SNOMED CT® Release Format 1 (RF1) Guide - ihtsdo

344 | SNOMED CT Release Format 1 (RF1) Guide July 2012
4.5.5.1.1. Recursive testing of subtype Relationships
It is possible to determine whether one concept subsumes another concept by recursively following every
possible sequences of | is a | Relationships from a candidate concept until the predicate concept is reached
or until all possible paths have been exhausted.
This approach is far too slow to deliver effective implementations in all environments in which it has been
tested to date.
4.5.5.1.2. Semantic type Identifiers and hierarchy flags
Flags added to the internal representation of each Concept can be used to indicate the set of high-level
concept nodes of which that concept is a subtype. A concept can only subsume concepts that include the
same set of high-level concept flags.This approach can reduce the number of tests that need to be performed
to recursively test the subtype relationships:
• If a candidate does not have all the high-level node flags that the predicate has, no further tests are needed.
The candidate is not a subtype of the predicate.
• Even if a candidate shares the high-level node flags with the predicate, any path that reaches a concept
that does not share those flags need not be further tested.
While faster than the unaided recursive testing approach, this is too slow to deliver effective implementations
and is not scalable.
4.5.5.1.3. Use of proprietary database features
Some databases include additional features to support the recursive testing of a chain of hierarchical
relationships. Other methods of optimization that may be applied to allow more rapid computation of subtype
descendant relationships are outlined in the following subsections.
Current experiences of databases that support this type of approach indicate that (while easy to implement)
the performance is substantially inferior to use of branch-numbering or transitive closure (see Precomputed
Transitive Closure table ).
4.5.5.1.4. Branch numbering
The internal representation of each Concept can be extended to include a branch-number and a set of
branch-number -ranges.
A branch-numbering algorithm can then be applied when each release of SNOMED CT is imported.
A typical branch-numbering algorithm processes the subtype hierarchy in the following way:
• A depth first tree walk is performed starting from the root Concept (branch-number 1) and an incrementing
number is applied to each Concept when it is encountered for the first time.
• After the branch numbers have been computed a further tree walk allocates one or more branch-number
ranges to each Concept with any subtype descendants:
• Many Concepts will have a single branch number range containing all their descendants.
• Some Concepts will have several non-contiguous ranges of descendant Concept branch numbers:
• This is because a Concept may have multiple supertypes.Therefore, the descendants of a Concept
may have branch numbers that were allocated as a result of their relationship to another ancestor
Concept. However, the path from any Concept to the root Concept always converges at or before
the top-level Concept. Therefore, multiple ranges coalesce when reaching more general common
supertype ancestors.
• At run time, rather than needing to traverse many subtype Relationships, the branch number of each
Concept is tested for inclusion in the branch number range of the putative ancestor.
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