Separate reinitialization of shared parallel-scan state from ExecReScan.

Previously, the parallel executor logic did reinitialization of shared
state within the ExecReScan code for parallel-aware scan nodes.  This is
problematic, because it means that the ExecReScan call has to occur
synchronously (ie, during the parent Gather node's ReScan call).  That is
swimming very much against the tide so far as the ExecReScan machinery is
concerned; the fact that it works at all today depends on a lot of fragile
assumptions, such as that no plan node between Gather and a parallel-aware
scan node is parameterized.  Another objection is that because ExecReScan
might be called in workers as well as the leader, hacky extra tests are
needed in some places to prevent unwanted shared-state resets.

Hence, let's separate this code into two functions, a ReInitializeDSM
call and the ReScan call proper.  ReInitializeDSM is called only in
the leader and is guaranteed to run before we start new workers.
ReScan is returned to its traditional function of resetting only local
state, which means that ExecReScan's usual habits of delaying or
eliminating child rescan calls are safe again.

As with the preceding commit 7df2c1f8d, it doesn't seem to be necessary
to make these changes in 9.6, which is a good thing because the FDW and
CustomScan APIs are impacted.

Discussion: https://postgr.es/m/CAA4eK1JkByysFJNh9M349u_nNjqETuEnY_y1VUc_kJiU0bxtaQ@mail.gmail.com

1 files changed, 15 insertions, 23 deletions

diff --git a/src/backend/executor/nodeIndexscan.c b/src/backend/executor/nodeIndexscan.c
index 404076d5930..638b17b07cb 100644
--- a/src/backend/executor/nodeIndexscan.c
+++ b/src/backend/executor/nodeIndexscan.c
@@ -24,6 +24,7 @@
  *		ExecIndexRestrPos		restores scan position.
  *		ExecIndexScanEstimate	estimates DSM space needed for parallel index scan
  *		ExecIndexScanInitializeDSM initialize DSM for parallel indexscan
+ *		ExecIndexScanReInitializeDSM reinitialize DSM for fresh scan
  *		ExecIndexScanInitializeWorker attach to DSM info in parallel worker
  */
 #include "postgres.h"
@@ -577,18 +578,6 @@ ExecIndexScan(PlanState *pstate)
 void
 ExecReScanIndexScan(IndexScanState *node)
 {
-	bool		reset_parallel_scan = true;
-
-	/*
-	 * If we are here to just update the scan keys, then don't reset parallel
-	 * scan.  We don't want each of the participating process in the parallel
-	 * scan to update the shared parallel scan state at the start of the scan.
-	 * It is quite possible that one of the participants has already begun
-	 * scanning the index when another has yet to start it.
-	 */
-	if (node->iss_NumRuntimeKeys != 0 && !node->iss_RuntimeKeysReady)
-		reset_parallel_scan = false;
-
 	/*
 	 * If we are doing runtime key calculations (ie, any of the index key
 	 * values weren't simple Consts), compute the new key values.  But first,
@@ -614,21 +603,11 @@ ExecReScanIndexScan(IndexScanState *node)
 			reorderqueue_pop(node);
 	}
 
-	/*
-	 * Reset (parallel) index scan.  For parallel-aware nodes, the scan
-	 * descriptor is initialized during actual execution of node and we can
-	 * reach here before that (ex. during execution of nest loop join).  So,
-	 * avoid updating the scan descriptor at that time.
-	 */
+	/* reset index scan */
 	if (node->iss_ScanDesc)
-	{
 		index_rescan(node->iss_ScanDesc,
 					 node->iss_ScanKeys, node->iss_NumScanKeys,
 					 node->iss_OrderByKeys, node->iss_NumOrderByKeys);
-
-		if (reset_parallel_scan && node->iss_ScanDesc->parallel_scan)
-			index_parallelrescan(node->iss_ScanDesc);
-	}
 	node->iss_ReachedEnd = false;
 
 	ExecScanReScan(&node->ss);
@@ -1717,6 +1696,19 @@ ExecIndexScanInitializeDSM(IndexScanState *node,
 }
 
 /* ----------------------------------------------------------------
+ *		ExecIndexScanReInitializeDSM
+ *
+ *		Reset shared state before beginning a fresh scan.
+ * ----------------------------------------------------------------
+ */
+void
+ExecIndexScanReInitializeDSM(IndexScanState *node,
+							 ParallelContext *pcxt)
+{
+	index_parallelrescan(node->iss_ScanDesc);
+}
+
+/* ----------------------------------------------------------------
  *		ExecIndexScanInitializeWorker
  *
  *		Copy relevant information from TOC into planstate.