntblspc == 0)
+       {
+               table_endscan(scan);
+               table_close(rel, AccessShareLock);
+               return;
+       }
 
-                       XLogBeginInsert();
-                       XLogRegisterData((char *) &xlrec, sizeof(xl_dbase_drop_rec));
+       tablespace_ids = (Oid *) palloc(ntblspc * sizeof(Oid));
+       i = 0;
+       foreach(cell, ltblspc)
+               tablespace_ids[i++] = lfirst_oid(cell);
 
-                       (void) XLogInsert(RM_DBASE_ID,
-                                                         XLOG_DBASE_DROP | XLR_SPECIAL_REL_UPDATE);
-               }
+       /* Record the filesystem change in XLOG */
+       {
+               xl_dbase_drop_rec xlrec;
 
-               pfree(dstpath);
+               xlrec.db_id = db_id;
+               xlrec.ntablespaces = ntblspc;
+
+               XLogBeginInsert();
+               XLogRegisterData((char *) &xlrec, MinSizeOfDbaseDropRec);
+               XLogRegisterData((char *) tablespace_ids, ntblspc * sizeof(Oid));
+
+               (void) XLogInsert(RM_DBASE_ID,
+                                                 XLOG_DBASE_DROP | XLR_SPECIAL_REL_UPDATE);
        }
 
+       list_free(ltblspc);
+       pfree(tablespace_ids);
+
        table_endscan(scan);
        table_close(rel, AccessShareLock);
 }
@@ -2197,8 +2221,7 @@ dbase_redo(XLogReaderState *record)
        {
                xl_dbase_drop_rec *xlrec = (xl_dbase_drop_rec *) XLogRecGetData(record);
                char       *dst_path;
-
-               dst_path = GetDatabasePath(xlrec->db_id, xlrec->tablespace_id);
+               int                     i;
 
                if (InHotStandby)
                {
@@ -2228,11 +2251,17 @@ dbase_redo(XLogReaderState *record)
                /* Clean out the xlog relcache too */
                XLogDropDatabase(xlrec->db_id);
 
-               /* And remove the physical files */
-               if (!rmtree(dst_path, true))
-                       ereport(WARNING,
-                                       (errmsg("some useless files may be left behind in old database directory \"%s\"",
-                                                       dst_path)));
+               for (i = 0; i < xlrec->ntablespaces; i++)
+               {
+                       dst_path = GetDatabasePath(xlrec->db_id, xlrec->tablespace_ids[i]);
+
+                       /* And remove the physical files */
+                       if (!rmtree(dst_path, true))
+                               ereport(WARNING,
+                                               (errmsg("some useless files may be left behind in old database directory \"%s\"",
+                                                               dst_path)));
+                       pfree(dst_path);
+               }
 
                if (InHotStandby)
                {

diff --git a/src/include/commands/dbcommands_xlog.h b/src/include/commands/dbcommands_xlog.h
index 46be8a615accc6a9fb05e157ee637b248188f7b0..4e8a442c89f6330527d73e8f95f9c4057ad6af27 100644 (file)
--- a/src/include/commands/dbcommands_xlog.h
+++ b/src/include/commands/dbcommands_xlog.h
@@ -32,10 +32,11 @@ typedef struct xl_dbase_create_rec
 
 typedef struct xl_dbase_drop_rec
 {
-       /* Records dropping of a single subdirectory incl. contents */
        Oid                     db_id;
-       Oid                     tablespace_id;
+       int                     ntablespaces;           /* number of tablespace IDs */
+       Oid                     tablespace_ids[FLEXIBLE_ARRAY_MEMBER];
 } xl_dbase_drop_rec;
+#define MinSizeOfDbaseDropRec offsetof(xl_dbase_drop_rec, tablespace_ids)
 
 extern void dbase_redo(XLogReaderState *rptr);
 extern void dbase_desc(StringInfo buf, XLogReaderState *rptr);