FWIW a fix for this has been posted to all active branches:
Author: Andres Freund <andres@anarazel.de>
Branch: master [fd6a3f3ad] 2015-02-26 12:50:07 +0100
Branch: REL9_4_STABLE [d72115112] 2015-02-26 12:50:07 +0100
Branch: REL9_3_STABLE [abce8dc7d] 2015-02-26 12:50:07 +0100
Branch: REL9_2_STABLE [d67076529] 2015-02-26 12:50:07 +0100
Branch: REL9_1_STABLE [5c8dabecd] 2015-02-26 12:50:08 +0100
Branch: REL9_0_STABLE [82e0d6eb5] 2015-02-26 12:50:08 +0100
Reconsider when to wait for WAL flushes/syncrep during commit. Up to now RecordTransactionCommit() waited for
WALto be flushed (if synchronous_commit != off) and to be synchronously replicated (if enabled), even if a
transactiondid not have a xid assigned. The primary reason for that is that sequence's nextval() did not assign a
xid,but are worthwhile to wait for on commit. This can be problematic because sometimes read only transactions
do write WAL, e.g. HOT page prune records. That then could lead to read only transactions having to wait during
commit.Not something people expect in a read only transaction. This lead to such strange symptoms as backends
beingseemingly stuck during connection establishment when all synchronous replicas are down. Especially annoying
whensaid stuck connection is the standby trying to reconnect to allow syncrep again... This behavior also is
involvedin a rather complicated <= 9.4 bug where the transaction started by catchup interrupt processing waited for
syncrepusing latches, but didn't get the wakeup because it was already running inside the same overloaded signal
handler.Fix the issue here doesn't properly solve that issue, merely papers over the problems. In 9.5 catchup
interruptsaren't processed out of signal handlers anymore. To fix all this, make nextval() acquire a top level
xid,and only wait for transaction commit if a transaction both acquired a xid and emitted WAL records. If only a
xidhas been assigned we don't uselessly want to wait just because of writes to temporary/unlogged tables; if only WAL
has been written we don't want to wait just because of HOT prunes. The xid assignment in nextval() is unlikely to
causeoverhead in real-world workloads. For one it only happens SEQ_LOG_VALS/32 values anyway, for another only
usageof nextval() without using the result in an insert or similar is affected. Discussion:
20150223165359.GF30784@awork2.anarazel.de, 369698E947874884A77849D8FE3680C2@maumau,
5CF4ABBA67674088B3941894E22A0D25@maumau Per complaint from maumau and Thom Brown Backpatch all the way back;
9.0doesn't have syncrep, but it seems better to be consistent behavior across all maintained branches.
--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
