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diff --git a/html/xbc6.htm b/html/xbc6.htm new file mode 100755 index 0000000..f5cf75d --- /dev/null +++ b/html/xbc6.htm @@ -0,0 +1,150 @@ +<!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> |