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Commit276bea2

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Ensure that seqscans check for interrupts at least once per page.
If a seqscan encounters many consecutive pages containing only dead tuples,it can remain in the loop in heapgettup for a long time, and there was noCHECK_FOR_INTERRUPTS anywhere in that loop. This meant there werereal-world situations where a query would be effectively uncancelable forlong stretches. Add a check placed to occur once per page, which should beenough to provide reasonable response time without adding any measurableoverhead.Report and patch by Merlin Moncure (though I tweaked it a bit).Back-patch to all supported branches.
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‎src/backend/access/heap/heapam.c

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@@ -219,6 +219,13 @@ heapgetpage(HeapScanDesc scan, BlockNumber page)
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scan->rs_cbuf=InvalidBuffer;
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}
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/*
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* Be sure to check for interrupts at least once per page. Checks at
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* higher code levels won't be able to stop a seqscan that encounters
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* many pages' worth of consecutive dead tuples.
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*/
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CHECK_FOR_INTERRUPTS();
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/* read page using selected strategy */
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scan->rs_cbuf=ReadBufferExtended(scan->rs_rd,MAIN_FORKNUM,page,
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RBM_NORMAL,scan->rs_strategy);

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