RE: Disable WAL logging to speed up data loading

From: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
> XLogBeginInsert();
> XLogSetRecrodFlags(XLOG_MARK_ESSENTIAL);  # new flag value
> XLOGInsert(....);

Oh, sounds like a nice idea.  That's more flexible by allowing WAL-emitting modules to specify which WAL records are
mandatoryeven when wal_level is none. 

For example, gistXLogAssignLSN() adds the above flag like this:

    XLogBeginInsert();
    XLogSetRecordFlags(XLOG_MARK_UNIMPORTANT | XLOG_MARK_ESSENTIAL);
    XLogRegisterData((char *) &dummy, sizeof(dummy));

(Here's a word play - unimportant but essential, what's that?)

And the filter in XLogInsert() becomes:

+    if (wal_level == WAL_LEVEL_NONE &&
+        !((rmid == RM_XLOG_ID && info == XLOG_CHECKPOINT_SHUTDOWN) ||
+          (rmid == RM_XLOG_ID && info == XLOG_PARAMETER_CHANGE) ||
+          (rmid == RM_XACT_ID && info == XLOG_XACT_PREPARE) ||
+          (curinsert_flags & XLOG_MARK_ESSENTIAL)))

Or,

+    if (wal_level == WAL_LEVEL_NONE &&
+         !(curinsert_flags & XLOG_MARK_ESSENTIAL))

and add the new flag when emitting XLOG_CHECKPOINT_ONLINE, XLOG_PARAMETER_CHANGE and XLOG_PREPARE records.  I think
bothhave good reasons: the former centralizes the handling of XACT and XLOG RM WAL records (as the current XLOG module
doesalready), and the latter delegates the decision to each module.  Which would you prefer?  (I kind of like the
former,but this is a weak opinion.) 


Regards
Takayuki Tsunakawa