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From: Kyotaro Horiguchi <[email protected]>
To: [email protected]
Cc: [email protected]
Subject: Re: TRAP: FailedAssertion("HaveRegisteredOrActiveSnapshot()", File: "toast_internals.c", Line: 670, PID: 19403)
Date: Tue, 29 Mar 2022 17:06:21 +0900 (JST)
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>

At Mon, 28 Mar 2022 18:36:46 +0900 (JST), Kyotaro Horiguchi <[email protected]> wrote in 
> Inserting EnsurePortalSnapshotExists() to RunFromStore fixes this but
> I'm not sure where is the right place to do this..

Then, I found that portal->holdSnapshot is that.  I came up with the
attached.  It does the follows:

1. Teach PlannedStmtRequiresSnapshot() to return true for FetchStmt.

2. Use holdSnapshot in RunFromStore if any.


The rerpducer is reduced to as small as the following.

CREATE TABLE t (a text);
INSERT INTO t VALUES('some random text');
BEGIN;
DECLARE c CURSOR FOR SELECT * FROM t;
FETCH ALL IN c;

But I haven't come up with a reasonable way to generate the 'some
random text' yet.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center


Attachments:

  [text/x-patch] use_holdsnapshot_to_read_cursor.patch (1.4K, ../[email protected]/2-use_holdsnapshot_to_read_cursor.patch)
  download | inline diff:
diff --git a/src/backend/tcop/pquery.c b/src/backend/tcop/pquery.c
index 5aa5a350f3..8354029f2a 100644
--- a/src/backend/tcop/pquery.c
+++ b/src/backend/tcop/pquery.c
@@ -1068,6 +1068,18 @@ RunFromStore(Portal portal, ScanDirection direction, uint64 count,
 
 	dest->rStartup(dest, CMD_SELECT, portal->tupDesc);
 
+	/*
+	 * If holdSnapshot is set, that means we should use the snapshot to read
+	 * this store.
+	 */
+	if (portal->holdSnapshot)
+	{
+		Assert(portal->portalSnapshot == 0);
+		PushActiveSnapshotWithLevel(portal->holdSnapshot,
+									portal->createLevel);
+		portal->portalSnapshot = GetActiveSnapshot();
+	}
+	
 	if (ScanDirectionIsNoMovement(direction))
 	{
 		/* do nothing except start/stop the destination */
@@ -1114,6 +1126,13 @@ RunFromStore(Portal portal, ScanDirection direction, uint64 count,
 
 	dest->rShutdown(dest);
 
+	if (portal->holdSnapshot)
+	{
+		Assert(portal->portalSnapshot == GetActiveSnapshot());
+		PopActiveSnapshot();
+		portal->portalSnapshot = NULL;
+	}
+
 	ExecDropSingleTupleTableSlot(slot);
 
 	return current_tuple_count;
@@ -1756,7 +1775,6 @@ PlannedStmtRequiresSnapshot(PlannedStmt *pstmt)
 		IsA(utilityStmt, VariableShowStmt) ||
 		IsA(utilityStmt, ConstraintsSetStmt) ||
 	/* efficiency hacks from here down */
-		IsA(utilityStmt, FetchStmt) ||
 		IsA(utilityStmt, ListenStmt) ||
 		IsA(utilityStmt, NotifyStmt) ||
 		IsA(utilityStmt, UnlistenStmt) ||

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  Cc: [email protected], [email protected], [email protected]
  Subject: Re: TRAP: FailedAssertion("HaveRegisteredOrActiveSnapshot()", File: "toast_internals.c", Line: 670, PID: 19403)
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