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-<!DOCTYPE HTML PUBLIC>
-<HTML>
-<TITLE>Xbase DBMS Chapter 6</TITLE>
-<BODY BGCOLOR=#FFFFFF>
-<H2><p align="center">NDX Indices</p></H2>
-<p align="center">Chapter Updated 4/12/04</p><hr>
-
-The objective of this chapter is to provide information regarding the
-basic concepts of how .NDX index files work in the Xbase environment.<br><br>
-
-The information in this chapter has been gathered by searching the internet
-and by examining the structure of known good NDX indexes.<br><br>
-
-<h4>NDX Index File Characteristics</h4>
-
-<li>NDX indices maintain keys in ascending sort order only.<br><br>
-<li>NDX indices support <em>unique</em> or <em>non unique</em> keys.<br><br>
-
-<em>Unique</em> keys must be unique. The database update routines will
-fail if an attempt to add a non-unique key is performed.<br><br>
-
-<em>Non-unique</em> 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.<br><br>
-
-<li>NDX indexes are automatically updated by the Xbase library after the
-indices are opened.<br><br>
-
-<li>Character keys are left justified and padded on the right with spaces.<br><br>
-
-<li>Numeric keys are stored as eight byte double values.<br><br>
-
-<h4>NDX File Internals</h4>
-
-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.<br><br>
-
-<li>The <em>Head Node</em> 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.<br><br>
-
-
-<TABLE BORDER>
-<CAPTION ALIGN="TOP"><h3>NDX Header Node</H3></CAPTION>
-<TR VALIGN="BASELINE">
-<TR><TH ALIGN="LEFT">Type<TD>Size<TD>Field Name<TD>Description
-<TR><TH ALIGN="LEFT">xbLong<TD>4<TD>StartNode<TD>This identifies the root node of
- the index. The Header node is node 0.
-<TR><TH ALIGN="LEFT">xbLong<TD>4<TD>Total Nodes<TD>This is the count of the total
- nodes in the index. The count includes the header node.
-<TR><TH ALIGN="LEFT">xbLong<TD>4<TD>NoOfKeys<TD>Total number of keys in the index +1
-<TR><TH ALIGN="LEFT">xbUShort<TD>2<TD>KeyLen<TD>The index key length
-<TR><TH ALIGN="LEFT">xbUShort<TD>2<TD>KeysPerNode<TD>The maximum number of keys per node
-<TR><TH ALIGN="LEFT">xbUShort<TD>2<TD>KeyType<TD>Type of key<br>
-00 - Character<br>01 - Numeric
-<TR><TH ALIGN="LEFT">xbLong<TD>4<TD>Keysize<TD>Key record size + 8
-<TR><TH ALIGN="LEFT">char<TD>1<TD>Unknown<TD>Reserved
-<TR><TH ALIGN="LEFT">char<TD>1<TD>Unique<TD>Unique indicator<br>
-00 - Not Unique - XB_NON_UNIQUE<br>01 - Unique - XB_UNIQUE
-<TR><TH ALIGN="LEFT">char<TD>488<TD>KeyExpression<TD>Key expression string
-<TR><TH ALIGN="LEFT"><TD>512<TD><TD>Total bytes in node
-</TABLE>
-<br><br>
-The following structure is used by the Xbase NDX routines:
-<xmp>
- 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 */
- }
-</xmp>
-<br><br>
-
-<h4>Interior and Leaf Nodes</h4>
-
-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.<br><br>
-
-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.<br><br>
-
-Leaf nodes have 0 in the LeftNodeNo field but do have a value in the
-DbfRecNo field which points to a DFB record.<br><br>
-
-
-<TABLE BORDER>
-<CAPTION ALIGN="TOP"><h3>NDX Interior Node and Leaf Node Structure</H3></CAPTION>
-<TR VALIGN="BASELINE">
-<TR><TH ALIGN="LEFT">Type<TD>Size<TD>Field Name<TD>Description
-<TR><TH ALIGN="LEFT">xbLong<TD>4<TD>NoOfKeysThisNode<TD>The number of key values in this node.
-<TR><TH ALIGN="LEFT">char<TD>508<TD>KeyRec<TD>A repeating structure of
- pointers and keys. See the next table for the KeyRec structure.
-</TABLE>
-<br><br>
-<TABLE BORDER>
-<CAPTION ALIGN="TOP"><h3>KeyRec Structure</H3></CAPTION>
-<TR VALIGN="BASELINE">
-<TR><TH ALIGN="LEFT">Type<TD>Size<TD>Field Name<TD>Description
-<TR><TH ALIGN="LEFT">xbLong<TD>4<TD>LeftNodeNo<TD>The node number of the lower node
- for this key. 0 in Leaf Nodes.
-<TR><TH ALIGN="LEFT">xbLong<TD>4<TD>DbfRecNo<TD>The DBF record number for this key.
- 0 in Interior Nodes.
-<TR><TH ALIGN="LEFT">char<TD>KeyLen<TD>KeyValue<TD>The key value.
-</TABLE>
-
-<br><br>
-For those interested in knowing how the Xbase DBMS manipulates and
-navigates index files, the following discussion may be helpfull.<br><br>
-
-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.
-
-<hr>
-<p><img src="xbase.jpg"><br><hr>
-</BODY>
-</HTML>