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
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
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
pass additional data to the
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
method, and the appropriate element to the
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
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
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
When using CREATE OPERATOR CLASS with the compare function, the storage types of the operator class must match the types of the compare's Datum parameter a and b. If they do not, you will see some obscure and potentially difficult to diagnose crashes, such as a segment fault.