| PostgreSQL 8.4.5 Documentation | ||||
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The GIN interface has a high level of abstraction, requiring the access method implementer only to implement the semantics of the data type being accessed. The GIN layer itself takes care of concurrency, logging and searching the tree structure.
All it takes to get a GIN access method working is to implement four (or five) user-defined methods, which define the behavior of keys in the tree and the relationships between keys, indexed values, and indexable queries. In short, GIN combines extensibility with generality, code reuse, and a clean interface.
The four methods that an operator class for GIN must provide are:
Compares keys (not indexed values!) and returns an integer less than zero, zero, or greater than zero, indicating whether the first key is less than, equal to, or greater than the second.
Returns an array of keys given a value to be indexed. The number of returned keys must be stored into *nkeys.
       Returns an array of keys given a value to be queried; that is,
       query is the value on the right-hand side of an
       indexable operator whose left-hand side is the indexed column.
       n is the strategy number of the operator within the
       operator class (see Section 34.14.2).
       Often, extractQuery will need
       to consult n to determine the data type of
       query and the key values that need to be extracted.
       The number of returned keys must be stored into *nkeys.
       If the query contains no keys then extractQuery
       should store 0 or -1 into *nkeys, depending on the
       semantics of the operator.  0 means that every
       value matches the query and a full-index scan should be
       performed (but see Section 52.5).
       -1 means that nothing can match the query, and
       so the index scan can be skipped entirely.
       pmatch is an output argument for use when partial match
       is supported.  To use it, extractQuery must allocate
       an array of *nkeys booleans and store its address at
       *pmatch.  Each element of the array should be set to TRUE
       if the corresponding key requires partial match, FALSE if not.
       If *pmatch is set to NULL then GIN assumes partial match
       is not required.  The variable is initialized to NULL before call,
       so this argument can simply be ignored by operator classes that do
       not support partial match.
       extra_data is an output argument that allows
       extractQuery to pass additional data to the
       consistent and comparePartial methods.
       To use it, extractQuery must allocate
       an array of *nkeys Pointers and store its address at
       *extra_data, then store whatever it wants to into the
       individual pointers.  The variable is initialized to NULL before
       call, so this argument can simply be ignored by operator classes that
       do not require extra data.  If *extra_data is set, the
       whole array is passed to the consistent method, and
       the appropriate element to the comparePartial method.
      
       Returns TRUE if the indexed value satisfies the query operator with
       strategy number n (or might satisfy, if the recheck
       indication is returned).  The check array has length
       nkeys, which is the same as the number of keys previously
       returned by extractQuery for this query datum.
       Each element of the
       check array is TRUE if the indexed value contains the
       corresponding query key, ie, if (check[i] == TRUE) the i-th key of the
       extractQuery result array is present in the indexed value.
       The original query datum (not the extracted key array!) is
       passed in case the consistent method needs to consult it.
       extra_data is the extra-data array returned by
       extractQuery, or NULL if none.
       On success, *recheck should be set to TRUE if the heap
       tuple needs to be rechecked against the query operator, or FALSE if
       the index test is exact.
      
Optionally, an operator class for GIN can supply a fifth method:
       Compare a partial-match query to an index key.  Returns an integer
       whose sign indicates the result: less than zero means the index key
       does not match the query, but the index scan should continue; zero
       means that the index key does match the query; greater than zero
       indicates that the index scan should stop because no more matches
       are possible.  The strategy number n of the operator
       that generated the partial match query is provided, in case its
       semantics are needed to determine when to end the scan.  Also,
       extra_data is the corresponding element of the extra-data
       array made by extractQuery, or NULL if none.
      
  To support "partial match" queries, an operator class must
  provide the comparePartial method, and its
  extractQuery method must set the pmatch
  parameter when a partial-match query is encountered.  See
  Section 52.3.2 for details.