Allow hash joins to be interrupted while searching hash table for match.

Per experimentation with a recent example, in which unreasonable amounts
of time could elapse before the backend would respond to a query-cancel.

This might be something to back-patch, but the patch doesn't apply cleanly
because this code was rewritten for 9.1.  Given the lack of field
complaints I won't bother for now.

Cédric Villemain

src/backend/executor/nodeHashjoin.c

@@ -19,6 +19,7 @@
 #include "executor/hashjoin.h"
 #include "executor/nodeHash.h"
 #include "executor/nodeHashjoin.h"
+#include "miscadmin.h"
 #include "utils/memutils.h"
 
 
@@ -262,6 +263,13 @@ ExecHashJoin(HashJoinState *node)
 
 			case HJ_SCAN_BUCKET:
 
+				/*
+				 * We check for interrupts here because this corresponds to
+				 * where we'd fetch a row from a child plan node in other
+				 * join types.
+				 */
+				CHECK_FOR_INTERRUPTS();
+
 				/*
 				 * Scan the selected hash bucket for matches to current outer
 				 */