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Commit524dbc1

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Avoid superfluous work for commits during logical slot creation.
Before955a684 logical decoding snapshot maintenance needed tocope with transactions it might not have seen in their entirety. Forsuch transactions we'd to assume they modified the catalog (could havehappened before we were watching), and thus a new snapshot had to bebuilt, and distributed to concurrently running transactions.That's problematic because building a new snapshot isn't that cheap ,especially as the the array of committed transactions needs to besorted. When creating a slot on a server with a lot of transactions,this could make logical slot creation infeasibly expensive.After955a684 there's no need to deal with transaction thataren't guaranteed to be fully observable. That allows to avoidbuilding snapshots for transactions that haven't modified catalog,even before reaching consistency.While this isn't necessarily a bugfix, slot creation being impossiblein some production workloads, is severe enough to warrantbackpatching.Author: Andres Freund, based on a quite different patch from Petr JelinekAnalyzed-By: Petr JelinekReviewed-By: Petr JelinekDiscussion:https://postgr.es/m/f37e975c-908f-858e-707f-058d3b1eb214@2ndquadrant.comBackpatch: 9.4-, where logical decoding has been introduced
1 parent955a684 commit524dbc1

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1 file changed

+68
-56
lines changed

1 file changed

+68
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lines changed

‎src/backend/replication/logical/snapbuild.c‎

Lines changed: 68 additions & 56 deletions
Original file line numberDiff line numberDiff line change
@@ -929,105 +929,122 @@ SnapBuildCommitTxn(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid,
929929
{
930930
intnxact;
931931

932-
boolforced_timetravel= false;
932+
boolneeds_snapshot= false;
933+
boolneeds_timetravel= false;
933934
boolsub_needs_timetravel= false;
934-
booltop_needs_timetravel= false;
935935

936936
TransactionIdxmax=xid;
937937

938938
/*
939-
* If we couldn't observe every change of a transaction because it was
940-
* already running at the point we started to observe we have to assume it
941-
* made catalog changes.
942-
*
943-
* This has the positive benefit that we afterwards have enough
944-
* information to build an exportable snapshot that's usable by pg_dump et
945-
* al.
939+
* Transactions preceding BUILDING_SNAPSHOT will neither be decoded, nor
940+
* will they be part of a snapshot. So we don't need to record anything.
946941
*/
942+
if (builder->state==SNAPBUILD_START||
943+
(builder->state==SNAPBUILD_BUILDING_SNAPSHOT&&
944+
TransactionIdPrecedes(xid,SnapBuildNextPhaseAt(builder))))
945+
{
946+
/* ensure that only commits after this are getting replayed */
947+
if (builder->start_decoding_at <=lsn)
948+
builder->start_decoding_at=lsn+1;
949+
return;
950+
}
951+
947952
if (builder->state<SNAPBUILD_CONSISTENT)
948953
{
949954
/* ensure that only commits after this are getting replayed */
950955
if (builder->start_decoding_at <=lsn)
951956
builder->start_decoding_at=lsn+1;
952957

953958
/*
954-
* We could avoid treating !SnapBuildTxnIsRunning transactions as
955-
* timetravel ones, but we want to be able to export a snapshot when
956-
* we reached consistency.
959+
* If building an exportable snapshot, force xid to be tracked, even
960+
* if the transaction didn't modify the catalog.
957961
*/
958-
forced_timetravel= true;
959-
elog(DEBUG1,"forced to assume catalog changes for xid %u because it was running too early",xid);
962+
if (builder->building_full_snapshot)
963+
{
964+
needs_timetravel= true;
965+
}
960966
}
961967

962968
for (nxact=0;nxact<nsubxacts;nxact++)
963969
{
964970
TransactionIdsubxid=subxacts[nxact];
965971

