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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:
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.
Datum *extractValue(Datum inputValue, int32 *nkeys)Returns an array of keys given a value to be indexed. The number of returned keys must be stored into *nkeys.
Datum *extractQuery(Datum query, int32 *nkeys,
StrategyNumber n, bool **pmatch, Pointer **extra_data) 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 35.14.2).
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 53.5).
-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.
bool consistent(bool check[], StrategyNumber n, Datum query,
int32 nkeys, Pointer extra_data[], bool *recheck) 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:
int comparePartial(Datum partial_key, Datum key, StrategyNumber n,
Pointer extra_data) 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 53.3.2 for details.