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Reduce impact of btree page reuse on Hot Standby by fixing off-by-1 error.

WAL records of type XLOG_BTREE_REUSE_PAGE were generated using alatestRemovedXid one higher than actually needed because xid used waspage opaque->btpo.xact rather than an actually removed xid.Noticed on an otherwise quiet system by Noah Misch.Noah Misch and Simon Riggs

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src/backend/access/nbtree
- nbtpage.c

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+11

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`‎src/backend/access/nbtree/nbtpage.c`

Lines changed: 11 additions & 1 deletion

Original file line number	Diff line number	Diff line change
`@@ -560,9 +560,19 @@ _bt_getbuf(Relation rel, BlockNumber blkno, int access)`
`560`	`560`	`*/`
`561`	`561`	`if (XLogStandbyInfoActive())`
`562`	`562`	`{`
	`563`	`+TransactionIdlatestRemovedXid;`
	`564`	`+`
`563`	`565`	`BTPageOpaqueopaque= (BTPageOpaque)PageGetSpecialPointer(page);`
`564`	`566`
`565`		`-_bt_log_reuse_page(rel,blkno,opaque->btpo.xact);`
	`567`	`+/*`
	`568`	`+ * opaque->btpo.xact is the threshold value not the`
	`569`	`+ * value to measure conflicts against. We must retreat`
	`570`	`+ * by one from it to get the correct conflict xid.`
	`571`	`+ */`
	`572`	`+latestRemovedXid=opaque->btpo.xact;`
	`573`	`+TransactionIdRetreat(latestRemovedXid);`
	`574`	`+`
	`575`	`+_bt_log_reuse_page(rel,blkno,latestRemovedXid);`
`566`	`576`	`}`
`567`	`577`
`568`	`578`	`/* Okay to use page. Re-initialize and return it */`

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Commite1cd66f

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`‎src/backend/access/nbtree/nbtpage.c`

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