Only WAL-log the modified portion in an UPDATE, if possible.

When a row is updated, and the new tuple version is put on the same page as
the old one, only WAL-log the part of the new tuple that's not identical to
the old. This saves significantly on the amount of WAL that needs to be
written, in the common case that most fields are not modified.

Amit Kapila, with a lot of back and forth with me, Robert Haas, and others.

src/backend/access/transam/xlog.c

@@ -2335,6 +2335,29 @@ XLogRecPtrToBytePos(XLogRecPtr ptr)
 	return result;
 }
 
+/*
+ * Determine whether the buffer referenced has to be backed up.
+ *
+ * Since we don't yet have the insert lock, fullPageWrites and forcePageWrites
+ * could change later, so the result should be used for optimization purposes
+ * only.
+ */
+bool
+XLogCheckBufferNeedsBackup(Buffer buffer)
+{
+	bool		doPageWrites;
+	Page		page;
+
+	page = BufferGetPage(buffer);
+
+	doPageWrites = XLogCtl->Insert.fullPageWrites || XLogCtl->Insert.forcePageWrites;
+
+	if (doPageWrites && PageGetLSN(page) <= RedoRecPtr)
+		return true;			/* buffer requires backup */
+
+	return false;				/* buffer does not need to be backed up */
+}
+
 /*
  * Determine whether the buffer referenced by an XLogRecData item has to
  * be backed up, and if so fill a BkpBlock struct for it.  In any case