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Use GetCurrentTimestamp instead of instr_time in GlobalSnapshotGenerate.

CLOCK_MONOTONIC is not ok because it is time since server start, which meansvalues between nodes are totally uncorrelated. So resort to use usual real timeclock GetCurrentTimestamp (gettimeofday); we should now ensure monotonicity onour own, but this was done previously anyway.Ideally, we should use clock_gettime with CLOCK_REALTIME which might providenanosecond resolution.

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src/backend/access/transam
- global_snapshot.c

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`‎src/backend/access/transam/global_snapshot.c`

Lines changed: 4 additions & 5 deletions

Original file line number	Diff line number	Diff line change
`@@ -369,23 +369,22 @@ GlobalSnapshotToXmin(GlobalCSN snapshot_global_csn)`
`369`	`369`	`* GlobalSnapshotGenerate`
`370`	`370`	`*`
`371`	`371`	`* Generate GlobalCSN which is actually a local time. Also we are forcing`
`372`		`- * this time to be always increasing. Since now it is not uncommon to have`
`373`		`- * millions of read transactions per second we are trying to use nanoseconds`
	`372`	`+ * this time to be always increasing.`
	`373`	`+ * XXX since now it is not uncommon to have`
	`374`	`+ * millions of read transactions per second should use nanoseconds`
`374`	`375`	`* if such time resolution is available.`
`375`	`376`	`*/`
`376`	`377`	`GlobalCSN`
`377`	`378`	`GlobalSnapshotGenerate(boollocked)`
`378`	`379`	`{`
`379`		`-instr_timecurrent_time;`
`380`	`380`	`GlobalCSNglobal_csn;`
`381`	`381`
`382`	`382`	`Assert(track_global_snapshots\|\|global_snapshot_defer_time>0);`
`383`	`383`
`384`	`384`	`/*`
`385`	`385`	`* TODO: create some macro that add small random shift to current time.`
`386`	`386`	`*/`
`387`		`-INSTR_TIME_SET_CURRENT(current_time);`
`388`		`-global_csn= (GlobalCSN)INSTR_TIME_GET_NANOSEC(current_time);`
	`387`	`+global_csn=GetCurrentTimestamp()1000;/ nsec per usec */`
`389`	`388`
`390`	`389`	`/* TODO: change to atomics? */`
`391`	`390`	`if (!locked)`

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Commit248913f

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`‎src/backend/access/transam/global_snapshot.c`

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