Re: sequential scans that pick up only deleted records do not honor query cancel or timeout

Merlin Moncure <mmoncure@gmail.com> writes:
> On Tue, May 22, 2012 at 4:08 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> heapgetpage() seems like the most reasonable place to me, as there we'll
>> only be making the check once per page not once per tuple.

> ok. this fixes the issue:

Well, actually it needs to be a bit earlier than that or it won't stop
non-pageatatime scans (cf the return at line 230 in HEAD).  I was
thinking to place it at the point where we hold no buffer pin, just to
save a couple cycles in error cleanup:


diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 0d6fe3f0acd38a87af759bd3d5196da504202486..0c67156390a6a1bf1fee0bb39b96e0da8cd55dbb 100644
*** a/src/backend/access/heap/heapam.c
--- b/src/backend/access/heap/heapam.c
*************** heapgetpage(HeapScanDesc scan, BlockNumb
*** 222,227 ****
--- 222,234 ----
          scan->rs_cbuf = InvalidBuffer;
      }

+     /*
+      * Be sure to check for interrupts at least once per page.  Checks at
+      * higher code levels won't be able to stop a seqscan that encounters
+      * many pages' worth of consecutive dead tuples.
+      */
+     CHECK_FOR_INTERRUPTS();
+
      /* read page using selected strategy */
      scan->rs_cbuf = ReadBufferExtended(scan->rs_rd, MAIN_FORKNUM, page,
                                         RBM_NORMAL, scan->rs_strategy);


But thanks for verifying that a check in this function does fix the
issue for you.

            regards, tom lane