Re: Gather Merge - Mailing list pgsql-hackers

On Wed, Oct 5, 2016 at 11:35 AM, Rushabh Lathia
<rushabh.lathia@gmail.com> wrote:
> Hi hackers,
>
> Attached is the patch to implement Gather Merge.
>

Couple of review comments:

1.
ExecGatherMerge()
{
..
+ /* No workers? Then never mind. */
+ if (!got_any_worker
||
+ node->nreaders < 2)
+ {
+
ExecShutdownGatherMergeWorkers(node);
+ node->nreaders = 0;
+
}

Are you planning to restrict the use of gather merge based on number
of workers, if there is a valid reason, then I think comments should
be updated for same?

2.
+ExecGatherMerge(GatherMergeState * node){
..
+ if (!node->initialized)
+ {
+ EState   *estate = node->ps.state;
+
GatherMerge *gm = (GatherMerge *) node->ps.plan;
+
+ /*
+ * Sometimes we
might have to run without parallelism; but if parallel
+ * mode is active then we can try to
fire up some workers.
+ */
+ if (gm->num_workers > 0 && IsInParallelMode())
+
{
+ ParallelContext *pcxt;
+ bool got_any_worker =
false;
+
+ /* Initialize the workers required to execute Gather node. */
+
if (!node->pei)
+ node->pei = ExecInitParallelPlan(node-
>ps.lefttree,
+estate,
+gm->num_workers);
..
}

There is lot of common code between ExecGatherMerge and ExecGather.
Do you think it makes sense to have a common function to avoid the
duplicity?

I see there are small discrepancies in both the codes like I don't see
the use of single_copy flag, as it is present in gather node.

3.
+gather_merge_readnext(GatherMergeState * gm_state, int reader, bool force)
{
..
+ tup = gm_readnext_tuple(gm_state, reader, force, NULL);
+
+ /*
+* try to read more tuple into nowait mode and store it into the tuple
+ * array.
+*/
+ if (HeapTupleIsValid(tup))
+ fill_tuple_array(gm_state, reader);

How the above read tuple is stored in array?  In anycase the above
interface seems slightly awkward to me. Basically, I think what you
are trying to do here is after reading first tuple in waitmode, you
are trying to fill the array by reading more tuples.  So, can't we
push reading of this fist tuple into that function and name it as
form_tuple_array().

4.
+create_gather_merge_path(..)
{
..
+  0 /* FIXME: pathnode->limit_tuples*/);

What exactly you want to fix in above code?

5.+/* Tuple array size */
+#define MAX_TUPLE_STORE 10

Have you tried with other values of MAX_TUPLE_STORE?  If yes, then
what are the results?  I think it is better to add a comment why array
size is best for performance.


6.
+/* INTERFACE ROUTINES
+ * ExecInitGatherMerge - initialize the MergeAppend
node
+ * ExecGatherMerge - retrieve the next tuple from the node
+ *
ExecEndGatherMerge - shut down the MergeAppend node
+ *
ExecReScanGatherMerge - rescan the MergeAppend node

typo.  /MergeAppend/GatherMerge

7.
+static TupleTableSlot *gather_merge_getnext(GatherMergeState * gm_state);
+static HeapTuple
gm_readnext_tuple(GatherMergeState * gm_state, int nreader, bool
force, bool *done);

Formatting near GatherMergeState doesn't seem to be appropriate.  I
think you need to add GatherMergeState in typedefs.list and then run
pgindent again.

8.
+ /*
+ * Initialize funnel slot to same tuple descriptor as outer plan.
+ */
+ if
(!ExecContextForcesOids(&gm_state->ps, &hasoid))

I think in above comment, you mean Initialize GatherMerge slot.

9.
+ /* Does tuple array have any avaiable tuples? */
/avaiable/available

>
> Open Issue:
>
> - Commit af33039317ddc4a0e38a02e2255c2bf453115fd2 fixed the leak into
> tqueue.c by
> calling gather_readnext() into per-tuple context. Now for gather merge that
> is
> not possible, as we storing the tuple into Tuple array and we want tuple to
> be
> free only its get pass through the merge sort algorithm. One idea is, we can
> also call gm_readnext_tuple() under per-tuple context (which will fix the
> leak
> into tqueue.c) and then store the copy of tuple into tuple array.
>

Won't extra copy per tuple impact performance?  Is the fix in
mentioned commit was for record or composite types, if so, does
GatherMerge support such types and if it support, does it provide any
benefit over Gather?


-- 
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com