Re: [HACKERS] Parallel Index Scans

>+ /* Check if the scan for current scan keys is finished */

>+ if (so->arrayKeyCount < btscan->btps_arrayKeyCount)

>+ *status = false;

>I didn't clearly understand, in which scenario the arrayKeyCount is less

>than btps_arrayKeyCount?

Consider following array scan keys

select * from test2 where j=ANY(ARRAY[1000,2000,3000]);

By the time the current worker has finished reading heap tuples corresponding

to array key 1000(arrayKeyCount = 0), some other worker might have advanced the scan to the

array key 2000(btps_arrayKeyCount =1). In this case when the current worker fetches next page to scan,

it must advance its scan keys before scanning the next page of parallel scan.
I hope this helps.

>+BlockNumber

>+_bt_parallel_seize(IndexScanDesc scan, bool *status)

>The return value of the above function is validated only in _bt_first

>function, but in other cases it is not.
In other cases it is validated in _bt_readnextpage() which is called after

_bt_parallel_seize().

>From the function description,

>it is possible to return P_NONE for the workers also with status as

>true. I feel it is better to handle the P_NONE case internally only

>so that callers just check for the status. Am i missing anything?

In case of the next block being P_NONE and status true, the code

calls _bt_parallel_done() to notify other workers followed by

BTScanPosInvalidate(). Similar check for block = P_NONE also

happens in existing code. See following in _bt_readnextpage(),


            if (blkno == P_NONE || !so->currPos.moreRight)
            {
               _bt_parallel_done(scan);
                BTScanPosInvalidate(so->currPos);
                return false;
            }

So to keep it consistent with the existing code, the check

is kept outside _bt_parallel_seize().