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Don't force-assign transaction id when exporting a snapshot.

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Previously we required every exported transaction to have an xid
assigned. That was used to check that the exporting transaction is
still running, which in turn is needed to guarantee that that
necessary rows haven't been removed in between exporting and importing
the snapshot.

The exported xid caused unnecessary problems with logical decoding,
because slot creation has to wait for all concurrent xid to finish,
which in turn serializes concurrent slot creation.   It also
prohibited snapshots to be exported on hot-standby replicas.

Instead export the virtual transactionid, which avoids the unnecessary
serialization and the inability to export snapshots on standbys. This
changes the file name of the exported snapshot, but since we never
documented what that one means, that seems ok.

Author: Petr Jelinek, slightly editorialized by me
Reviewed-By: Andres Freund
Discussion: https://postgr.es/m/f598b4b8-8cd7-0d54-0939-adda763d8c34@2ndquadrant.com
This commit is contained in:
Andres Freund
2017-06-14 11:57:21 -07:00
parent b6966d4627
commit 6c2003f8a1
7 changed files with 115 additions and 67 deletions
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11
src/backend/storage/ipc/procarray.c
View File

@ -1793,14 +1793,15 @@ GetSnapshotData(Snapshot snapshot)
* Returns TRUE if successful, FALSE if source xact is no longer running.
*/
bool
ProcArrayInstallImportedXmin(TransactionId xmin, TransactionId sourcexid)
ProcArrayInstallImportedXmin(TransactionId xmin,
VirtualTransactionId *sourcevxid)
{
bool result = false;
ProcArrayStruct *arrayP = procArray;
int index;
Assert(TransactionIdIsNormal(xmin));
if (!TransactionIdIsNormal(sourcexid))
if (!sourcevxid)
return false;
/* Get lock so source xact can't end while we're doing this */
@ -1817,8 +1818,10 @@ ProcArrayInstallImportedXmin(TransactionId xmin, TransactionId sourcexid)
if (pgxact->vacuumFlags & PROC_IN_VACUUM)
continue;
xid = pgxact->xid; /* fetch just once */
if (xid != sourcexid)
/* We are only interested in the specific virtual transaction. */
if (proc->backendId != sourcevxid->backendId)
continue;
if (proc->lxid != sourcevxid->localTransactionId)
continue;
/*