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Improve comments for [Heap]CheckForSerializableConflictOut().
Rewrite the documentation of these functions, in light of recent bug fix
commit 5940ffb2
.
Back-patch to 13 where the check-for-conflict-out code was split up into
AM-specific and generic parts, and new documentation was added that now
looked wrong.
Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Discussion: https://postgr.es/m/db7b729d-0226-d162-a126-8a8ab2dc4443%40jepsen.io
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@ -8975,15 +8975,13 @@ heap_mask(char *pagedata, BlockNumber blkno)
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/*
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* HeapCheckForSerializableConflictOut
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* We are reading a tuple which has been modified. If it is visible to
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* us but has been deleted, that indicates a rw-conflict out. If it's
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* not visible and was created by a concurrent (overlapping)
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* serializable transaction, that is also a rw-conflict out,
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* We are reading a tuple. If it's not visible, there may be a
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* rw-conflict out with the inserter. Otherwise, if it is visible to us
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* but has been deleted, there may be a rw-conflict out with the deleter.
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*
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* We will determine the top level xid of the writing transaction with which
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* we may be in conflict, and check for overlap with our own transaction.
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* If the transactions overlap (i.e., they cannot see each other's writes),
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* then we have a conflict out.
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* we may be in conflict, and ask CheckForSerializableConflictOut() to check
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* for overlap with our own transaction.
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*
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* This function should be called just about anywhere in heapam.c where a
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* tuple has been read. The caller must hold at least a shared lock on the
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@ -4037,13 +4037,16 @@ CheckForSerializableConflictOutNeeded(Relation relation, Snapshot snapshot)
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/*
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* CheckForSerializableConflictOut
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* A table AM is reading a tuple that has been modified. After determining
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* that it is visible to us, it should call this function with the top
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* level xid of the writing transaction.
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* A table AM is reading a tuple that has been modified. If it determines
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* that the tuple version it is reading is not visible to us, it should
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* pass in the top level xid of the transaction that created it.
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* Otherwise, if it determines that it is visible to us but it has been
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* deleted or there is a newer version available due to an update, it
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* should pass in the top level xid of the modifying transaction.
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*
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* This function will check for overlap with our own transaction. If the
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* transactions overlap (i.e., they cannot see each other's writes), then we
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* have a conflict out.
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* This function will check for overlap with our own transaction. If the given
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* xid is also serializable and the transactions overlap (i.e., they cannot see
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* each other's writes), then we have a conflict out.
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*/
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void
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CheckForSerializableConflictOut(Relation relation, TransactionId xid, Snapshot snapshot)
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