PGPRO-2432: Improve reading of compressed WAL's if Postgrestries to read same record many times

Author: za-arthur

src/parsexlog.c

@@ -104,6 +104,9 @@ typedef struct XLogPageReadPrivate
 #ifdef HAVE_LIBZ
 	gzFile		gz_xlogfile;
 	char		gz_xlogpath[MAXPGPATH];
+
+	char		gz_buf[XLOG_BLCKSZ];
+	uint32		gz_prev_off;
 #endif
 } XLogPageReadPrivate;
 
@@ -1057,22 +1060,30 @@ SimpleXLogPageRead(XLogReaderState *xlogreader, XLogRecPtr targetPagePtr,
 #ifdef HAVE_LIBZ
 	else
 	{
-		if (gzseek(private_data->gz_xlogfile, (z_off_t) targetPageOff, SEEK_SET) == -1)
+		if (private_data->gz_prev_off != 0 &&
+			private_data->gz_prev_off == targetPageOff)
+			memcpy(readBuf, private_data->gz_buf, XLOG_BLCKSZ);
+		else
 		{
-			elog(WARNING, "Thread [%d]: Could not seek in compressed WAL segment \"%s\": %s",
-				 private_data->thread_num,
-				 private_data->gz_xlogpath,
-				 get_gz_error(private_data->gz_xlogfile));
-			return -1;
-		}
+			if (gzseek(private_data->gz_xlogfile, (z_off_t) targetPageOff, SEEK_SET) == -1)
+			{
+				elog(WARNING, "Thread [%d]: Could not seek in compressed WAL segment \"%s\": %s",
+					private_data->thread_num,
+					private_data->gz_xlogpath,
+					get_gz_error(private_data->gz_xlogfile));
+				return -1;
+			}
 
-		if (gzread(private_data->gz_xlogfile, readBuf, XLOG_BLCKSZ) != XLOG_BLCKSZ)
-		{
-			elog(WARNING, "Thread [%d]: Could not read from compressed WAL segment \"%s\": %s",
-				 private_data->thread_num,
-				 private_data->gz_xlogpath,
-				 get_gz_error(private_data->gz_xlogfile));
-			return -1;
+			if (gzread(private_data->gz_xlogfile, readBuf, XLOG_BLCKSZ) != XLOG_BLCKSZ)
+			{
+				elog(WARNING, "Thread [%d]: Could not read from compressed WAL segment \"%s\": %s",
+					private_data->thread_num,
+					private_data->gz_xlogpath,
+					get_gz_error(private_data->gz_xlogfile));
+				return -1;
+			}
+			private_data->gz_prev_off = targetPageOff;
+			memcpy(private_data->gz_buf, readBuf, XLOG_BLCKSZ);
 		}
 	}
 #endif
@@ -1131,6 +1142,7 @@ CleanupXLogPageRead(XLogReaderState *xlogreader)
 	{
 		gzclose(private_data->gz_xlogfile);
 		private_data->gz_xlogfile = NULL;
+		private_data->gz_prev_off = 0;
 	}
 #endif
 	private_data->xlogexists = false;