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

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Merlin Moncure <mmoncure(at)gmail(dot)com>
Cc: pgsql-bugs <pgsql-bugs(at)postgresql(dot)org>
Subject: Re: sequential scans that pick up only deleted records do not honor query cancel or timeout
Date: 2012-05-22 23:13:12
Message-ID: 12013.1337728392@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-bugs
Merlin Moncure <mmoncure(at)gmail(dot)com> writes:
> On Tue, May 22, 2012 at 4:08 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)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

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