NDX Indices

Chapter Updated 11/27/22

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 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.

    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
    TypeSizeField NameDescription
    xbLong4StartNodeThis identifies the root node of the index. The Header node is node 0.
    xbLong4Total NodesThis is the count of the total nodes in the index. The count includes the header node.
    xbLong4NoOfKeysTotal number of keys in the index +1
    xbUShort2KeyLenThe index key length
    xbUShort2KeysPerNodeThe maximum number of keys per node
    xbUShort2KeyTypeType of key
    00 - Character
    01 - Numeric
    xbLong4KeysizeKey record size + 8
    char1UnknownReserved
    char1UniqueUnique indicator
    00 - Not Unique - XB_NON_UNIQUE
    01 - Unique - XB_UNIQUE
    char488KeyExpressionKey expression string
    512Total 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
    TypeSizeField NameDescription
    xbLong4NoOfKeysThisNodeThe number of key values in this node.
    char508KeyRecA repeating structure of pointers and keys. See the next table for the KeyRec structure.


    KeyRec Structure
    TypeSizeField NameDescription
    xbLong4LeftNodeNoThe node number of the lower node for this key. 0 in Leaf Nodes.
    xbLong4DbfRecNoThe DBF record number for this key. 0 in Interior Nodes.
    charKeyLenKeyValueThe 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.