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+<!DOCTYPE HTML PUBLIC>
+<HTML>
+<TITLE>Xbase DBMS Chapter 8</TITLE>
+<BODY BGCOLOR=#FFFFFF>
+<H2><p align="center">MDX Indices</p></H2>
+<p align="center">Chapter Updated 12/02/22</p><hr>
+
+The objective of this chapter is to provide information regarding the
+basic concepts of how .MDX 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 <DX indexes.<br><br>
+
+<h4>MDX Index File Characteristics</h4>
+
+<li>MDX files are the same name as the corresponding DBF file with an MDX extension.
+<li>MDX files are automatically opened by the library when the DBF file is opened.
+<li>MDX index files (aka prod indices) contain from one to 47 tags, where each tag has it's own key characteristics.
+<li>MDX indices maintain keys in either ascending or descending sort order.
+<li>MDX indices support filtered keys.  For example, a filter of <b>.NOT. DELETED()</b> will keep deleted records out
+of the index tag.
+<li>MDX indices are automatically updated by the Xbase library after the
+indices are opened.
+
+<li>MDX indices support <em>unique</em> or <em>non unique</em> keys.<br><br>
+
+<em>Unique</em> keys must be unique if the UniqueKeyOption is not set to XB_EMULATE_DBASE.
+If the UniqueKeyOption is set to XB_EMULATE_DBASE, then the database update routines will 
+add a record to the table, but not add a corresponding duplicate key to the index tag.
+The UniqueKeyOption is off (don't allow duplicates) by default.
+<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>Character keys are left justified and padded on the right with spaces.
+<li>Numeric keys are stored as twelve byte BCD values.
+<li>Date keys are stored as eight byte double julian values.
+
+<h4>MDX File Internals</h4>
+
+The following information is not needed to use the library, it is just included
+for general information.<br><br>
+
+MDX files are comprised of 512 pages where multiple pages make a block.  The default 
+setting is 1024 blocks, each block containing two pages.<br><br>
+
+The first four pages contain:
+<li>Bytes 0 - 543 contain general file information.
+<li>Bytes 544 - 2047 is a 47 item table containing specific tag information.
+<br><br>
+
+Pages five and beyound:
+<li>Bytes 2048 and beyond contain tag header blocks, interior nodes and leaf nodes.
+
+<br><br>
+
+<h4>Interior and Leaf Nodes</h4>
+
+Interior Nodes and Leaf Nodes share the same structure in an NDX file with 
+the exception that interior nodes have a non zero number immediately
+after the rightmost key on the node.
+
+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 MDX 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 per tag in the file.  Leaf nodes 
+contain DBF record numbers which point to the location of the record 
+in the DBF file.<br><br>
+
+<br><br>
+
+<hr>
+<p><img src="xbase.jpg"><br><hr>
+</BODY>
+</HTML>