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Teach nulltestsel() that system columns are never NULL.

While it's perhaps unlikely that users would write an explicit testlike "ctid IS NULL", this function is also used in range estimation,and an incorrect answer can throw off the results for tight ranges.Anyway it's not much code so we might as well do it.Edmund HornerDiscussion:https://postgr.es/m/CAMyN-kCa3BFUFrCTtQeprxTU1anCd3Pua7zXstGCKq4pXgjukw@mail.gmail.com

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src/backend/utils/adt
- selfuncs.c

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`‎src/backend/utils/adt/selfuncs.c`

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Original file line number	Diff line number	Diff line change
`@@ -1797,6 +1797,15 @@ nulltestsel(PlannerInfo root, NullTestType nulltesttype, Node arg,`
`1797`	`1797`	`return (Selectivity)0;/* keep compiler quiet */`
`1798`	`1798`	`}`
`1799`	`1799`	`}`
	`1800`	`+elseif (vardata.var&&IsA(vardata.var,Var)&&`
	`1801`	`+ ((Var*)vardata.var)->varattno<0)`
	`1802`	`+{`
	`1803`	`+/*`
	`1804`	`+ * There are no stats for system columns, but we know they are never`
	`1805`	`+ * NULL.`
	`1806`	`+ */`
	`1807`	`+selec= (nulltesttype==IS_NULL) ?0.0 :1.0;`
	`1808`	`+}`
`1800`	`1809`	`else`
`1801`	`1810`	`{`
`1802`	`1811`	`/*`

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Commitff750ce

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`‎src/backend/utils/adt/selfuncs.c`

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