Re: Parallel bitmap heap scan

Reviewing 0003:

How about adding a regression test?

bitmap_subplan_mark_shared could use castNode(), which seems like it
would be better style.  Maybe some other places, too.

+         <entry><literal>ParallelBitmapPopulate</></entry>
+         <entry>Waiting for the leader to populate the TidBitmap.</entry>
+        </row>
+        <row>

If you build the documentation, you'll find that this doesn't come out
right; you need to add 1 to the value of the nearest preceding
"morerows".  (I fixed a similar issue with 0001 while committing.)

+        /*---------------
+         * Check the current state
+         * If state is
+         * BM_INITIAL     : We become the leader and set it to BM_INPROGRESS
+         * BM_INPROGRESS : We need to wait till leader creates bitmap
+         * BM_FINISHED   : bitmap is ready so no need to wait
+         *---------------

The formatting of this comment is slightly off - the comment for
BM_INITIAL isn't aligned the same as the others.  But I would just
delete the whole comment, since more or less it recapitulates the
function header comment anyway.

I wonder if BitmapShouldInitializeSharedState couldn't be written a
little more compactly overall, like this:

{
    SharedBitmapState   state;

    while (1)
    {
        SpinLockAcquire(&pstate->mutex);
        state = pstate->state;
        if (pstate->state == BM_INITIAL)
            pstate->state = BM_INPROGRESS;
        SpinLockRelease(&pstate->mutex);

        /* If we are leader or leader has already created a TIDBITMAP */
        if (state != BM_INPROGRESS)
            break;

        /* Sleep until leader finishes creating the bitmap */
        ConditionVariableSleep(&pstate->cv, WAIT_EVENT_PARALLEL_BITMAP_SCAN);
    }

    ConditionVariableCancelSleep();

    return (state == BM_INITIAL);
}

+                /*
+                 * By this time we have already populated the TBM and
+                 * initialized the shared iterators so set the state to
+                 * BM_FINISHED and wake up others.
+                 */
+                SpinLockAcquire(&pstate->mutex);
+                pstate->state = BM_FINISHED;
+                SpinLockRelease(&pstate->mutex);
+                ConditionVariableBroadcast(&pstate->cv);

I think it would be good to have a function for this, like
BitmapDoneInitializingSharedState(), and just call that function here.

+            SpinLockAcquire(&pstate->mutex);
+
+            /*
+             * Recheck under the mutex, If some other process has already done
+             * the enough prefetch then we need not to do anything.
+             */
+            if (pstate->prefetch_pages >= pstate->prefetch_target)
+                SpinLockRelease(&pstate->mutex);
+                return;
+            SpinLockRelease(&pstate->mutex);

I think it would be clearer to write this as:

SpinLockAcquire(&pstate->mutex);
do_prefetch = (pstate->prefetch_pages >= pstate->prefetch_target);
SpinLockRelease(&pstate->mutex);
if (!do_prefetch)
   return;

Then it's more obvious what's going on with the spinlock.  But
actually what I would do is also roll in the increment to prefetch
pages in there, so that you don't have to reacquire the spinlock after
calling PrefetchBuffer:

bool do_prefetch = false;
SpinLockAcquire(&pstate->mutex);
if (pstate->prefetch_pages < pstate->prefetch_target)
{
    pstate->prefetch_pages++;
    do_prefetch = true;
}
SpinLockRelease(&pstate->mutex);

That seems like it will reduce the amount of excess prefetching
considerably, and also simplify the code and cut the spinlock
acquisitions by 50%.

Overall I think this is in pretty good shape.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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