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Commitb452900

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Revert "Add single-item cache when looking at topmost XID of a subtrans XID"
This reverts commit06f5295 as per issues with this approach, both interms of efficiency impact and stability. First, contrary to thesingle-item cache for transaction IDs in transam.c, the cache may finishby not be hit for a long time, and without an invalidation mechanism toclear it, it would cause inconsistent results on wraparound forexample. Second, the use of SubTransGetTopmostTransaction() for thecaching has a limited impact on performance. SubTransGetParent() couldhave more impact, though the benchmarking of the single-item approachstill needs to be proved, particularly under the conditions where SLRUlookups are stressed in parallel with overflowed snapshots (aka morethan 64 subxids generated, for example).After discussion with Andres Freund.Discussion:https://postgr.es/m/20220524235250.gtt3uu5zktfkr4hv@alap3.anarazel.de
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‎src/backend/access/transam/subtrans.c

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Original file line numberDiff line numberDiff line change
@@ -54,14 +54,6 @@
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#defineTransactionIdToPage(xid) ((xid) / (TransactionId) SUBTRANS_XACTS_PER_PAGE)
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#defineTransactionIdToEntry(xid) ((xid) % (TransactionId) SUBTRANS_XACTS_PER_PAGE)
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/*
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* Single-item cache for results of SubTransGetTopmostTransaction(). It's
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* worth having such a cache because we frequently find ourselves repeatedly
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* checking the same XID, for example when scanning a table just after a
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* bulk insert, update, or delete.
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*/
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staticTransactionIdcachedFetchSubXid=InvalidTransactionId;
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staticTransactionIdcachedFetchTopmostXid=InvalidTransactionId;
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/*
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* Link to shared-memory data structures for SUBTRANS control
@@ -163,13 +155,6 @@ SubTransGetTopmostTransaction(TransactionId xid)
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/* Can't ask about stuff that might not be around anymore */
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Assert(TransactionIdFollowsOrEquals(xid,TransactionXmin));
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/*
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* Before going to the subtrans log, check our single item cache to see if
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* we know the result from a previous/recent request.
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*/
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if (TransactionIdEquals(xid,cachedFetchSubXid))
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returncachedFetchTopmostXid;
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while (TransactionIdIsValid(parentXid))
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{
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previousXid=parentXid;
@@ -189,9 +174,6 @@ SubTransGetTopmostTransaction(TransactionId xid)
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Assert(TransactionIdIsValid(previousXid));
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cachedFetchSubXid=xid;
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cachedFetchTopmostXid=previousXid;
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returnpreviousXid;
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}
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