Clarify docs on hot standby lock release

Andres Freund and Simon Riggs

diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
index d38164caa7358cf0f73656e11484a7e8463d8a61..8c0d7b0ea957744edde57568e70d80cf34856a01 100644 (file)
--- a/src/backend/storage/ipc/procarray.c
+++ b/src/backend/storage/ipc/procarray.c
@@ -1564,6 +1564,11 @@ ProcArrayInstallImportedXmin(TransactionId xmin, TransactionId sourcexid)
  * We don't worry about updating other counters, we want to keep this as
  * simple as possible and leave GetSnapshotData() as the primary code for
  * that bookkeeping.
+ *
+ * Note that if any transaction has overflowed its cached subtransactions
+ * then there is no real need include any subtransactions. That isn't a
+ * common enough case to worry about optimising the size of the WAL record,
+ * and we may wish to see that data for diagnostic purposes anyway.
  */
 RunningTransactions
 GetRunningTransactionData(void)

diff --git a/src/backend/storage/ipc/standby.c b/src/backend/storage/ipc/standby.c
index 43f74112733772089a33e39b623572b916fe2d35..905d33148a96d3f023f027c398338b0b428bd3cd 100644 (file)
--- a/src/backend/storage/ipc/standby.c
+++ b/src/backend/storage/ipc/standby.c
@@ -540,6 +540,10 @@ StandbyTimeoutHandler(void)
  * RelationLockList, so we can keep track of the various entries made by
  * the Startup process's virtual xid in the shared lock table.
  *
+ * We record the lock against the top-level xid, rather than individual
+ * subtransaction xids. This means AccessExclusiveLocks held by aborted
+ * subtransactions are not released as early as possible on standbys.
+ *
  * List elements use type xl_rel_lock, since the WAL record type exactly
  * matches the information that we need to keep track of.
  *
@@ -673,8 +677,8 @@ StandbyReleaseAllLocks(void)
 
 /*
  * StandbyReleaseOldLocks
- *             Release standby locks held by XIDs that aren't running, as long
- *             as they're not prepared transactions.
+ *             Release standby locks held by top-level XIDs that aren't running,
+ *             as long as they're not prepared transactions.
  */
 void
 StandbyReleaseOldLocks(int nxids, TransactionId *xids)