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Commit53f1ca5

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Allow hint bits to be set sooner for temporary and unlogged tables.
We need not wait until the commit record is durably on disk, becausein the event of a crash the page we're updating with hint bits willbe gone anyway. Per off-list report from Heikki Linnakangas, thiscan significantly degrade the performance of unlogged tables; I wasable to show a 2x speedup from this patch on a pgbench run with scalefactor 15. In practice, this will mostly help small, heavily updatedtables, because on larger tables you're unlikely to run into the samerow again before the commit record makes it out to disk.
1 parentb6335a3 commit53f1ca5

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3 files changed

+38
-5
lines changed

3 files changed

+38
-5
lines changed

‎src/backend/storage/buffer/bufmgr.c

Lines changed: 29 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -1929,6 +1929,35 @@ RelationGetNumberOfBlocksInFork(Relation relation, ForkNumber forkNum)
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returnsmgrnblocks(relation->rd_smgr,forkNum);
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}
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/*
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* BufferIsPermanent
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*Determines whether a buffer will potentially still be around after
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*a crash. Caller must hold a buffer pin.
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*/
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bool
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BufferIsPermanent(Bufferbuffer)
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{
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volatileBufferDesc*bufHdr;
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/* Local buffers are used only for temp relations. */
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if (BufferIsLocal(buffer))
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return false;
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/* Make sure we've got a real buffer, and that we hold a pin on it. */
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Assert(BufferIsValid(buffer));
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Assert(BufferIsPinned(buffer));
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/*
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* BM_PERMANENT can't be changed while we hold a pin on the buffer, so
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* we need not bother with the buffer header spinlock. Even if someone
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* else changes the buffer header flags while we're doing this, we assume
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* that changing an aligned 2-byte BufFlags value is atomic, so we'll read
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* the old value or the new value, but not random garbage.
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*/
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bufHdr=&BufferDescriptors[buffer-1];
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return (bufHdr->flags&BM_PERMANENT)!=0;
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}
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/* ---------------------------------------------------------------------
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*DropRelFileNodeBuffers
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*

‎src/backend/utils/time/tqual.c

Lines changed: 7 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -82,10 +82,12 @@ static bool XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot);
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* Set commit/abort hint bits on a tuple, if appropriate at this time.
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*
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* It is only safe to set a transaction-committed hint bit if we know the
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* transaction's commit record has been flushed to disk. We cannot change
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* the LSN of the page here because we may hold only a share lock on the
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* buffer, so we can't use the LSN to interlock this; we have to just refrain
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* from setting the hint bit until some future re-examination of the tuple.
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* transaction's commit record has been flushed to disk, or if the table is
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* temporary or unlogged and will be obliterated by a crash anyway. We
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* cannot change the LSN of the page here because we may hold only a share
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* lock on the buffer, so we can't use the LSN to interlock this; we have to
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* just refrain from setting the hint bit until some future re-examination
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* of the tuple.
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*
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* We can always set hint bits when marking a transaction aborted.(Some
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* code in heapam.c relies on that!)
@@ -113,7 +115,7 @@ SetHintBits(HeapTupleHeader tuple, Buffer buffer,
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/* NB: xid must be known committed here! */
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XLogRecPtrcommitLSN=TransactionIdGetCommitLSN(xid);
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116-
if (XLogNeedsFlush(commitLSN))
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if (XLogNeedsFlush(commitLSN)&&BufferIsPermanent(buffer))
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return;/* not flushed yet, so don't set hint */
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}
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‎src/include/storage/bufmgr.h

Lines changed: 2 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -192,6 +192,8 @@ extern void DropDatabaseBuffers(Oid dbid);
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#defineRelationGetNumberOfBlocks(reln) \
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RelationGetNumberOfBlocksInFork(reln, MAIN_FORKNUM)
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externboolBufferIsPermanent(Bufferbuffer);
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#ifdefNOT_USED
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externvoidPrintPinnedBufs(void);
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#endif

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