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Ignore CatalogSnapshot when checking COPY FREEZE prerequisites.

Browse Source 
This restores the ability, essentially lost in commit
ffaa44cb559db332baeee7d25dedd74a61974203, to use COPY FREEZE under
REPEATABLE READ isolation.  Back-patch to 9.4, like that commit.

Reviewed by Tom Lane.

Discussion: https://postgr.es/m/CA+TgmoahWDm-7fperBxzU9uZ99LPMUmEpSXLTw9TmrOgzwnORw@mail.gmail.com
This commit is contained in:
Noah Misch 2017-11-05 09:25:52 -08:00
parent 86bc521811
commit bab3a714b6
2 changed files with 23 additions and 3 deletions
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18
src/backend/commands/copy.c
View File

@ -2394,13 +2394,25 @@ CopyFrom(CopyState cstate)
/*
* Optimize if new relfilenode was created in this subxact or one of its
* committed children and we won't see those rows later as part of an
* earlier scan or command. This ensures that if this subtransaction
* aborts then the frozen rows won't be visible after xact cleanup. Note
* earlier scan or command. The subxact test ensures that if this subxact
* aborts then the frozen rows won't be visible after xact cleanup. Note
* that the stronger test of exactly which subtransaction created it is
* crucial for correctness of this optimization.
* crucial for correctness of this optimization. The test for an earlier
* scan or command tolerates false negatives. FREEZE causes other sessions
* to see rows they would not see under MVCC, and a false negative merely
* spreads that anomaly to the current session.
*/
if (cstate->freeze)
{
/*
* Tolerate one registration for the benefit of FirstXactSnapshot.
* Scan-bearing queries generally create at least two registrations,
* though relying on that is fragile, as is ignoring ActiveSnapshot.
* Clear CatalogSnapshot to avoid counting its registration. We'll
* still detect ongoing catalog scans, each of which separately
* registers the snapshot it uses.
*/
InvalidateCatalogSnapshot();
if (!ThereAreNoPriorRegisteredSnapshots() || !ThereAreNoReadyPortals())
ereport(ERROR,
(errcode(ERRCODE_INVALID_TRANSACTION_STATE),

8
src/backend/utils/time/snapmgr.c
View File

@ -1645,6 +1645,14 @@ DeleteAllExportedSnapshotFiles(void)
FreeDir(s_dir);
}
/*
* ThereAreNoPriorRegisteredSnapshots
* Is the registered snapshot count less than or equal to one?
*
* Don't use this to settle important decisions. While zero registrations and
* no ActiveSnapshot would confirm a certain idleness, the system makes no
* guarantees about the significance of one registered snapshot.
*/
bool
ThereAreNoPriorRegisteredSnapshots(void)
{