From 4875a3dd9b183dcd2256e2abfc4ccf7484c233b4 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?J=C3=B6rg=20Frings-F=C3=BCrst?= Date: Wed, 7 Dec 2022 13:17:14 +0100 Subject: New upstream version 4.0.2 --- html/xbc6.htm | 150 ---------------------------------------------------------- 1 file changed, 150 deletions(-) delete mode 100755 html/xbc6.htm (limited to 'html/xbc6.htm') diff --git a/html/xbc6.htm b/html/xbc6.htm deleted file mode 100755 index f5cf75d..0000000 --- a/html/xbc6.htm +++ /dev/null @@ -1,150 +0,0 @@ - - -Xbase DBMS Chapter 6 - -

NDX Indices

-

Chapter Updated 4/12/04

- -The objective of this chapter is to provide information regarding the -basic concepts of how .NDX index files work in the Xbase environment.

- -The information in this chapter has been gathered by searching the internet -and by examining the structure of known good NDX indexes.

- -

NDX Index File Characteristics

- -
  • NDX indices maintain keys in ascending sort order only.

    -
  • NDX indices support unique or non unique keys.

    - -Unique keys must be unique. The database update routines will -fail if an attempt to add a non-unique key is performed.

    - -Non-unique Keys are not required to be unique, duplicate -keys are allowed if the index is created with the XB_NOT_UNIQUE -setting. Duplicate keys are stored in record number order.

    - -
  • NDX indexes are automatically updated by the Xbase library after the -indices are opened.

    - -
  • Character keys are left justified and padded on the right with spaces.

    - -
  • Numeric keys are stored as eight byte double values.

    - -

    NDX File Internals

    - -NDX files are comprised of two or more 512 byte blocks or nodes of -information. There are three types of nodes: Head Nodes, Interior -Nodes and Leaf Nodes.

    - -
  • The Head Node is the first node in the file starting at -position zero (0) and contains information about the NDX file. There -is only one Head Node in each index and it always starts at the -beginning of the file.

    - - - - - -

    NDX Header Node

    TypeSizeField NameDescription -
    xbLong4StartNodeThis identifies the root node of - the index. The Header node is node 0. -
    xbLong4Total NodesThis is the count of the total - nodes in the index. The count includes the header node. -
    xbLong4NoOfKeysTotal number of keys in the index +1 -
    xbUShort2KeyLenThe index key length -
    xbUShort2KeysPerNodeThe maximum number of keys per node -
    xbUShort2KeyTypeType of key
    -00 - Character
    01 - Numeric -
    xbLong4KeysizeKey record size + 8 -
    char1UnknownReserved -
    char1UniqueUnique indicator
    -00 - Not Unique - XB_NON_UNIQUE
    01 - Unique - XB_UNIQUE -
    char488KeyExpressionKey expression string -
    512Total bytes in node -
    -

    -The following structure is used by the Xbase NDX routines: - - struct NdxHeadNode{ - xbLong StartNode; /* header node is node 0 */ - xbLong TotalNodes; /* includes header node */ - xbLong NoOfKeys; /* actual count + 1 */ - xbUShort KeyLen; /* length of key data */ - xbUShort KeysPerNode; /* max number of keys per node */ - xbUShort KeyType; /* 00 = Char, 01 = Numeric */ - xbLong KeySize; /* KeyLen + 8 */ - char Reserved1; /* Not sure about this one */ - char Unique; /* 00 = not unique, 01 = unique*/ - char KeyExpression[488]; /* key definition */ - } - -

    - -

    Interior and Leaf Nodes

    - -Interior Nodes and Leaf Nodes share the same structure in an NDX file. -The difference between the two types is that interior nodes point to -other interior nodes or leaf nodes and leaf nodes point to records in -a DBF file. Interior nodes are optional nodes in an NDX file, -however if there are more than a few keys in the index there will -certainly be one or more interior nodes in the file. There will -always be at least one leaf node in the file. Leaf nodes contain DBF -record numbers which point to the location of the record in the -DBF file.

    - -Interior nodes have field LeftNodeNo valued which points to the node -which points to the keys which are less than the key value in the KeyVal -field. There is one more LeftNodeNo value in the node than there are keys. -The Last LeftNodeNo points to the node which is greater than the highest -key value in the node. Interior nodes have 0 in the value for the -DbfRecNo field.

    - -Leaf nodes have 0 in the LeftNodeNo field but do have a value in the -DbfRecNo field which points to a DFB record.

    - - - - - -

    NDX Interior Node and Leaf Node Structure

    TypeSizeField NameDescription -
    xbLong4NoOfKeysThisNodeThe number of key values in this node. -
    char508KeyRecA repeating structure of - pointers and keys. See the next table for the KeyRec structure. -
    -

    - - - -

    KeyRec Structure

    TypeSizeField NameDescription -
    xbLong4LeftNodeNoThe node number of the lower node - for this key. 0 in Leaf Nodes. -
    xbLong4DbfRecNoThe DBF record number for this key. - 0 in Interior Nodes. -
    charKeyLenKeyValueThe key value. -
    - -

    -For those interested in knowing how the Xbase DBMS manipulates and -navigates index files, the following discussion may be helpfull.

    - -Xbase DBMS navigates through NDX files by using an in-memory chain -of nodes of the current location / key in use. It starts by reading the -Head Node of the index, which points to the first node of the file. The -first node of the file will be a leaf node if the index is small or will -be an interior node if the index has more than one leaf node. The first -interior node is loaded into memory, added to the node chain and points -to the next node to read. The node is made up of one or more keys. If -it is a leaf node, the logic looks for a matching key on the node. -Otherwise, if it is an interior node, the logic looks at the keys until the -search key is greater than or equal to the key in the node and then -traverses down the tree to the next node. It continues down the tree, -adding the nodes to the in-memory node chain until it reaches the correct -leaf node. If it finds a matching key in the leaf node, it returns a -XB_FOUND condition. If it doesn't find an exact match in the leaf node, it -returns a XB_NOT_FOUND condition and stops on the key which is greater than -the search key given. - -
    -


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