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Commit1c7675a

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Fix wrong data table horizon computation during backend startup.
When ComputeXidHorizons() was called before MyDatabaseOid is set,e.g. because a dead row in a shared relation is encountered duringInitPostgres(), the horizon for normal tables was computed tooaggressively, ignoring all backends connected to a database.During subsequent pruning in a data table the too aggressive horizoncould end up still being used, possibly leading to still needed tuplesbeing removed. Not good.This is a bug indc7420c, which the test added in94bc27b madevisible, if run with force_parallel_mode set to regress. In that casethe bug is reliably triggered, because "pruning_query" is run in aparallel worker and the start of that parallel worker is likely toencounter a dead row in pg_database.The fix is trivial: Compute a more pessimistic data table horizon ifMyDatabaseId is not yet known.Author: Andres FreundDiscussion:https://postgr.es/m/20201029040030.p4osrmaywhqaesd4@alap3.anarazel.de
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‎src/backend/storage/ipc/procarray.c

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@@ -1757,9 +1757,16 @@ ComputeXidHorizons(ComputeXidHorizonsResult *h)
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* the shared horizon. But in recovery we cannot compute an accurate
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* per-database horizon as all xids are managed via the
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* KnownAssignedXids machinery.
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*
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* Be careful to compute a pessimistic value when MyDatabaseId is not
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* set. If this is a backend in the process of starting up, we may not
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* use a "too aggressive" horizon (otherwise we could end up using it
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* to prune still needed data away). If the current backend never
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* connects to a database that is harmless, because
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* data_oldest_nonremovable will never be utilized.
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*/
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if (in_recovery||
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proc->databaseId==MyDatabaseId||
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MyDatabaseId==InvalidOid||proc->databaseId==MyDatabaseId||
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proc->databaseId==0)/* always include WalSender */
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{
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h->data_oldest_nonremovable=

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