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Optimize memory access in GetRunningTransactionData()

e85662d made GetRunningTransactionData() calculate the oldest runningtransaction id within the current database. This commit optimized thiscalculation by performing a cheap transaction id comparison before fetchingthe process database id, while the latter could cause extra cache misses.Reported-by: Noah MischDiscussion:https://postgr.es/m/20240630231816.bf.nmisch%40google.com

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src/backend/storage/ipc
- procarray.c

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+11

-6

lines changed

`‎src/backend/storage/ipc/procarray.c`

Lines changed: 11 additions & 6 deletions

Original file line number	Diff line number	Diff line change
`@@ -2753,8 +2753,6 @@ GetRunningTransactionData(void)`
`2753`	`2753`	`*/`
`2754`	`2754`	`for (index=0;index<arrayP->numProcs;index++)`
`2755`	`2755`	`{`
`2756`		`-intpgprocno=arrayP->pgprocnos[index];`
`2757`		`-PGPROC*proc=&allProcs[pgprocno];`
`2758`	`2756`	`TransactionIdxid;`
`2759`	`2757`
`2760`	`2758`	`/* Fetch xid just once - see GetNewTransactionId */`
`@@ -2776,11 +2774,18 @@ GetRunningTransactionData(void)`
`2776`	`2774`	`oldestRunningXid=xid;`
`2777`	`2775`
`2778`	`2776`	`/*`
`2779`		`- * Also, update the oldest running xid within the current database.`
	`2777`	`+ * Also, update the oldest running xid within the current database. As`
	`2778`	`+ * fetching pgprocno and PGPROC could cause cache misses, we do cheap`
	`2779`	`+ * TransactionId comparison first.`
`2780`	`2780`	`*/`
`2781`		`-if (proc->databaseId==MyDatabaseId&&`
`2782`		`-TransactionIdPrecedes(xid,oldestDatabaseRunningXid))`
`2783`		`-oldestDatabaseRunningXid=xid;`
	`2781`	`+if (TransactionIdPrecedes(xid,oldestDatabaseRunningXid))`
	`2782`	`+{`
	`2783`	`+intpgprocno=arrayP->pgprocnos[index];`
	`2784`	`+PGPROC*proc=&allProcs[pgprocno];`
	`2785`	`+`
	`2786`	`+if (proc->databaseId==MyDatabaseId)`
	`2787`	`+oldestDatabaseRunningXid=xid;`
	`2788`	`+}`
`2784`	`2789`
`2785`	`2790`	`if (ProcGlobal->subxidStates[index].overflowed)`
`2786`	`2791`	`suboverflowed= true;`

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Commit6897f0e

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`‎src/backend/storage/ipc/procarray.c`

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