Xbase DBMS Chapter 6

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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

  Type | Size | Field Name | Description
  xbLong | 4 | StartNode | This identifies the root node of the index. The Header node is node 0.
  xbLong | 4 | Total Nodes | This is the count of the total nodes in the index. The count includes the header node.
  xbLong | 4 | NoOfKeys | Total number of keys in the index +1
  xbUShort | 2 | KeyLen | The index key length
  xbUShort | 2 | KeysPerNode | The maximum number of keys per node
  xbUShort | 2 | KeyType | Type of key 00 - Character 01 - Numeric
  xbLong | 4 | Keysize | Key record size + 8
  char | 1 | Unknown | Reserved
  char | 1 | Unique | Unique indicator 00 - Not Unique - XB_NON_UNIQUE 01 - Unique - XB_UNIQUE
  char | 488 | KeyExpression | Key expression string
  | 512 |  | Total 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

  Type | Size | Field Name | Description
  xbLong | 4 | NoOfKeysThisNode | The number of key values in this node.
  char | 508 | KeyRec | A repeating structure of pointers and keys. See the next table for the KeyRec structure.

  ### KeyRec Structure

  Type | Size | Field Name | Description
  xbLong | 4 | LeftNodeNo | The node number of the lower node for this key. 0 in Leaf Nodes.
  xbLong | 4 | DbfRecNo | The DBF record number for this key. 0 in Interior Nodes.
  char | KeyLen | KeyValue | The 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.