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Diffstat (limited to 'src/backend/storage/ipc/procarray.c')
| -rw-r--r-- | src/backend/storage/ipc/procarray.c | 12 |
1 files changed, 12 insertions, 0 deletions
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c index 36701e61891..f4b8ad700a1 100644 --- a/src/backend/storage/ipc/procarray.c +++ b/src/backend/storage/ipc/procarray.c @@ -578,6 +578,18 @@ TransactionIdIsActive(TransactionId xid) * This is also used to determine where to truncate pg_subtrans. allDbs * must be TRUE for that case, and ignoreVacuum FALSE. * + * Note: it's possible for the calculated value to move backwards on repeated + * calls. The calculated value is conservative, so that anything older is + * definitely not considered as running by anyone anymore, but the exact + * value calculated depends on a number of things. For example, if allDbs is + * TRUE and there are no transactions running in the current database, + * GetOldestXmin() returns latestCompletedXid. If a transaction begins after + * that, its xmin will include in-progress transactions in other databases + * that started earlier, so another call will return an lower value. The + * return value is also adjusted with vacuum_defer_cleanup_age, so increasing + * that setting on the fly is an easy way to have GetOldestXmin() move + * backwards. + * * Note: we include all currently running xids in the set of considered xids. * This ensures that if a just-started xact has not yet set its snapshot, * when it does set the snapshot it cannot set xmin less than what we compute. |