966972
/*
967-
*If we're forcing timetravel we also need visibility information
968-
*about subtransaction, so keep track of subtransaction's state.
973+
*Add subtransaction to base snapshot if catalog modifying, we don't
974+
*distinguish to toplevel transactions there.
969975
*/
970-
if (forced_timetravel)
976+
if (ReorderBufferXidHasCatalogChanges(builder->reorder,subxid))
971977
{
978+
sub_needs_timetravel= true;
979+
needs_snapshot= true;
980+
981+
elog(DEBUG1,"found subtransaction %u:%u with catalog changes",
982+
xid,subxid);
983+
972984
SnapBuildAddCommittedTxn(builder,subxid);
985+
973986
if (NormalTransactionIdFollows(subxid,xmax))
974987
xmax=subxid;
975988
}
976-
977989
/*
978-
* Add subtransaction to base snapshot if it DDL, we don't distinguish
979-
* to toplevel transactions there.
990+
* If we're forcing timetravel we also need visibility information
991+
* about subtransaction, so keep track of subtransaction's state, even
992+
* if not catalog modifying. Don't need to distribute a snapshot in
993+
* that case.
980994
*/
981-
elseif (ReorderBufferXidHasCatalogChanges(builder->reorder,subxid))
995+
elseif (needs_timetravel)
982996
{
983-
sub_needs_timetravel= true;
984-
985-
elog(DEBUG1,"found subtransaction %u:%u with catalog changes.",
986-
xid,subxid);
987-
988997
SnapBuildAddCommittedTxn(builder,subxid);
989-
990998
if (NormalTransactionIdFollows(subxid,xmax))
991999
xmax=subxid;
9921000
}
9931001
}
9941002

995-
if (forced_timetravel)
1003+
/* if top-level modified catalog, it'll need a snapshot */
1004+
if (ReorderBufferXidHasCatalogChanges(builder->reorder,xid))
9961005
{
997-
elog(DEBUG2,"forced transaction %u to do timetravel.",xid);
998-
1006+
elog(DEBUG2,"found top level transaction %u, with catalog changes",
1007+
xid);
1008+
needs_snapshot= true;
1009+
needs_timetravel= true;
9991010
SnapBuildAddCommittedTxn(builder,xid);
10001011
}
1001-
/* add toplevel transaction to base snapshot */
1002-
elseif (ReorderBufferXidHasCatalogChanges(builder->reorder,xid))
1012+
elseif (sub_needs_timetravel)
10031013
{
1004-
elog(DEBUG2,"found top level transaction %u, with catalog changes!",
1005-
xid);
1006-
1007-
top_needs_timetravel= true;
1014+
/* track toplevel txn as well, subxact alone isn't meaningful */
10081015
SnapBuildAddCommittedTxn(builder,xid);
10091016
}
1010-
elseif (sub_needs_timetravel)
1017+
elseif (needs_timetravel)
10111018
{
1012-
/* mark toplevel txn as timetravel as well */
1019+
elog(DEBUG2,"forced transaction %u to do timetravel",xid);
1020+
10131021
SnapBuildAddCommittedTxn(builder,xid);
10141022
}
10151023

1016-
/* if there's any reason to build a historic snapshot, do so now */
1017-
if (forced_timetravel||top_needs_timetravel||sub_needs_timetravel)
1024+
if (!needs_timetravel)
10181025
{
1019-
/*
1020-
* Adjust xmax of the snapshot builder, we only do that for committed,
1021-
* catalog modifying, transactions, everything else isn't interesting
1022-
* for us since we'll never look at the respective rows.
1023-
*/
1024-
if (!TransactionIdIsValid(builder->xmax)||
1025-
TransactionIdFollowsOrEquals(xmax,builder->xmax))
1026-
{
1027-
builder->xmax=xmax;
1028-
TransactionIdAdvance(builder->xmax);
1029-
}
1026+
/* record that we cannot export a general snapshot anymore */
1027+
builder->committed.includes_all_transactions= false;
1028+
}
1029+
1030+
Assert(!needs_snapshot||needs_timetravel);
10301031

1032+
/*
1033+
* Adjust xmax of the snapshot builder, we only do that for committed,
1034+
* catalog modifying, transactions, everything else isn't interesting
1035+
* for us since we'll never look at the respective rows.
1036+
*/
1037+
if (needs_timetravel&&
1038+
(!TransactionIdIsValid(builder->xmax)||
1039+
TransactionIdFollowsOrEquals(xmax,builder->xmax)))
1040+
{
1041+
builder->xmax=xmax;
1042+
TransactionIdAdvance(builder->xmax);
1043+
}
1044+
1045+
/* if there's any reason to build a historic snapshot, do so now */
1046+
if (needs_snapshot)
1047+
{
10311048
/*
10321049
* If we haven't built a complete snapshot yet there's no need to hand
10331050
* it out, it wouldn't (and couldn't) be used anyway.
@@ -1059,11 +1076,6 @@ SnapBuildCommitTxn(SnapBuild *builder, XLogRecPtr lsn, TransactionId xid,
10591076
/* add a new Snapshot to all currently running transactions */
10601077
SnapBuildDistributeNewCatalogSnapshot(builder,lsn);
10611078
}
1062-
else
1063-
{
1064-
/* record that we cannot export a general snapshot anymore */
1065-
builder->committed.includes_all_transactions= false;
1066-
}
10671079
}
10681080

10691081

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