Hi Alexander, On Sat, Jan 10, 2026 at 12:47 PM Xuneng Zhou <[email protected]> wrote: > > On Fri, Jan 9, 2026 at 9:44 PM Xuneng Zhou <[email protected]> wrote: > > > > Hi, > > > > On Fri, Jan 9, 2026 at 4:42 AM Alexander Korotkov <[email protected]> > > wrote: > > > > > > On Thu, Jan 8, 2026 at 6:29 PM Xuneng Zhou <[email protected]> wrote: > > > > On Thu, Jan 8, 2026 at 10:19 PM Alexander Korotkov > > > > <[email protected]> wrote: > > > > > I see, you were right. This is not related to the MyProc->xmin. > > > > > ResolveRecoveryConflictWithTablespace() calls > > > > > GetConflictingVirtualXIDs(InvalidTransactionId, InvalidOid). That > > > > > would kill WAIT FOR LSN query independently on its xmin. > > > > > > > > I think the concern is valid --- conflicts like > > > > PROCSIG_RECOVERY_CONFLICT_SNAPSHOT could occur and terminate the > > > > backend if the timing is unlucky. It's more difficult to reproduce > > > > though. A check for the log containing "conflict with recovery" would > > > > likely catch these conflicts as well. > > > > > > Yes, I found multiple reasons why xmin gets temporarily set during > > > processing of WAIT FOR LSN query. I'll soon post a draft patch to fix > > > that. > > > > > > > > I guess your > > > > > patch is the only way to go. It's clumsy to wrap WAIT FOR LSN call > > > > > with retry loop, but it would still consume less resources than > > > > > polling. > > > > > > > > > > > > > Assuming recovery conflicts are relatively rare in tap tests, except > > > > for the explicitly designed tests like 031_recovery_conflict and the > > > > narrow timing window that the standby has not caught up while the wait > > > > for gets invoked, a simple fallback seems appropriate to me. > > > > > > Yes, I see. Seems acceptable given this seems the only feasible way to > > > go. > > > > > > > Here is the updated patch with recovery conflicts handled. > > V2 corrected the commit message to state " if the WAIT FOR LSN session > is interrupted by a recovery conflict (e.g., DROP TABLESPACE > triggering conflicts on all backends),". In this case, the statement > is canceled when possible; in some states (idle in transaction or > subtransaction) the session may be terminated. >
The attached patch avoids a syscache lookup while constructing the tuple descriptor for WAIT FOR LSN, so that a catalog snapshot is not re-established after the wait finishes. The standard output path (printtup) may still briefly establish a catalog snapshot during result emission, but this seems acceptable: the snapshot window is narrow to emit a single row. A fully catalog-free output path would require either bypassing the DestReceiver lifecycle (breaking layering) or adding a custom receiver (added complexity for marginal benefit). The current approach is simpler and might be sufficient unless output-phase conflicts are observed a lot in practice. Does this make sense to you? -- Best, Xuneng
v1-0001-Avoid-syscache-lookup-in-WAIT-FOR-LSN-tuple-descr.patch
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