1. An apparatus for providing tree structure indexing, said apparatus comprising: an integrated circuit implemented in hardware; a querying arrangement, wherein said querying arrangement queries based on a tree pattern; and an indexing arrangement, said indexing arrangement is adapted to: provide a set of XML elements containing a pattern similar to the queried tree pattern; represent the queried tree pattern via a sequence prior to responding to a query by said querying arrangement, wherein the representing comprises: taking into account the distribution of XML data during sequencing to ensure that an order of each node in the sequence cannot be solely bound by the tree pattern the node represents; unifying notation and encoding in the sequence at least one element node, attribute node, and value node of the tree pattern, wherein the value node is encoded via a hash value; determining and including an order of a plurality of encoded nodes in the sequence, wherein the order is relied upon to rebuild the tree pattern; using path-encoded sequence representation whereby each node in the tree pattern is encoded by a path leading from a root node to an end node; providing a constraint for use in path encoded-representation based on whether identical sibling nodes are present in the tree pattern, wherein said constraint consists of: a first constraint ƒ 1 (p i ,p j )≡p i ⊂p j used where there are no identical nodes which is a boolean function ƒ(•,•) that satisfies the following condition: ∀p j εT and ∀t⊂p j , there exists one and only one p i εT such that ƒ(p i ,p j )=true and p i =t., wherein T= p l ,. .. , p n is a sequence of path-encoded nodes; and ƒ(p i ,p j ) embodies the ancestor-descendant relationship between nodes p i and p j and ƒ(p i ,p j ) evaluates to true if p i is an ancestor of p j ; and a forward prefix constraint where there is at least one identical sibling nodes present which eliminates ambiguity of identical sibling nodes, said forward prefix constraint defined by p k ⊂p i is a forward prefix of p i if ∀p j =p k ,i<k<j and there does not exist p j =p k ,k<j<i._ƒ 2 (p i ,p j )≡p i , wherein p i is the forward prefix of p j ; and p l ,. .. , p n is a sequence of path-encoded nodes; and applying the constraint to the path-encoded sequence to arrive at a constraint sequence which maps to a unique tree pattern; provide query-equivalence between a tree pattern match and a subsequence match via subsequence matching and at least one constraint sequence; respond to a query by said querying arrangement via said subsequence matching; wherein constraint sequence matching is used to avoid false alarms without join operations and post-processing.