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 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 index operator class for
GIN must provide are:
int compare(Datum a, Datum b)
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 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 33.14.2).
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.
bool consistent(bool check[], StrategyNumber n, Datum query)
Returns TRUE if the indexed value satisfies the query operator with
strategy number n (or may satisfy, if the operator is
marked RECHECK in the operator class). The check array has
the same length as the number of keys previously returned by
extractQuery for this query. 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.
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