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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 @@ -<!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> |