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document pattern of branch splits

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Jason E. Aten 2020-12-08 20:50:29 +00:00
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rbf/README.md
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@ -108,3 +108,97 @@ change as development progresses:
- WAL support is deferred
## Pattern of branch splits when adding data in ascending sorted order.
In this example, fan-out is restricted to 2 to make the
drawings easy and the splits obvious. The data leaves are only allowed one
roaring.Container key (ckey) in this example.
Each frame adds the next datum: A,B,C,D,E,... in order.
The letter represent data leaves, while
numbers represent branch pages. The one exception is the
first frame where the root is a leaf with data.
Every frame after has normal branch at the root.
NB: there are only three places pages get written on addition:
a) writeRoot
b) putLeafCell
c) putBranchCells
---------------------------------------------
add A:
root
3
A
---------------------------------------------
add B:
root
3
4 5
A B
---------------------------------------------
add C: this sequence of updates occurs
1) putLeafCell writes leaf B to page 5
2) putLeafCell writes leaf C to page 6
3) putBranchCells writes branch page 7 with children 4,5
4) putBranchCells writes branch page 8 with child 6
5) writeRoot writes branch cells to pgno 3, children: 7,8
root
3
7 8
4 5 6
A B C
---------------------------------------------
add D:
root
3
7 8
4 5 6 9
A B C D
---------------------------------------------
add E:
root
3
12 13
7 8 11
4 5 6 9 10
A B C D E
---------------------------------------------
add F:
root
3
12 13
7 8 11
4 5 6 9 10 14
A B C D E F
---------------------------------------------
add G:
root
3
12 13
7 8 11 16
4 5 6 9 10 14 15
A B C D E F G
---------------------------------------------
add H:
root
3
12 13
7 8 11 16
4 5 6 9 10 14 15 17
A B C D E F G H
---------------------------------------------
add I:
root
3
21 22
12 13 20
7 8 11 16 19
4 5 6 9 10 14 15 17 18
A B C D E F G H I
---------------------------------------------