Wednesday, January 2, 2008

index Lesson#3: B+ tree index -- key-pointer pairs

Nodes: root page, leaf pages and branch(or intermediate) pages. It's a -> t r e e < -. Physically, Each node is an index-PAGE on disk. There's another post on these PAGES.

Logically, a node contains key-pointer pairs. I call these "objects". You can represent them better in xml. A leaf page probably contains thousands of pairs, which read like
name rowId
-------------
Tom -> 12989
May -> 2390
May -> 907
May -> 2090 // non-unique index
This is, of course, an index on firstName.

Now you see that In a leaf node, an object consists of
1) a key ie a value in the index column. (consider single-column index for simplicity.)
2) a pointer to exactly 1 table ROW. pointer is a physical address, known as row-id in sybase, ROWID in oracle, RID in DB2

In a sybase non-clustered index, each object contains a ptr to 1 row, not 1 page.

In non-leaf nodes, an object consist of
1) a key
2) a pointer to a child PAGE (Consider a unique index for simplicity)

http://www.itworld2.com/frmdsBplus.aspx
http://db.ucsd.edu/CSE232W99/Indexing/sld031.htm
http://infocenter.sybase.com/help/index.jsp?topic=/com.sybase.dc20020_1251/html/databases/X59082.htm

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