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Prevent synchronized scanning when systable_beginscan chooses a heapscan.

The only interesting-for-performance case wherein we force heapscan hereis when we're rebuilding the relcache init file, and the only such casethat is likely to be examining a catalog big enough to be syncscanned isRelationBuildTupleDesc. But the early-exit optimization in that code getsbroken if we start the scan at a random place within the catalog, so thatallowing syncscan is actually a big deoptimization if pg_attribute is large(at least for the normal case where the rows for core system catalogs havenever been changed since initdb). Hence, prevent syncscan here. Per mytesting pursuant to complaints from Jeff Frost and Greg Sabino Mullane,though neither of them seem to have actually hit this specific problem.Back-patch to 8.3, where syncscan was introduced.

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src/backend/access/index
- genam.c

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`‎src/backend/access/index/genam.c`

Lines changed: 10 additions & 1 deletion

Original file line number	Diff line number	Diff line change
`@@ -293,7 +293,16 @@ systable_beginscan(Relation heapRelation,`
`293`	`293`	`}`
`294`	`294`	`else`
`295`	`295`	`{`
`296`		`-sysscan->scan=heap_beginscan(heapRelation,snapshot,nkeys,key);`
	`296`	`+/*`
	`297`	`+ * We disallow synchronized scans when forced to use a heapscan on a`
	`298`	`+ * catalog. In most cases the desired rows are near the front, so`
	`299`	`+ * that the unpredictable start point of a syncscan is a serious`
	`300`	`+ * disadvantage; and there are no compensating advantages, because`
	`301`	`+ * it's unlikely that such scans will occur in parallel.`
	`302`	`+ */`
	`303`	`+sysscan->scan=heap_beginscan_strat(heapRelation,snapshot,`
	`304`	`+nkeys,key,`
	`305`	`+ true, false);`
`297`	`306`	`sysscan->iscan=NULL;`
`298`	`307`	`}`
`299`	`308`

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Commit532fe28

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`‎src/backend/access/index/genam.c`

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