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nbtree: Use only one notnullkey ScanKeyData.

_bt_first need only store one ScanKeyData struct on the stack for thepurposes of building an IS NOT NULL key based on an implied NOT NULLconstraint. We don't need INDEX_MAX_KEYS-many ScanKeyData structs.This saves us a little over 2KB in stack space. It's possible that thishas some performance benefit. It also seems simpler and more direct.It isn't possible for more than a single index attribute to need its ownimplied IS NOT NULL key: the first such attribute/IS NOT NULL key alwaysmakes _bt_first stop adding additional boundary keys to startKeys[].Using INDEX_MAX_KEYS-many ScanKeyData entries was (at best) misleading.Author: Peter Geoghegan <pg@bowt.ie>Reviewed-By: Mircea Cadariu <cadariu.mircea@gmail.com>Discussion:https://postgr.es/m/CAH2-Wzm=1kJMSZhhTLoM5BPbwQNWxUj-ynOEh=89ptDZAVgauw@mail.gmail.com

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src/backend/access/nbtree
- nbtsearch.c

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lines changed

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

Lines changed: 6 additions & 8 deletions

Original file line number	Diff line number	Diff line change
`@@ -892,9 +892,9 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)`
`892`	`892`	`OffsetNumberoffnum;`
`893`	`893`	`BTScanInsertDatainskey;`
`894`	`894`	`ScanKeystartKeys[INDEX_MAX_KEYS];`
`895`		`-ScanKeyDatanotnullkeys[INDEX_MAX_KEYS];`
	`895`	`+ScanKeyDatanotnullkey;`
`896`	`896`	`intkeysz=0;`
`897`		`-StrategyNumberstrat_total;`
	`897`	`+StrategyNumberstrat_total=InvalidStrategy;`
`898`	`898`	`BlockNumberblkno=InvalidBlockNumber,`
`899`	`899`	`lastcurrblkno;`
`900`	`900`
`@@ -1034,7 +1034,6 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)`
`1034`	`1034`	`* need to be kept in sync.`
`1035`	`1035`	`*----------`
`1036`	`1036`	`*/`
`1037`		`-strat_total=BTEqualStrategyNumber;`
`1038`	`1037`	`if (so->numberOfKeys>0)`
`1039`	`1038`	`{`
`1040`	`1039`	`AttrNumbercurattr;`
`@@ -1122,16 +1121,15 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)`
`1122`	`1121`	`ScanDirectionIsForward(dir) :`
`1123`	`1122`	`ScanDirectionIsBackward(dir)))`
`1124`	`1123`	`{`
`1125`		`-/Yes, so build the key in notnullkeys[keysz] /`
`1126`		`-bkey=&notnullkeys[keysz];`
	`1124`	`+/Final startKeys[] entry will be deduced NOT NULL key /`
	`1125`	`+bkey=&notnullkey;`
`1127`	`1126`	`ScanKeyEntryInitialize(bkey,`
`1128`	`1127`	`(SK_SEARCHNOTNULL \|SK_ISNULL \|`
`1129`	`1128`	`(impliesNN->sk_flags&`
`1130`	`1129`	`(SK_BT_DESC \|SK_BT_NULLS_FIRST))),`
`1131`	`1130`	`curattr,`
`1132`		`- ((impliesNN->sk_flags&SK_BT_NULLS_FIRST) ?`
`1133`		`-BTGreaterStrategyNumber :`
`1134`		`-BTLessStrategyNumber),`
	`1131`	`+ScanDirectionIsForward(dir) ?`
	`1132`	`+BTGreaterStrategyNumber :BTLessStrategyNumber,`
`1135`	`1133`	`InvalidOid,`
`1136`	`1134`	`InvalidOid,`
`1137`	`1135`	`InvalidOid,`

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

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