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Commitc2b3218

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Update comments on rd_newRelfilenodeSubid.
Ensure comments accurately reflect state of codegiven new understanding, and recent changes.Include example code from Noah Misch toillustrate how rd_newRelfilenodeSubid can bereset deterministically. No code changes.
1 parentae9aba6 commitc2b3218

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2 files changed

+26
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2 files changed

+26
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‎src/backend/commands/copy.c

Lines changed: 15 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -1963,8 +1963,18 @@ CopyFrom(CopyState cstate)
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* routine first.
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*
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* As mentioned in comments in utils/rel.h, the in-same-transaction test
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* is not completely reliable, since in rare cases rd_createSubid or
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* rd_newRelfilenodeSubid can be cleared before the end of the transaction.
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* is not always set correctly, since in rare cases rd_newRelfilenodeSubid
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* can be cleared before the end of the transaction. The exact case is
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* when a relation sets a new relfilenode twice in same transaction, yet
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* the second one fails in an aborted subtransaction, e.g.
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*
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* BEGIN;
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* TRUNCATE t;
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* SAVEPOINT save;
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* TRUNCATE t;
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* ROLLBACK TO save;
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* COPY ...
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*
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* However this is OK since at worst we will fail to make the optimization.
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*
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* Also, if the target file is new-in-transaction, we assume that checking
@@ -1994,10 +2004,9 @@ CopyFrom(CopyState cstate)
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* which subtransaction created it is crucial for correctness
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* of this optimisation.
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*
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* Note that because the test is unreliable in case of relcache reset
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* we cannot guarantee that we can honour the request to FREEZE.
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* If we cannot honour the request we do so silently, firstly to
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* avoid noise for the user and also to avoid obscure test failures.
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* As noted above rd_newRelfilenodeSubid is not set in all cases
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* where we can apply the optimization, so in those rare cases
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* where we cannot honour the request we do so silently.
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*/
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if (cstate->freeze&&
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ThereAreNoPriorRegisteredSnapshots()&&

‎src/include/utils/rel.h

Lines changed: 11 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -90,11 +90,20 @@ typedef struct RelationData
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/*
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* rd_createSubid is the ID of the highest subtransaction the rel has
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* survived into; or zero if the rel was not created in the current top
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* transaction. Thisshould be relied on only for optimization purposes;
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*it is possible for new-ness tobe "forgotten"(eg, after CLUSTER).
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* transaction. Thiscan benow berelied on, whereas previously it
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*couldbe "forgotten"in earlier releases.
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* Likewise, rd_newRelfilenodeSubid is the ID of the highest
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* subtransaction the relfilenode change has survived into, or zero if not
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* changed in the current transaction (or we have forgotten changing it).
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* rd_newRelfilenodeSubid can be forgotten when a relation has multiple
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* new relfilenodes within a single transaction, with one of them occuring
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* in a subsequently aborted subtransaction, e.g.
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* BEGIN;
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* TRUNCATE t;
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* SAVEPOINT save;
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* TRUNCATE t;
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* ROLLBACK TO save;
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* -- rd_newRelfilenode is now forgotten
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*/
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SubTransactionIdrd_createSubid;/* rel was created in current xact */
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SubTransactionIdrd_newRelfilenodeSubid;/* new relfilenode assigned in

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