diff options
Diffstat (limited to 'src/backend/access/transam/xlog.c')
| -rw-r--r-- | src/backend/access/transam/xlog.c | 32 |
1 files changed, 31 insertions, 1 deletions
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c index 3f393eb10de..144d609bdb7 100644 --- a/src/backend/access/transam/xlog.c +++ b/src/backend/access/transam/xlog.c @@ -7,7 +7,7 @@ * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group * Portions Copyright (c) 1994, Regents of the University of California * - * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.212 2005/07/29 03:25:53 momjian Exp $ + * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.213 2005/07/29 19:29:59 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -4914,6 +4914,36 @@ GetRedoRecPtr(void) } /* + * GetRecentNextXid - get the nextXid value saved by the most recent checkpoint + * + * This is currently used only by the autovacuum daemon. To check for + * impending XID wraparound, autovac needs an approximate idea of the current + * XID counter, and it needs it before choosing which DB to attach to, hence + * before it sets up a PGPROC, hence before it can take any LWLocks. But it + * has attached to shared memory, and so we can let it reach into the shared + * ControlFile structure and pull out the last checkpoint nextXID. + * + * Since we don't take any sort of lock, we have to assume that reading a + * TransactionId is atomic ... but that assumption is made elsewhere, too, + * and in any case the worst possible consequence of a bogus result is that + * autovac issues an unnecessary database-wide VACUUM. + * + * Note: we could also choose to read ShmemVariableCache->nextXid in an + * unlocked fashion, thus getting a more up-to-date result; but since that + * changes far more frequently than the controlfile checkpoint copy, it would + * pose a far higher risk of bogus result if we did have a nonatomic-read + * problem. + * + * A (theoretically) completely safe answer is to read the actual pg_control + * file into local process memory, but that certainly seems like overkill. + */ +TransactionId +GetRecentNextXid(void) +{ + return ControlFile->checkPointCopy.nextXid; +} + +/* * This must be called ONCE during postmaster or standalone-backend shutdown */ void |